Chapter 7: Plant Growth and Mineral Nutrition
In the early years of the twentieth century, a scientist in Calcutta sat watching a plant — and complained that nothing seemed to be happening. Jagadish Chandra Bose knew perfectly well that the seedling in front of him was growing. Yet growth in plants is so unhurried that a human eye, watching for an hour, sees nothing at all. Most people accepted this as a limitation of plants. Bose treated it as a limitation of instruments.
So he built a better instrument. His crescograph used a system of levers and a recording surface to magnify the movement of a growing tip thousands of times — by some accounts up to ten thousand times — until the invisible creeping of a shoot became a visible, traceable line. For the first time, growth could be watched almost as it happened. Bose used the device to show that a plant’s growth is not a fixed, mechanical thing: speed it up, slow it down, poison it, chill it, and the recorded line changes shape in response.
That single idea sits underneath this whole chapter. A plant is not simply getting bigger. It is dividing, stretching, maturing, sensing light and cold, moving hormones about, and pulling minerals out of the soil — all at once, and all in a controlled order.
Diagram
Fig. 7.0 – A seedling clamped in a crescograph-style setup, with a lever arm magnifying the tip’s upward growth onto a smoked recording drum; label seedling, growing tip, lever system, recording drum, traced growth curve
Keep these questions in mind as you read. This chapter will answer every one of them.
7.1 Plant Growth
Growth is one of the defining features of a living organism. As a phenomenon, growth has two aspects — a quantitative aspect and a qualitative aspect. Looking at both together is what separates real biological growth from a stone simply getting bigger as more material piles onto it.
The Two Faces of Growth
The quantitative aspect speaks of measurable increase: increase in the length, breadth, size, volume, body mass or dry weight, and in the number of cells. These are things you can put a number to. Growth, seen as a quantitative change, is the final end product of successful metabolism — a plant can only build new material if its metabolic reactions have supplied enough energy and raw material to build with.
The qualitative aspect talks about the change in the nature of growth — not just “more,” but “different.” Here, development is an ordered change or progress, while differentiation leads a cell to a higher and more complex state. A growing seedling does not merely get taller; its cells take on distinct identities and its form changes.
Putting both aspects together gives us the working definition of growth used throughout this chapter.
The word irreversible matters. A wilted plant that swells back up after watering has not “grown” — it has simply regained water it lost. Growth cannot be undone by removing water, because new cells and new wall material have actually been laid down.
Where Plants Grow
In multicellular (vascular) plants, growth is indeterminate — it occurs throughout life, indefinitely, with no fixed final size for the plant as a whole. This is also described as open growth, because the plant keeps the option of adding new organs open for as long as it lives. An animal, in contrast, reaches a definite adult size and stops; this is the key difference the recall question above points to.
But growth in a plant is not spread evenly over the whole body. It is restricted to some specific regions called meristems, which are the regions where new cells are constantly and continuously produced.
Because a meristem holds on to the power of division, a plant can go on adding fresh cells at particular sites long after the rest of its body has matured. Meristems are of three types based on their location, as shown in FC 7.1 — Apical, Intercalary and Lateral.
Flowchart
FC 7.1 – Classification of meristems by location: Meristem → (branch 1) Apical – at apices of root and shoot; (branch 2) Intercalary – at node or base of internode; (branch 3) Lateral – laterally along the axis (vascular cambium)
Their positions on the plant body are worth fixing in your mind, since each position explains what that meristem can do — as seen in Fig. 7.1a.
Diagram
Fig. 7.1a – Outline of a young dicot plant marking the three meristems: apical meristem at root tip and shoot tip, intercalary meristem at the node/base of internode, lateral meristem (vascular cambium) as a strip inside the stem
The Three Meristems and What They Do
The apical meristem lies at the apices (tips) of the root and the shoot. In vascular plants, growth is restricted to these apices; the apical meristem is responsible for growth in length or height, and for the differentiation of cell types. It contributes to the primary growth of the plant — the growth that makes a plant taller and its roots deeper.
The intercalary meristem is located at the node, or at the base of an internode of the stem. It is primarily responsible for increasing the length of internodes, and also for the formation of leaf primordia and lateral buds. This is the meristem that lets grasses shoot back up after grazing or mowing, because the growing region sits low down rather than only at the tip.
The lateral meristem is located laterally, along the axis of dicotyledons and gymnosperms. It occurs as a strip within the vascular bundles of the stem of dicots, where it is called the vascular cambium. It is responsible for the increase in the girth (thickness) of the stem, through the addition of secondary vascular tissues.
Growth That Stops, and Growth That Does Not
Indeterminate growth is not the whole story even within a single plant. The root and the stem show indefinite or indeterminate growth. However, in organs like leaves, flowers and fruits, growth is determinate — they grow only up to a certain genetically destined size and then stop. A mango leaf does not keep expanding all season; it reaches its species-typical size and halts.
In unicellular plants, growth is uniform and determinate. There is only one cell, and it grows to a set size before dividing, so there is no question of adding new organs indefinitely.
Growth Is a Recurring Idea
Growth, development and differentiation are not three unrelated words but three linked descriptions of the same living process — increase in bulk, ordered progress in form, and specialisation of cells. The rest of this chapter examines how a plant achieves all three: in ordered phases (Section 7.2), under specific conditions (Section 7.3), at measurable rates (Sections 7.4 and 7.5), and under the chemical control of hormones and the environmental control of light and temperature.
7.2 Phases of Growth
A meristematic cell does not turn into a mature root or stem cell in one step. It passes through a fixed sequence of events: it divides, then it enlarges, and finally it becomes specialised. The cells in the meristem divide, enlarge and get differentiated, and corresponding to these three stages there are three phases of growth.
Because the sequence is fixed, the three phases can also be seen laid out in space along a growing root tip — the youngest cells sit closest to the tip and the oldest, most specialised cells sit furthest from it. Notice in Fig. 7.1 how the zones follow one another from the root tip upwards, and how the position of the radicle changes between the beginning (A) and the end (B) of a period of growth.
Diagram
Fig. 7.1 – Phases of growth in root: a germinating seedling showing plumule and radicle, with the radicle marked at the beginning (A) and at the end (B); label from tip upwards — zone of cell formation, zone of cell elongation, zone of cell differentiation, zone of mature cells
Flowchart
FC 7.2 – Phases of growth in sequence: Meristematic cell → Phase of cell division/formation (Lag phase) → Phase of cell enlargement/elongation (Exponential or Log phase) → Phase of cell maturation/differentiation (Stationary phase) → mature, specialised cell
A. Phase of Cell Division / Formation
Cells of the meristem are thin walled and non-vacuolated, with a prominent nucleus and granular cytoplasm. Each of these features suits a cell whose only job is to divide: a thin wall can be split and rebuilt quickly, the absence of a large vacuole leaves the cytoplasm dense and packed with the machinery of division, and a prominent nucleus reflects the constant demand for DNA replication and control of mitosis.
The meristematic cell — particularly a cambial cell — undergoes mitotic division to form two new cells. Mitosis is the type of nuclear division that produces two daughter cells with exactly the same chromosome number and the same genetic content as the parent cell, so both daughters are fully equipped to carry on.
What happens next is the key to why a meristem never runs out. One of the two cells remains meristematic, keeping the power to divide again. The other cell leaves the cycle and goes on to enlargement and differentiation. In this way the meristem renews itself while still supplying the plant with new cells.
In this phase, the rate of growth occurs at a slower pace. Cells are being manufactured, but they are small, so the plant’s measurable size changes very little. This slow start is called the lag phase.
B. Phase of Cell Enlargement / Elongation
The newly formed cell now becomes vacuolated, osmotically active and turgid due to the absorption of water. A vacuole forms and fills with cell sap; the solutes dissolved in that sap make the cell osmotically active, so water moves in by osmosis. The inward push of this water against the cell wall is turgidity.
The turgidity results in the enlargement of the cell — both lengthwise and breadthwise. Water, in other words, does the physical work of stretching the cell, which is a remarkably cheap way for a plant to gain size compared with building whole new cells.
But a stretching cell would simply burst if its wall did not keep pace. In this phase new wall materials and other materials are synthesised to cope up with the enlargement, so the wall expands and is reinforced as the cell grows.
Since each cell is now adding a large volume very quickly, the growth rate in this phase occurs at an accelerated pace. This is the exponential or log phase.
C. Phase of Cell Maturation / Differentiation
The enlarged cell now becomes specialised to perform a specific function and attains maturity — both morphological (in its form and structure) and physiological (in the work it does). A cell in this phase may thicken its wall to become a supporting fibre, lose its contents to become a water-conducting element, or fill with chloroplasts to become a photosynthetic cell.
In this phase, the rate of growth slows down and comes to a steady state. This is the stationary phase. Growth in bulk has essentially finished here; what the cell gains now is competence at a job.
7.3 Conditions for Growth
Meristems can divide, cells can elongate, and cells can mature — but none of this happens unless the surroundings allow it. Growth is the end product of successful metabolism, and metabolism has requirements. The different environmental and physiological conditions necessary for growth include water, supply of nutrients, temperature, oxygen, carbon/nitrogen ratio, gravitational force, light, and growth hormones. Together these conditions are summarised in Fig. 7.3a.
Diagram
Fig. 7.3a – A young plant at the centre with arrows pointing inward from the eight conditions for growth: water, nutrients, temperature, oxygen, carbon/nitrogen ratio, gravitational force, light, growth hormones
The chief conditions are explained below.
Water
Water is the essential component of protoplasm and maintains the turgidity of the cell. You have already seen why turgidity matters: without water entering the vacuole, the cell simply cannot stretch, and the entire phase of cell enlargement stalls.
Water also acts as the aqueous medium for biochemical reactions. Enzymes work in solution, and the substrates they act on must be dissolved before they can meet an enzyme at all. A dehydrated cell is therefore not just a limp cell — it is a metabolically slowed cell.
Supply of Nutrients
Microelements and macroelements are the nutrients required for the proper growth of the plant. Microelements are needed only in traces, macroelements in larger quantity, and each has a specific job to do in the plant body. A shortage of even one of them shows up as a visible disorder in growth. These elements, their roles and their deficiency symptoms are dealt with in detail in Section 7.12.
Temperature
Growth depends on enzyme-catalysed reactions, and enzymes are sensitive to temperature. Optimum temperature for growth ranges between 25–35 °C. Below this range, reactions become too slow to support active division and elongation; well above it, enzymes begin to lose their shape and stop working.
Oxygen
Oxygen is essential for respiration and the release of energy. Cell division, the synthesis of new wall material, and the active uptake of nutrients all consume energy in the form of ATP, and aerobic respiration is where that ATP comes from. An oxygen-starved root cannot pay for its own growth.
Light
Light is very much essential for germination of seed and for photosynthesis. Photosynthesis supplies the sugars that are the raw material and the fuel for all growth, so light is, indirectly, the source of the plant’s building material. Light also acts as a signal rather than only as an energy source — a role you will meet again in photoperiodism (Section 7.10).
Gravitational Force
Gravitational force decides the direction of growth of the shoot and the root. It does not supply energy or material; it supplies information. The shoot grows away from the pull of gravity and the root grows towards it, which is why a root reliably heads down into the soil and a shoot reliably heads up into the light.
Carbon/Nitrogen Ratio and Growth Hormones
The carbon/nitrogen ratio — the balance between carbon-rich compounds (mainly carbohydrates from photosynthesis) and nitrogen-rich compounds (mainly proteins) in the plant — influences the pattern of growth, and so is listed among the conditions for growth.
Finally, growth hormones are internal, physiological conditions rather than external ones. They are chemical signals made by the plant itself that promote, inhibit or modify growth, and they decide how the plant responds to all the external conditions listed above. They are taken up fully in Section 7.9.
7.4 Growth Rate and Types of Growth
Growth Rate
We have seen what growth is and where it happens. The next question a scientist asks is how fast. Growth rate is the increased growth per unit time. It is also called the efficiency index, because it tells us how efficiently the plant is converting its metabolism into new bulk.
Rate of growth can be measured by an increase in the size and area of different plant organs, such as the leaf, the flower and the fruit. In other words, growth rate is not one single measurement — it depends on which organ and which parameter you have chosen to follow.
Absolute and Relative Growth Rate
The ratio of change in the cell number (\(dn\)) over the time interval (\(dt\)) is called the Absolute growth rate (AGR).
\[\text{AGR} = \frac{dn}{dt}\]
Here \(dn\) is the change in cell number and \(dt\) is the interval of time over which that change took place; the ratio therefore gives the number of new cells appearing per unit time. Stated more generally, AGR is the measurement and comparison of total growth per unit time.
The AGR, when divided by the total number of cells present in the medium (\(n\)), gives the Relative growth ratio (RGR).
\[\text{RGR} = \frac{\text{AGR}}{n}\]
Alternatively, RGR refers to the growth of a particular system per unit time, expressed on a common basis; or it is the ratio of growth in the given time to the initial growth. The word relative is the whole point — RGR asks not “how much was added?” but “how much was added, compared with how much was already there?”
Both quantities are useful in describing the dynamics of cell growth in culture, where a scientist needs to know both the absolute output of a culture and how vigorously each existing cell is contributing to it.
Comparing Two Growing Leaves
We can also observe quantitative comparison between the growth of a living system in two ways. Measurement and comparison of total growth per unit time is called the Absolute growth rate (AGR), whereas the growth of the given system per unit time, expressed on a common basis per unit initial parameter, is called the Relative growth rate (RGR). The difference between the two becomes obvious the moment we compare two leaves of unequal size, as in Fig. 7.4.
Diagram
Fig. 7.4 – Growth rate comparison: a small leaf A of 5 cm² enlarging to A¹ of 10 cm², and a large leaf B of 50 cm² enlarging to B¹ of 55 cm²; both gains marked as 5 cm²
In the above example, two leaves ‘A’ and ‘B’ are of different sizes but show the same absolute increase in area in a given time. Both leaves grow and increase their area by 5 cm² to produce the ‘A¹’ and ‘B¹’ leaves. Their AGR is identical.
Their RGR, however, is not. Leaf ‘A’ of 5 cm² in size grows 5 cm²/day, so its RGR would be 100%. If the leaf is 50 cm² in size and the growth rate/day is 5 cm², then its RGR would become 10%.
Types of Growth
There are two types of growth, viz., arithmetic growth and geometric growth. The difference between them comes down to a single question: after a cell divides, do both daughter cells keep dividing, or only one?
Flowchart
FC 7.3 – Two fates after mitosis: Meristematic cell divides → (branch A, arithmetic growth) one daughter keeps dividing + one daughter differentiates and matures → constant rate; (branch B, geometric growth) both daughters keep dividing and redividing → doubling, exponential rate
a. Arithmetic Growth
Here, the rate of growth is constant, and the increase in growth occurs in arithmetic progression, i.e. 2, 4, 6, 8 cms, etc. In this type of growth, the rate of growth is constant — the same amount is added in every equal interval of time.
This happens because, after mitosis, one of the daughter cells continues to divide while the other cell takes part in differentiation and maturation. Only one cell is left dividing at each round, so the supply of new cells stays steady instead of multiplying. For example, elongation of a root at a constant rate best explains arithmetic growth.
A linear curve is obtained when growth is plotted against time. Arithmetic growth is expressed mathematically by an equation.
It is expressed as:
\[L_t = L_o + rt\]
Where \(L_t\) = Length at time ‘t’ \(L_o\) = Length at time ‘zero’ \(r\) = Growth rate \(t\) = Time of growth
Length here is measured in units of length (SI unit: metre, though centimetres are used in practice), \(t\) in units of time, and \(r\) therefore in length per unit time. When a graph of length (\(L\)) is plotted against time (\(t\)), a linear curve is obtained, as indicated in Graph 7.2.
Diagram
Graph 7.2 – Constant linear growth: height of the plant (L) on the y-axis against time (t) on the x-axis, giving a straight ascending line, with a small right-angled triangle on the line marking rise ‘a’ over run ‘b’ to show the constant slope
b. Geometric Growth
A cell divides mitotically into two. Here, both the daughter cells continue to divide and redivide repeatedly. Such growth is called geometric growth. Since no daughter cell drops out to mature, the number of dividing cells doubles each round: 1 becomes 2, 2 becomes 4, 4 becomes 8, and so on.
Here, growth rate is slow initially but later on there is a rapid growth at an exponential rate. The early slowness is deceptive — doubling a small number gives a small increase, but once the population of cells is large, each doubling adds an enormous number of cells. Geometric growth can be expressed mathematically by an equation.
\[W_1 = W_o e^{rt}\]
Where \(W_1\) = Final size \(W_o\) = Initial size \(r\) = Growth rate \(t\) = Time of growth \(e\) = Base of natural logarithm
Both the arithmetic and the geometric patterns are shown in Fig. 7.3, along with the stages during embryo development in which the two phases appear one after the other.
Diagram
Fig. 7.3 – Growth rate: (a) arithmetic growth, one cell of each pair continuing to divide while the other stops; (b) geometric growth, all cells dividing and redividing (1, 2, 4, 8 … 16); (c) stages during embryo development showing an early geometric phase in which all cells divide, followed by an arithmetic phase in which some cells lose the capacity to divide; use one shade for cells capable of division and another for cells that have lost the capacity to divide
How Growth Is Actually Measured
Growth in plants can be measured in terms of several different parameters, and the one chosen depends on the organ being studied.
- Increase in the number of cells produced — e.g. a single maize root apical meristem can give rise to more than 17,500 new cells/hour.
- Increase in surface area of the leaf — e.g. growth of a dorsiventral leaf.
- Increase in length — e.g. growth of a pollen tube.
- Increase in volume of a fruit — e.g. in a watermelon flower, the ovary after fertilization increases in its size/volume by up to 3,50,000 times.
- Increase in girth of shoot.
- Increase in dry weight of organ.
Notice how badly a single yardstick would serve here. Measuring a pollen tube by its girth, or a watermelon by its cell count, would tell us nothing useful. The parameter must match what the organ is actually doing as it grows.
Methods for Measuring Linear Growth
Various methods for the measurement of linear growth of stem and radicle are as follows.
- Direct method: Growth can easily be measured with the help of an ordinary measuring scale. It is a simple method.
- Horizontal microscope: It is used to measure growth in fields.
- Auxanometer: This equipment is used for precise measurement of linear growth of a shoot. There are two types of auxanometer, viz. (a) Arc auxanometer and (b) Pfeffer’s auxanometer.
- Crescograph: It records primary growth very accurately. It magnifies growth up to 10,000 times, giving information of growth per second. It was developed by Sir J. C. Bose.
7.5 Growth Curve
A growth curve is a graphic representation of the total growth plotted against time. Once growth has been measured — by any of the parameters and instruments of the previous section — plotting those measurements against time turns a column of numbers into a shape, and the shape immediately tells us what kind of growth is going on.
There are three types of curves, viz. Linear, Exponential and Sigmoid. The arithmetic growth curve is linear, while the geometric growth curve is exponential.
The Exponential Curve
Geometric growth, in which every cell divides and redivides, produces the exponential curve. Because the number of dividing cells doubles at each round, the curve rises gently at first and then climbs almost vertically — giving it the familiar J-shape seen in Graph 7.5.
Diagram
Graph 7.5 – Exponential growth curve: population size on the y-axis against year on the x-axis, giving a J-shaped curve that stays flat and low at first and then rises steeply
The Sigmoid Curve
In a real plant, however, the exponential rise cannot continue forever — space, water, nutrients and light eventually run short, and cells stop dividing and begin to mature. What we actually observe is a combination of all three phases of growth.
Corresponding to the three distinct phases of growth, growth rates differ. In the lag phase, growth rate is slow. In the exponential (log) phase, growth rate is faster and reaches its maximum. In the stationary phase, growth rate gradually slows down. When a graph of rate of growth against time is plotted for these three phases of growth, an ‘S’-shaped, i.e. sigmoid, curve is obtained, as shown in Graph 7.6.
Diagram
Graph 7.6 – Sigmoid growth curve: size/weight of the organ on the y-axis against time on the x-axis, giving an S-shaped curve; label the flat beginning as lag phase, the steeply rising middle as exponential phase, and the flattening top as stationary phase
The sigmoid curve is really the growth story of the previous sections drawn as a single line. Its slow beginning is the phase of cell division, when cells are being made but are still small. Its steep middle is the phase of cell enlargement, when water-filled cells stretch rapidly. Its flattening top is the phase of cell maturation, when cells specialise and stop adding bulk.
Grand Period of Growth
The total time (period) required for all phases to occur is called the Grand Period of Growth.
This single term therefore covers the whole life-span of a growing cell, organ or plant from the beginning of the lag phase to the end of the stationary phase — in graphical terms, the full width of the sigmoid curve along the time axis.
7.6 Differentiation, De-Differentiation, Re-Differentiation
A plant cell’s career is not a one-way street. It can specialise, it can un-specialise, and it can then specialise all over again. These three moves — differentiation, de-differentiation and re-differentiation — are what give a plant its remarkable ability to repair itself and to keep adding new tissue for as long as it lives.
a. Differentiation
Differentiation is the maturation of cells derived from the apical meristem of the root and the shoot. The permanent change in the structure and function of cells, leading to maturation, is called differentiation.
During cell differentiation, a cell undergoes few to major anatomical and physiological changes. Anatomical changes are changes in the cell’s build — its wall, its shape, the spaces around it. Physiological changes are changes in what it does — the reactions it carries out and the substances it handles. For example, parenchyma in hydrophytes (water plants) develops large schizogenous interspaces — air spaces formed by the splitting apart of neighbouring cells — which provide mechanical support, buoyancy and aeration.
There is a price for this specialisation. The maturation is at the cost of the capacity to divide and redivide. A cell that has committed itself to a job gives up its power of division to do it.
b. De-differentiation
Remarkably, that loss of dividing power need not be final. The living differentiated cell, which has lost the capacity to divide, may regain the same as per the need and divide again. Thus a permanent (mature) cell undergoes de-differentiation and becomes meristematic once more.
Note the word living. A cell that has died in the course of differentiation — a vessel element, for instance — can never come back. Only a living mature cell, such as parenchyma, can be recalled to duty.
Two familiar examples show this. Interfascicular cambium is formed from parenchyma cells lying between the vascular bundles, and cork cambium is formed from parenchyma cells of the innermost layer of the cortex. In both cases, ordinary mature parenchyma cells switch back into a dividing, meristematic state — creating a brand new meristem where there was none.
c. Re-differentiation
The cells produced by de-differentiation once again lose the capacity to divide and mature to perform a specific function. This is called re-differentiation.
For example, secondary xylem and secondary phloem are formed from the de-differentiated cambium present in the vascular bundle. The cambium divides, and its products then re-differentiate into conducting tissue — which is precisely how a stem thickens year after year.
Flowchart
FC 7.4 – Cycle of differentiation: Meristematic cell → differentiation → mature cell (loses capacity to divide) → de-differentiation → meristematic again (e.g. interfascicular cambium, cork cambium) → division → re-differentiation → mature cell with a specific function (e.g. secondary xylem, secondary phloem) → and the cycle can begin again
7.7 Development
Growth tells us that a plant is getting bigger. Differentiation tells us that its cells are becoming specialised. Development is the wider word that holds both of these — and everything else that happens to a plant from the moment a seed sprouts to the moment the plant dies.
Development refers to the ordered or progressive changes in shape, form and degree of complexity. It includes all the changes occurring in sequence, from the germination of the seed up to senescence or death, during the life cycle of the plant.
Thus development includes growth, morphogenesis, maturation and senescence. Two of these words are new. Morphogenesis is the process by which the plant acquires its form — the shaping of roots, stems, leaves and flowers into a recognisable body plan. Senescence is the ordered ageing of a cell, organ or whole plant, ending in death.
The point to hold on to is the word ordered. Development is not a random accumulation of changes. Each step follows the one before it, in a fixed sequence, as shown in FC 7.5.
Flowchart
FC 7.5 – Sequence of development: Seed germination → Meristem → Cell division → Plasmatic growth → Cell elongation → Cell maturation → Mature cell → Senescence → Death
Read down that sequence and you will recognise almost every idea of this chapter so far. Seed germination brings the meristem into action; the meristem supplies cell division; plasmatic growth builds up the living protoplasm of the new cells; cell elongation stretches them; cell maturation specialises them. Development simply adds the two ends that growth alone does not describe — the beginning at germination, and the ending in senescence and death.
7.8 Plasticity
A plant cannot walk away from a bad situation. Rooted in one spot, it must instead change itself to suit the conditions it finds — and it turns out to be extraordinarily good at doing so. This ability has a name.
Plasticity is the capacity of being moulded, formed or modelled. It is the ability of plants to form different kinds of structures — that is, to change — in response to different environmental (external) or internal stimuli, in various phases of life.
Notice what plasticity does not mean. It does not mean that the plant’s genes change. The same genome is being read in different ways under different conditions, so that one plant, at different times or in different places, can build visibly different organs.
Heterophylly — Different Leaves on One Plant
The clearest example is heterophylly: different forms of leaves borne by the same plant. In many plants, the juvenile stage and the mature stage show different forms of leaves on the same plant — for example, heterophylly in cotton, coriander and larkspur (Delphinium). Here the trigger is internal: the plant’s own phase of life decides the shape of the leaf. This is called intrinsic plasticity, and it is found in coriander and cotton.
The trigger can also be external. Environmental heterophylly is shown by Ranunculus flabellasis (buttercup). In this plant, leaves that develop under water are finely dissected into narrow segments, while leaves that develop above water in the air are broad and undivided — even though both kinds of leaf grow on the same individual. Heterophylly is thus exhibited in the same plant in different growth phases or under different environmental conditions, as seen in Fig. 7.8.
Diagram
Fig. 7.8 – Heterophylly: (1) juvenile leaf and (2) adult leaf of the same plant showing intrinsic heterophylly; (3) broad leaf formed in terrestrial habitat and (4) finely dissected leaf formed in water habitat, showing environmental heterophylly
7.9 Growth Hormones
Water, light, temperature and minerals set the stage for growth. But something inside the plant must decide where growth happens, how much, and when it stops. That internal control is chemical.
The term ‘hormone’ was coined first by Starling (1906) in animal physiology. The internal factors that influence growth are called growth hormones or growth regulators, as they inhibit, promote or modify the growth of the plant. The word regulator is chosen deliberately: these substances do not only push growth forward, they also hold it back and redirect it.
Growth promoters are auxins, gibberellins (GA) and cytokinins (CK). Growth inhibitors in plants are ethylene and abscissic acid (ABA). All phytohormones are growth regulators.
Flowchart
FC 7.6 – Classification of plant growth regulators: Growth regulators (phytohormones) → (branch 1) Growth promoters → auxins, gibberellins (GA), cytokinins (CK); (branch 2) Growth inhibitors → ethylene, abscissic acid (ABA)
According to Thimann and Pincus (1948), plant hormones are organic substances produced naturally in higher plants, affecting growth or other physiological functions at a site remote from their place of production, and active in very minute (optimum) amount.
Three conditions in that definition are worth pausing on, because each is exam-critical. The substance must be organic and naturally produced in the plant. It must act at a site remote from its place of production — which means it must be transported. And it must be effective in very minute amounts — a hormone is a signal, not a raw material. Hormones are transported through phloem parenchyma (Phillips 1971).
a. Auxins (Auxien = to grow)
The very name tells you what auxins do — auxien means “to grow.” F. W. Went, in 1931, used this term first. Auxin was isolated from the urine of a person suffering from pellagra (Kogl and H. Smit, 1931).
In plants, auxin is synthesized in the growing tips or meristematic regions, from where it is transported to other plant parts. The most common and important natural auxin is Indole-3-acetic acid (IAA). Tryptophan, an amino acid, is the primary precursor of IAA in plants — that is, the raw material from which the plant builds it.
Auxin is the first hormone to be discovered in plants, and it is primarily responsible for cell elongation. Look back at the phase of cell enlargement in Section 7.2 and you will see how central that single action is to a plant’s growth in length.
Auxin shows polar transport — that is, it moves in one direction only, and not freely both ways. In the stem this is basipetal transport: from the tip downwards, towards the base. This one-way movement is the reason a hormone made at the shoot tip can control what happens far below it.
Besides the natural auxin, synthetic auxins are now widely used, such as IBA (Indole butyric acid), NAA (Naphthalene acetic acid) and 2,4-D (Dichloro phenoxy acetic acid).
How Auxin Was Discovered
The Danish plant physiologist Boysen-Jensen (1910) cut off the coleoptile tip and inserted a thin plate of gelatin between the tip and the cut stump. The coleoptile still bent towards unilateral light — showing that whatever passed down from the tip could diffuse through gelatin, and was therefore a chemical, not an electrical or mechanical signal.
Paal (1919) cut off the tip of a coleoptile and replaced it asymmetrically — off to one side — on the cut stump. The coleoptile bent away from the side bearing the tip, and it did so even in the dark. So the chemical from the tip causes bending by itself; light merely decides where it goes.
F. W. Went (1928) finally captured it. He cut off coleoptile tips and placed them on small agar blocks, so that the substance could diffuse out into the agar. After some time, he placed these agar blocks asymmetrically on cut coleoptile stumps — and bending occurred, exactly as if a real tip were present. He had demonstrated a substance that could diffuse into agar and act on its own. Went named this substance auxin.
Diagram
Fig. 7.9a – The classic coleoptile experiments in four columns: Darwin (intact coleoptile bends; top of coleoptile removed — no bending; light-proof cap on tip — no bending); Boysen-Jensen (tip removed, gelatin inserted, bends towards light); Paal (tip removed and placed on one side, bending in absence of light); Went (coleoptile tips on agar; tips removed and agar cut into blocks; agar block placed on one side and bending occurs)
Flowchart
FC 7.7 – Logic of the coleoptile experiments: Darwin (stimulus arises in the tip and travels down) → Boysen-Jensen (stimulus diffuses through gelatin, so it is chemical) → Paal (asymmetric tip causes bending even in the dark, so the chemical itself bends the shoot) → Went (chemical collected in agar; agar alone causes bending) → substance named auxin
Physiological Effects and Applications of Auxin
The primary effect of auxin is cell enlargement. Every other effect below is, in one way or another, a consequence of auxin telling particular cells to grow, divide or stop.
In most of the higher plants, the growing apical bud inhibits the growth of the lateral buds. This is called apical dominance. Auxin made at the shoot tip travels basipetally down the stem and suppresses the buds it passes, so the plant grows tall rather than bushy.
Auxin stimulates the growth of stem and root. It induces multiplication of cells, and is hence used in tissue culture experiments to produce callus — an unorganised mass of dividing cells, produced by exactly the de-differentiation described in Section 7.6.
It stimulates the formation of lateral and adventitious roots. It also promotes root elongation in low concentration, and shooting at higher concentration — so the same hormone gives opposite outcomes at different doses, which is a point examiners like to test. It hastens early rooting in propagation by ‘cutting’.
Synthetic auxins are marketed as herbicides, e.g. 2,4-D (2,4-dichlorophenoxy acetic acid). It kills dicot weeds without affecting monocot crop plants — which is why a wheat or maize field can be sprayed to clear broad-leaved weeds while the crop itself stands unharmed.
Seedless fruits like orange, lemon, grapes and banana are produced by the application of auxin, i.e. induced parthenocarpy — the development of fruit without fertilisation, and hence without seeds.
Auxins promote cell division in the cambium and also cause early differentiation of xylem and phloem.
A foliar spray of NAA and 2,4-D induces flowering in litchi and pineapple. Likewise, it prevents premature fruit drop in apples, pear and oranges, and also prevents the formation of the abscission layer — the special layer of cells at the base of a leaf, flower or fruit whose breakdown lets that organ fall off.
Auxins play a role in elongation of the cell. They are known to increase the rate of respiration. Auxins break dormancy in seed and promote quick germination.
b. Gibberellins
Gibberellin is another growth promoting hormone, and is abundant in the root tip and developing seeds. Unlike auxin, it shows non-polar transport through vascular tissue — it can move freely in any direction, both up and down the plant.
In plants, gibberellin is synthesized in young leaves, seeds, roots and stem tips. Gibberellins are synthesized from mevalonic acid. More than 150 chemical types are known so far. GA₃ (= gibberellic acid) is the most common and biologically active form. Chemically, it contains a gibbane ring — a cyclic diterpene with four isoprene units.
Physiological Effects and Applications of Gibberellins
Dormancy of the bud can be broken by gibberellin treatment.
Gibberellin can promote seed germination in cereals like barley and wheat by synthesizing the hydrolysing enzyme amylase, which breaks down stored starch to produce sugar. This is a beautiful example of a hormone working by switching on an enzyme: the seed’s starch reserve is useless to the embryo until amylase converts it into sugar the embryo can actually respire and grow on.
The most striking effect of gibberellin is the elongation of the stem, in which the internodes increase in length, converting a genotypic dwarf into a phenotypically tall plant. When gibberellins are applied to genetically dwarf plants like maize, the stem rapidly elongates and acquires the height of the normal tall varieties of maize. Note carefully what has and has not changed: the plant’s genes are still those of a dwarf; only its appearance (phenotype) has become tall.
It also promotes bolting, i.e. the elongation of internodes just prior to flowering in plants with a rosette habit, e.g. beet and cabbage. A rosette plant keeps its internodes so short that the leaves sit in a tight circle near the ground; gibberellin stretches those internodes and shoots the flowering stalk upwards.
Gibberellin causes parthenocarpy in tomato, apple and pear, and flowering in long day plants. It is used to increase the fruit size and bunch length of grapes.
Application of gibberellins overcomes the requirement of vernalization — the cold treatment needed by some plants before they can flower (Section 7.11).
Usually, gibberellin inhibits the growth of root, delays senescence and prevents abscission. It also breaks dormancy of seed and hastens germination. Application of gibberellin causes the production of male flowers on a female plant.
c. Cytokinin
Cytokinin is another growth hormone; its central action is to promote cytokinesis — the division of the cell during cell division. Letham coined the term cytokinin.
Chemically, kinins are 6-furfuryl amino purine. The first natural cytokinin was obtained from unripe maize grains by Letham et al.; it is known as Zeatin. 6-benzyl adenine is a synthetic cytokinin hormone. Seven different types of cytokinins are recorded from plants. Natural cytokinins are also reported from plants like banana flowers, apple and tomato fruits, and coconut milk.
Physiological Effects and Applications of Cytokinin
Besides cell division, cytokinin also promotes cell enlargement. High cytokinin promotes shooting.
The most important idea here is not cytokinin acting alone, but cytokinin acting in ratio with auxin. A low ratio of cytokinin to auxin induces root development, but a high ratio causes buds and shoot to develop. The cytokinin–auxin ratio and their interactions control morphogenic differentiation — that is, they decide what organ a mass of dividing cells will actually become.
Cytokinin promotes the growth of lateral buds and controls apical dominance by cell division; in fact, cytokinin reverses the apical dominance effect that auxin imposes. It delays the senescence (ageing) and abscission processes in plant organs — this was reported by Richmond and Lang (1957), and the delay of ageing by cytokinin is known after them as the Richmond–Lang effect.
Formation of interfascicular cambium and the expansion of cells are other functions of cytokinin. It also breaks dormancy and promotes the germination of seeds. It induces RNA synthesis and the formation of interfascicular cambium.
d. Ethylene
Ethylene is the only gaseous growth regulator — a simple gaseous hydrocarbon. Being a gas, it can drift from one fruit to a neighbouring one, which no other hormone can do.
Its history was assembled by several workers. Denny (1924) reported that ethylene is effective in fruit ripening. Crocker (1930) proposed that ethylene is the plant hormone responsible for fruit ripening. Gane (1934) established that plants naturally synthesize ethylene — which settled the matter, because until then it could have been argued that ethylene was merely an outside chemical that happened to affect fruits.
The most widely used compound as a source of ethylene is ethephon. In the plant, ethylene is synthesized in roots, the shoot apical meristem, and ripening fruits.
Physiological Effects and Applications of Ethylene
Ethylene promotes the ripening of fruits like bananas, apples and mangoes. It stimulates the initiation of lateral roots in plants and breaks the dormancy of bud and seed.
It accelerates the abscission activity in leaves, flowers and fruits by the formation of the abscission layer. Note the direct opposition to auxin here, which prevents the formation of that same layer.
Ethylene inhibits the growth of lateral buds and causes apical dominance, and it retards flowering. It is associated with the enhancement of the process of senescence of plant organs. It inhibits flowering in most of the plants — except pineapple, where it in fact induces flowering, which is why this exception is worth remembering.
It causes epinasty — the drooping of leaves and flowers, caused by the upper side of the leaf stalk growing faster than the lower side, so that the leaf curves downward.
Ethylene increases the activity of the enzyme chlorophyllase, causing the degreening effect in banana and Citrus fruits.
e. Abscissic Acid (ABA)
Abscissic acid is a natural growth inhibiting hormone. Its story is unusual, because two groups arrived at the same molecule from two completely different directions.
ABA is synthesized in leaves, fruits, roots, seeds, etc. Chemically, it is a 15-carbon sesquiterpenoid, and it is synthesized from mevalonic acid — the same starting material as gibberellin, which is a neat reminder that a common precursor can lead to hormones with opposite effects.
Physiological Effects and Applications of ABA
ABA promotes the abscission of leaves and induces dormancy in many plants. It controls the dormancy in buds and seeds by inhibiting growth processes — a dormant seed is not a dead seed, but one that has been actively told not to germinate yet.
It accelerates the senescence of leaves, flowers and fruits. It inhibits and delays cell division and cell elongation, and suppresses cambium activity by inhibiting mitosis in the vascular cambium.
ABA can cause the efflux of K⁺ ions from the guard cells, and this results in the closure of stomata. When potassium ions leave the guard cells, water follows them out by osmosis, the guard cells lose their turgidity, and the stomatal pore closes. Because closing the stomata cuts down the loss of water vapour, ABA is known as an antitranspirant.
ABA acts as a stress hormone by inducing the plant to bear adverse environmental conditions. Trace the chain: drought → ABA is produced → stomata close → water is conserved → growth is slowed and dormancy is imposed → the plant survives the bad season instead of exhausting itself trying to grow through it.
It inhibits flowering in long day plants but stimulates flowering in short day plants.
Auxin and ABA — Two Ends of the Same Scale
Notice how each test is built on the hormone’s own signature effect: auxin bends a coleoptile, gibberellin switches on amylase, cytokinin holds back the loss of chlorophyll, and ABA blocks the very amylase that gibberellin switches on.
7.10 Photoperiodism
Higher plants reproduce sexually by producing special structures called flowers. At some point in its life, a plant stops merely adding leaves and stems and switches over to making flowers: plants exhibit a transition from vegetative growth to reproductive growth, during which flowers are produced.
That switch must be timed correctly. A plant that flowers in the wrong season will find no pollinators, or will have its fruits killed by frost. Like vegetative growth, reproductive growth is influenced by several environmental and nutritional factors — and among the environmental factors, light and temperature exert a profound influence on flowering.
The influence of the duration of light on flowering is known as Photoperiodism, and the influence of temperature is known as Vernalization (Section 7.11).
Light as a Signal, Not Just as Food
Light, as an environmental factor, influences the germination of seed, vegetative growth, photosynthesis, and much else. Light as a factor has three aspects, viz. Quality (the wavelength or colour of the light), Intensity (how bright it is) and Duration (how long it lasts each day).
It is the duration of light that has a profound effect on flowering in higher plants. This is a striking idea: the plant is not using this light as a source of energy at all. It is reading the length of the day as a message about the season.
How Photoperiodism Was Recognised
Garner and Allard worked through the obvious explanations one by one. They studied the effects of different temperatures, of nutrition, of soil moisture, and so on, in respect of flowering. None of these was found to regulate flowering.
They then examined the effect of day length on flowering by using artificial illumination — lengthening or shortening the light period at will, independently of the season. Experimentally, they found that the exposure to a specific duration of light (i.e. the photoperiod) had a profound influence on flowering. They concluded that the relative length of the day was the most crucial factor in the growth and development of flowers, and for this they coined the term photoperiodism.
Based on the photoperiodic response, plants were classified into three categories, viz. Short Day Plants (SDP), Long Day Plants (LDP) and Day Neutral Plants (DNP).
Flowchart
FC 7.8 – Classification on the basis of photoperiodic response: Plant → (branch 1) day length shorter than critical photoperiod needed → Short Day Plant (long night plant), e.g. Chrysanthemum, soybean; (branch 2) day length longer than critical photoperiod needed → Long Day Plant (short night plant), e.g. wheat, radish; (branch 3) flowering independent of photoperiod → Day Neutral Plant, e.g. tomato, maize
a. Short Day Plants (SDP)
These plants usually flower during winter and late summer, when the day length is shorter than the critical photoperiod.
The critical photoperiod is a threshold, not a fixed number of hours common to all plants — each species has its own. What matters for a short day plant is that the day must be shorter than this threshold.
Short day plants are also called long night plants, because they require a long, uninterrupted dark period (night) for flowering. And “uninterrupted” is meant literally: if the dark period is interrupted even by a flash of light, an SDP will not flower.
Some of the short day plants are Dahlia, Aster, Tobacco, Chrysanthemum, Soybean (Glycine max) and Cocklebur (Xanthium).
b. Long Day Plants (LDP)
Plants that flower usually during summer are called long day plants. They require a longer duration of light than the critical photoperiod for flowering.
They are called short night plants, as they require a short dark period. Consistently with this, when a long dark period is interrupted by a brief flash of light, LD plants can flower — the very treatment that blocks flowering in an SDP permits it in an LDP.
Examples are pea, radish, sugar beet, cabbage, spinach, wheat and poppy. The effect of interrupting the dark period in the two groups is shown in Fig. 7.9.
Diagram
Fig. 7.9 – Photoperiodism: horizontal bars showing light (yellow) and darkness (black) with the critical dark period marked by a dotted line. (a) A short day plant: flowers when the dark period is long and uninterrupted; does not flower when the long dark period is broken by a flash of light — labelled “requires a relatively long uninterrupted darkness.” (b) A long day plant: flowers when the dark period is short, and also flowers when a long dark period is broken by a flash of light — labelled “requires a relatively small duration of darkness / no darkness at all”
c. Day Neutral Plants (DNP)
These plants flower throughout the year round, independent of the duration of light (photoperiod). They do not require a specific photoperiod to flower. Therefore they are called day neutral plants, e.g. Cucumber, tomato, cotton, sunflower, maize and balsam.
Where the Signal Is Received, and How It Travels
If the plant is measuring day length, some organ must be doing the measuring — and it is not the bud that eventually flowers.
The leaf is the chief organ for receiving the photoperiodic stimulus, as demonstrated by Knoff (1934). Defoliated plants will not flower, even if the plants are exposed to the proper duration of light. Strip a plant of its leaves and it becomes blind to day length, however perfect the day length may be.
But the flower does not form on the leaf. So the message must travel. The photoperiodic stimulus is a chemical stimulus, transported through the phloem, and it is called florigen, which is hormonal in nature. Florigen is produced in the leaf, moves through the phloem to the bud, and there brings about the change from a vegetative bud into a flowering bud.
Flowchart
FC 7.9 – Path of the photoperiodic stimulus: Suitable photoperiod → perceived by the leaf (photochemical receptors — phytochromes) → florigen produced in the leaf → transported through the phloem → reaches the bud → vegetative bud converts to floral bud → flowering
The photochemical receptors in the leaves are the biloproteins (pigments) located in the cell membrane. These are called phytochromes. Blue wavelengths of light also influence flowering.
Phytochrome — the Pigment That Measures the Night
The leaves possess this light-receiving proteinaceous pigment, called phytochrome, which induces flowering. It exists in two interconvertible forms, viz. red (P\(_r\)) and far red (P\(_{fr}\)). When P\(_{fr}\) absorbs far red light, it is converted into P\(_r\), and vice versa. These are located in the cell membrane of green cells. The interconversion is shown in Fig. 7.9b.
Diagram
Fig. 7.9b – Phytochrome interconversion: P\(_r\) on the left and P\(_{fr}\) on the right; a forward arrow from P\(_r\) to P\(_{fr}\) labelled “660 nm (red)”; a reverse arrow from P\(_{fr}\) to P\(_r\) labelled “730 nm (far red)”; a second reverse arrow from P\(_{fr}\) back to P\(_r\) labelled “Darkness”
This two-form pigment is exactly the device a plant needs in order to keep time. During day time, P\(_{fr}\) accumulates in the plants — because sunlight is rich in red light, which drives the pigment towards the P\(_{fr}\) form. P\(_{fr}\) inhibits flowering in SDP, but initiates flowering in LDP.
During the dark period, P\(_{fr}\) slowly changes back into P\(_r\). And P\(_r\) stimulates or promotes flowering in SDP, while it inhibits flowering in LDP.
Now the whole system falls into place. A long night gives P\(_{fr}\) enough time to convert almost completely into P\(_r\) — so a short day plant, needing P\(_r\), flowers. A flash of red light in the middle of that night instantly pushes the pigment back to P\(_{fr}\) — the plant’s “clock” is reset, P\(_r\) never accumulates, and the SDP fails to flower. A long day plant, which needs P\(_{fr}\), is helped by exactly the same flash.
7.11 Vernalization (Yarovization)
Day length is one seasonal cue. Temperature is the other. Temperature, as an environmental factor, influences several physiological processes, including reproduction. Temperature as a factor has three cardinal points, viz. minimum, optimum and maximum — the lowest temperature at which a process can occur at all, the temperature at which it occurs fastest, and the highest temperature beyond which it stops.
What is surprising is that for some plants, flowering demands not warmth but cold. Vernalization is a low temperature (chilling) treatment that induces early flowering in plants, as was evidenced by Klippart (1918). Chouard (1960) defines vernalization as the acceleration of the ability to flower by chilling treatment. The term vernalization was coined by T. D. Lysenko (1928) for the effect of low temperature on flowering in plants.
Read the definition carefully. Vernalization does not cause flowering by itself — it makes the plant able to flower, and to do so sooner. The plant still needs suitable conditions afterwards to actually produce its flowers.
The Winter Wheat Puzzle
The strangeness appeared when the winter variety was sown in spring. It then did not produce flowers during that summer — but it did so in the next year’s summer, after it had sat through a winter. Something in the cold season was necessary before the plant could flower at all.
Klippart then tried the decisive experiment. He treated germinated seeds of ‘winter wheat’ with low temperature, and found that the treated seed behaved as spring wheat — it flowered in the same season, without waiting for a real winter. He concluded that winter wheat requires low-temperature exposure before the onset of flowering.
What Vernalization Involves
Vernalization is an influence of temperature on development and flowering. Many plants, such as cereals and crucifers, require a period of cold treatment for flowering.
In practice it is the method of inducing early flowering in plants by pretreatment of their seeds or seedlings at low temperature — 1–6 °C for one to one and a half months’ duration. Note both parts of this: the temperature must be low, and the exposure must be prolonged. A single cold night will not do; the plant is measuring a season, not a day.
The site of vernalization is believed to be the shoot apical meristem — which makes good sense, since this is the meristem that will later have to give rise to the flower. Generally, vernalization is effective at the seed stage in annual plants.
The vernalization stimulus is also a chemical stimulus, named vernalin. It can be transferred through grafting (Melcher, 1939) — that is, if a vernalized plant is grafted onto an unvernalized one, the second plant can be made to flower too. This is strong evidence that a mobile chemical, and not merely a physical state of the chilled tissue, carries the message.
Flowchart
FC 7.10 – Vernalization: Seed / seedling → chilling treatment at 1–6 °C for one to one and a half months → perceived at the shoot apical meristem → vernalin (chemical stimulus) formed → ability to flower is acquired → early flowering. (If followed by high temperature treatment → devernalization → ability to flower is lost)
Devernalization
The reversion of vernalization by high temperature treatment is called devernalization. The cold-acquired ability to flower can, in other words, be cancelled by a spell of heat that follows it.
Advantages of Vernalization
- Crop plants, by shortening the juvenile phase, attain early flowering. An earlier harvest can mean escaping a late-season drought or fitting a second crop into the same year.
- Crops can be cultivated in regions where they do not grow naturally. A variety that would normally never receive enough winter chilling in a warm region can be given that chilling artificially before sowing.
7.12 Mineral Nutrition
A plant builds its own food, but it cannot build its own atoms. The plant absorbs water, gases, minerals and nutrients from its surroundings. For the synthesis of their organic food, green plants need inorganic substances (elements), which are obtained from the soil in the form of minerals.
Minerals are the most commonly occurring solid, inorganic materials obtained from the earth’s crust. A plant, in other words, is partly built out of rock — weathered, dissolved, and drawn up through a root.
Chemical analysis of plant ash clearly indicates that a plant absorbs mineral elements from its surroundings — soil, air and water — for its use. When a plant is burnt, the organic matter goes off as gas and what remains behind is the ash: the mineral content the plant had taken up. About 36 to 40 different elements of the periodic table are used as minerals by plants.
These are absorbed in ionic (dissolved) form — as \(PO_4\), \(CO_3\), \(SO_4\), etc. — usually through the roots, and specifically through the regions of elongation and growth, where the root’s cells are young, thin-walled and in close contact with the soil solution.
Sources of Minerals
Plants derive the necessary elements from the atmosphere, the soil and water.
Carbon enters the plant as atmospheric carbon dioxide. The source of hydrogen is water, and oxygen comes from air and water. Carbon, hydrogen and oxygen are therefore not minerals in origin, even though they make up the bulk of every organic molecule in the plant — they come from air and water, not from the earth’s crust.
The source of nitrogen is the soil. The plant derives nitrogen from both mineral and non-mineral origin — a point taken further in Section 7.13.
Classification of Minerals
Earlier, on the basis of their requirement, minerals were classified as essential and non-essential.
Essential minerals are those that are indispensable — without them, plants cannot complete their life cycle, e.g. C, H, O, N, P, etc. These elements play structural and physiological roles, and their absence can produce or cause major deficiency symptoms. The non-essential elements are not indispensable, and they do not produce or cause any deficiency symptoms. This classification is obsolete now.
The difficulty with it was that “essential” was being decided rather loosely. What was needed was a strict test.
Macroelements and Microelements
Based on the quantity requirement, minerals are classified as minor or microelements and major or macroelements.
Microelements are required in traces, because they function in a catalytic role — e.g. Zn, Cu, Al, Si, Mn, B, Mo, Cl, etc., which act as co-factors. A co-factor is a non-protein helper that an enzyme needs in order to work; a single enzyme molecule can act on substrate after substrate, so a very small number of co-factor atoms can support a very large amount of chemistry. That is exactly why traces are enough.
Macroelements are required in large quantity. They mainly play nutritive and structural roles — e.g. C, H, O, P, Mg, N, K, S, Ca and Fe. These elements are built into the fabric of the plant itself: into proteins, nucleic acids, membranes and cell walls. Structural material is needed by the gram, not by the trace.
C, H and O are non-mineral major elements, since they come from air and water rather than from the soil. This classification, too, is not accepted now.
Nowadays, minerals are classified on the basis of their biochemical functions — that is, by the job the element actually does inside the plant, rather than by how much of it is required.
Table 7.10 — Roles of Mineral Elements in Plants
| Element | Region of plant in which required | Functions | Deficiency symptom |
|---|---|---|---|
| Nitrogen (\(NO_2^-\) or \(NO_3^-\) or \(NH_4^+\)) | Everywhere, particularly in meristematic tissues | Constituent of proteins, nucleic acids, vitamins, hormones, coenzymes, ATP, chlorophyll | Stunted growth, chlorosis |
| Phosphorus (\(H_2PO_4^-\) or \(HPO_4^{2-}\)) | Younger tissues; obtains it from older, metabolically less active cells | Constituent of cell membrane, certain proteins, all nucleic acids and nucleotides; required for all phosphorylation reactions | Poor growth, leaves dull green |
| Potassium (\(K^+\)) | Meristematic tissues, buds, leaves, root tips | Helps in determining anion–cation balance in cells; involved in protein synthesis, in formation of cell membrane, and in opening and closing of stomata; increases hardness; activates enzymes; helps in maintenance of turgidity of cells | Yellow edges to leaves, premature death |
| Calcium (\(Ca^{2+}\)) | Meristematic and differentiating tissues; accumulates in older leaves | Involved in selective permeability of cell membranes; activates certain enzymes required for development of stem and root apex; present as calcium pectate in the middle lamella of the cell wall | Stunted growth |
| Magnesium (\(Mg^{2+}\)) | Leaves; withdrawn from ageing leaves and exported to developing seeds | Activates enzymes in phosphate metabolism; constituent of chlorophyll; maintains ribosome structure | Chlorosis |
| Sulphur (\(SO_4^{2-}\)) | Stem and root tips; young leaves; remobilised during senescence | Constituent of certain proteins, vitamins (thiamine, biotin, CoA) and ferredoxin | Chlorosis |
| Iron (\(Fe^{3+}\)) | Everywhere; carried along leaf veins | Constituent of ferredoxin and cytochrome; activates catalase; required for synthesis of chlorophyll | Chlorosis |
| Manganese (trace) (\(Mn^{2+}\)) | Leaves and seeds | Activates certain enzymes (carboxylases) | Chlorosis, grey spots on leaves |
| Molybdenum (trace) (\(MoO_2^{2+}\), \(MoO^{3+}\)) | Everywhere, particularly in roots | Activates certain enzymes in nitrogen metabolism | Slight retardation of growth |
| Boron (trace) (\(BO_3^-\) or \(B_4O_7^{2-}\)) | Leaves and seeds | Required for uptake and utilisation of \(Ca^{2+}\), pollen germination, cell differentiation and carbohydrate translocation | Brown heart disease |
| Copper (trace) (\(Cu^{2+}\)) | Everywhere | Activates certain enzymes | Die-back of shoots |
| Zinc (trace) (\(Zn^{2+}\)) | Everywhere | Activates various enzymes, especially carboxylases; part of carbonic anhydrase and various dehydrogenases needed for auxin synthesis | Malformed leaves |
| Chlorine (\(Cl^-\)) | Everywhere | With \(Na^+\) and \(K^+\), helps to determine solute concentration and anion–cation balance in cells; essential for oxygen evolution in photosynthesis | Poor growth of the plant |
Symptoms of Mineral Deficiency in Plants
Any visible deviation from the normal structure and function of the plant is called a symptom or hunger sign. The plant, unable to speak, tells us in the only language it has: colour, shape and size.
The concentration of the essential elements below which plant growth is retarded is termed the critical concentration. The element is said to be deficient when it is present below the critical concentration.
Certain morphological changes are indicative of the deficiency of a particular element. Deficiency symptoms also depend on the mobility of the elements in the plant.
The deficiency symptoms appear first in young tissues when elements are relatively immobile, e.g. sulphur and calcium. Such elements, once fixed in an old leaf, cannot be pulled back out of it, so the plant cannot rescue its growing tips.
When the elements are actively mobilized within the plant and exported to young developing tissues, the deficiency symptoms are visible first in the older tissues (senescent leaves), e.g. nitrogen, magnesium and potassium. Here the plant sacrifices its old leaves to keep its new ones supplied.
Some important deficiency symptoms seen in plants are:
- Stunting: The growth is retarded. The stem appears condensed and short.
- Chlorosis: It is the loss or non-development of chlorophyll, resulting in the yellowing of leaves.
- Necrosis: It is the localized death of the tissue of leaves.
- Mottling: Appearance of green and non-green patches on the leaves.
- Abscission: Premature fall of flowers, fruits and leaves.
Toxicity of Micronutrients
Too little of an element is a problem, but so is too much. The mineral ion concentration which reduces the dry weight of tissues by 10% is considered as toxic.
Toxicity of one element may inhibit the uptake of another. For example, Mn competes with Fe and Mg for uptake, and also inhibits Ca translocation to the shoot apex. Thus, manganese toxicity develops the deficiency symptoms of Ca, Mg and Fe — an excess of one element produces a shortage of three others, which is precisely why a diagnosis based on symptoms alone can mislead.
The phenomenon where the toxic effect of one mineral is counterbalanced by another ion is called antagonism.
Mineral Salt Absorption
Most minerals in the soil are charged particles (ions), and hence they cannot pass freely across the cell membrane. The membrane’s interior is oily and non-polar, and a charged ion is repelled by it. Therefore most of the minerals are absorbed actively, with the expenditure of energy. Minerals can also be absorbed passively, without the expenditure of energy.
An important point: mineral ion absorption is independent of water absorption. The two processes happen in the same root at the same time, but a plant can take up water without taking up much mineral, and vice versa.
Mineral ion absorption can occur in two ways.
a. Passive Absorption
Movement of mineral ions into the root occurs by diffusion. Molecules or ions diffuse from a region of their higher concentration to a region of their lower concentration. The movement of mineral ions into root cells as a result of diffusion, without the expenditure of energy, is called passive absorption.
Passive absorption can take place by direct ion-exchange, indirect ion-exchange, mass flow, and Donnan equilibrium.
b. Donnan Equilibrium
Donnan equilibrium is based on the assumption that certain negatively charged ions (i.e. anions), after their entry into the cell, become fixed on the inner side of the cell membrane and cannot diffuse outside through the cell membrane.
Because these fixed anions carry negative charge that must be balanced, additional mobile cations are required to balance them. Obviously, the concentration of cations becomes more, due to this accumulation.
This kind of passive absorption of anions/cations from the cell exterior, against their own concentration gradient, in order to neutralize the effect of cations/anions, is called Donnan equilibrium.
Active Absorption
Uptake of mineral ions against the concentration gradient is called active absorption. Such movement requires an expenditure of energy by the absorbing cell. This energy is derived from respiration and is supplied through ATP.
The dependence on respiration can be demonstrated directly: when the roots are deprived of oxygen, they show a sudden drop in the active absorption of minerals. No oxygen means no aerobic respiration, no ATP, and no pumping.
The Path of a Mineral Ion Through the Plant
The mineral ions accumulated in the root hair pass into the cortex and finally reach the xylem.
The minerals in the xylem are then carried, along with water, to other parts of the plant along the transpiration stream — the upward pull of water created by evaporation from the leaves. They are subsequently assimilated into organic molecules, and then redistributed to other parts of the plant through the phloem.
Flowchart
FC 7.11 – Path of a mineral ion: Mineral ion in soil solution → absorbed by root hair (passively by diffusion or actively using ATP) → cortex → xylem → carried upward with water along the transpiration stream → reaches leaves and other organs → assimilated into organic molecules → redistributed to growing parts through the phloem
7.13 Nitrogen Cycle
The nitrogen cycle is the series of natural processes by which nitrogen enters successively from the air into organisms, through the soil, and back to the environment.
Plants use the photosynthetic product — the sugars — to make proteins. But sugars contain only carbon, hydrogen and oxygen, while every protein and every nucleic acid also contains nitrogen. To build proteins, plants need nitrogen.
Unfortunately, atmospheric nitrogen is very inert (non-reactive). Plants badly need nitrogen in a reactive form, usually as nitrate ions.
Why an Abundant Gas Is Still Scarce
Nitrogen is a limiting nutrient in agricultural ecosystems — the element that, more than any other, caps the yield of a crop. This sounds absurd for a gas that makes up most of the air, until we look at the molecule itself.
In the atmosphere, nitrogen exists as nitrogen atoms joined by a strong triple covalent bond (\(N \equiv N\)). Three bonds must be broken before a nitrogen atom can be used to build anything, and breaking them takes an enormous amount of energy. A crop can stand in an ocean of nitrogen and starve for want of it.
A regular supply of nitrogen to plants is therefore maintained through biological and physical processes, which together with the return pathways make up the nitrogen cycle shown in Fig. 7.11.
Diagram
Fig. 7.11 – Nitrogen cycle: atmospheric N₂ at the top, with arrows to non-biological nitrogen fixation and biological nitrogen fixation → nitrates in the soil (plus commercial fertilisers in soil) → NH₃ in plants → amino acids in plants → plant proteins → (food) animals → excreta and death → nitrogenous organic matter in soil → ammonifying bacteria in the soil → NH₃ in soil → nitrifying bacteria in the soil → nitrites and nitrates in the soil → denitrifying bacteria in soil → back to atmospheric N₂; also show combustion returning nitrogen to the atmosphere
Nitrogen Fixation
The atmosphere is the source of nitrogen, but it cannot be used directly. Nitrogen must first combine with C, H, N and O to form compounds before it can be used by living organisms.
The conversion of the free nitrogen (\(N_2\)) of the atmosphere into nitrogenous salts, so as to make it available for the plants, is called nitrogen fixation.
It is of two types: Physical and Biological fixation.
Flowchart
FC 7.12 – Types of nitrogen fixation: Nitrogen fixation → (branch 1) Physical fixation → by electric discharge/thunderstorm, and industrial (Haber–Bosch) fixation; (branch 2) Biological fixation → by prokaryotes (diazotrophs) → symbiotic (e.g. Rhizobium) or free living (e.g. cyanobacteria with heterocysts)
Physical Nitrogen Fixation
Physical nitrogen fixation occurs in several steps, and starts with the combination of atmospheric nitrogen with oxygen under the influence of electric discharge. Thunderstorms produce nitric oxide.
\[N_2 + O_2 \xrightarrow[\text{Thunder storm}]{\text{Electric discharge}} 2NO\]
Here \(N_2\) is nitrogen, \(O_2\) is oxygen, and \(NO\) is nitric oxide. The energy that a lightning stroke delivers is enough to break even the triple bond of nitrogen — which is exactly why so ordinary a chemical process needs so violent a trigger.
The nitric oxide is then oxidized to nitrogen peroxide in the presence of oxygen.
\[2NO + O_2 \xrightarrow{\text{Oxidation}} 2NO_2\]
Here \(NO_2\) is nitrogen peroxide.
During rains, the nitrogen peroxide combines with rain water to form nitrous acid and nitric acid, which come down to the ground along with the rain.
\[2NO_2 + \text{Rain water} \longrightarrow HNO_2 + HNO_3\]
Here \(HNO_2\) is nitrous acid and \(HNO_3\) is nitric acid.
On the ground, the alkali radicals of the soil react with nitric acid to produce nitrites and nitrates — the absorbable form that a root can actually take up.
\[HNO_3 + \text{Ca or K salts} \longrightarrow \text{Ca or K nitrates}\]
Industrial Nitrogen Fixation
Industrial nitrogen fixation occurs by the Haber–Bosch nitrate process, at high temperature and pressure.
\[N_2 + 3H_2 \xrightarrow[\text{200 atm}]{450^\circ C} 2NH_3\]
Here \(H_2\) is hydrogen and \(NH_3\) is ammonia; the reaction is carried out at 450 °C and 200 atmospheres of pressure. The extreme conditions are the price of breaking the triple bond without lightning.
Ammonia is then converted to urea, as urea is less toxic.
Biological Nitrogen Fixation
Biological nitrogen fixation is carried out by prokaryotes called ‘nitrogen fixers’ or ‘diazotrophs’. It accounts for nearly 70% of natural nitrogen fixation — far more than lightning ever manages.
Nitrogen fixers are either symbiotic or free living. The cyanobacteria fix a significant amount of nitrogen in specialized cells called heterocysts — thick-walled cells that keep oxygen out, because the nitrogen-fixing machinery is destroyed by oxygen.
Nitrogen fixation is a high energy requiring process (an endothermic reaction). Nitrogen fixers use 16 molecules of ATP to fix each molecule of nitrogen to form ammonia.
\[N_2 + 8H^+ + 8e^- + 16\,ATP \longrightarrow 2NH_3 + H_2 + 16\,ADP + 16\,P_i\]
Here \(H^+\) is a hydrogen ion (proton), \(e^-\) is an electron, \(ATP\) is adenosine triphosphate, \(ADP\) is adenosine diphosphate and \(P_i\) is inorganic phosphate. Count the ATP: sixteen molecules for a single molecule of nitrogen. This is the energetic cost of that triple bond, and it explains why nitrogen fixation only happens where a rich supply of energy is available — such as inside a root nodule, fed by sugars from the host plant.
The ammonia produced is then converted into amino acids.
Symbiotic N₂ Fixation
The best known nitrogen fixing symbiotic bacterium is Rhizobium. This soil living/dwelling bacterium forms root nodules in plants belonging to the family Fabaceae, e.g. beans, gram and groundnut.
The arrangement is a genuine partnership. The plant supplies the bacterium with sugars — the energy needed for those sixteen ATP molecules — and the bacterium supplies the plant with fixed nitrogen. Neither could do the job alone.
Nitrification
Plants absorb nitrogen chiefly as nitrate, so the ammonia produced by fixation and decay must be converted into nitrate. This conversion is nitrification.
Nitrification is an exothermic reaction — it releases energy, unlike fixation, which consumes it. Most of the soil bacteria participate in converting ammonia into nitrate, the form of nitrogen which can be used by plants and animals. This involves two steps, performed by two different types of bacteria.
First, a soil bacterium converts ammonia into nitrite, e.g. Nitrosomonas, Nitrosococcus, etc.
\[2NH_3 + 3O_2 \xrightarrow[\textit{Nitrosococcus}]{\textit{Nitrosomonas}} 2HNO_2 + 2H_2O\]
Then another type of soil bacterium, called Nitrobacter, adds a third oxygen atom to create nitrate.
\[2HNO_2 + O_2 \xrightarrow{\textit{Nitrobacter}} 2HNO_3\]
These bacteria are chemoautotrophs. By metabolizing nitrogen along with oxygen, they obtain energy to power their own life processes. They are not doing the plant a favour — they are earning a living, and the plant benefits from their waste product.
Flowchart
FC 7.13 – Nitrification in two steps: \(NH_3\) → (Nitrosomonas, Nitrosococcus) → nitrite (\(HNO_2\)) → (Nitrobacter) → nitrate (\(HNO_3\)) → absorbed by plant roots
Ammonification
After the death of plants and animals, various fungi, actinomycetes and some ammonifying bacteria decompose the tissues and convert organic nitrogen into amino acid, and then into ammonia, and back into the ecosystem. Ammonia (\(NH_4^+\)) is now available for uptake by plants and other micro-organisms for growth.
\[\text{Proteins} \xrightarrow[\text{decomposition}]{\text{Microbial}} \text{amino acids}\]
\[\text{Amino acids} \longrightarrow NH_3 + ROH\]
Here \(NH_3\) is ammonia and \(ROH\) represents an organic acid. Ammonification is the process that recycles nitrogen already inside the living world — without it, every nitrogen atom would be locked up in dead bodies, and fixation alone could never keep pace with the demand.
Nitrogen Assimilation
In the soil, nitrogen is present as nitrates, nitrites and ammonia (\(NH_4^+\)). It is absorbed by green plants and converted into nitrogenous organic compounds like amino acids, DNA, etc. This is known as nitrogen assimilation.
From plants, nitrogen — now in the form of biomolecules such as amino acids — enters the food chain, moves to animals, and then to decomposers through the death of animals.
Inside the plant, nitrates are first converted to ammonia. But ammonia is highly toxic, and so it is immediately used for conversion into amino acids, which are then transported to other parts of the plant for the synthesis of proteins. The plant, in effect, never lets ammonia accumulate.
\[NO_3^- + 2e^- + 2H^+ \longrightarrow NO_2^- + H_2O\]
\[NO_2^- + 8e^- + 8H^+ \longrightarrow NH_4^+ + 2H_2O\]
Here \(NO_3^-\) is the nitrate ion, \(NO_2^-\) the nitrite ion, and \(NH_4^+\) the ammonium ion. Notice that both steps are reductions — electrons are being added at each stage, taking nitrogen from its most oxidised form (nitrate) down to its most reduced form (ammonium), which is the form the plant can actually build into an amino acid.
Amino Acid Synthesis
Amino acids are the building blocks of proteins. The amino acids are synthesized through the following routes.
Reductive Amination
In reductive amination, ammonia reacts with alpha ketoglutaric acid to form glutamic acid (glutamate).
\[\alpha\text{-Ketoglutaric acid} + NH_4 + NADPH_2 \xrightarrow{\text{Glutamate dehydrogenase}} \text{Glutamate} + H_2O + NADP\]
Here \(NADPH_2\) is the reduced coenzyme that supplies the hydrogen (the reducing power), and \(NADP\) is its oxidised form. This single reaction is the doorway through which inorganic nitrogen enters the world of organic molecules — after it, the nitrogen atom is part of an amino acid.
Transamination
In transamination, the amino group of one amino acid (\(-CHNH_2\)) is transferred to the keto position (\(-CO\)) of another carboxylic acid.
\[\text{Glutamic acid} + \text{oxaloacetic acid} \xrightarrow[\text{Aminotransferase}]{\text{Glutamate aspartate}} \alpha\text{-Ketoglutaric acid} + \text{Aspartic acid}\]
Transamination is how a plant makes the other amino acids. Only one amino acid needs to be built from scratch out of ammonia; from there, the amino group can be handed on from molecule to molecule, generating the full range of amino acids the plant requires.
Amides
Ammonia may be absorbed by an amino acid to produce amides. The process is called amidation.
The amides are amino acids having two amino groups. The extra amino group is attached to the acidic group (\(-COOH\)) in the presence of ATP.
\[\text{Glutamic acid} + NH_4^+ + ATP \longrightarrow \text{alpha glutamine} + ADP\]
\[\text{Aspartic acid} + NH_4^+ + ATP \longrightarrow \text{Asparagine} + ADP\]
Amides like glutamine and asparagine are formed from glutamic acid and aspartic acid respectively, by the addition of another amino group to each. Amides are transported to other parts of the plant via the xylem vessels.
There is an elegant logic here. An amide carries two nitrogen atoms for the same carbon skeleton, so it is a more economical way of moving nitrogen around the plant — and it safely locks away ammonia, which would otherwise be toxic, in an organic form that can travel.
Denitrification
Denitrification is the process in which anaerobic bacteria convert soil nitrates back into nitrogen gas. Denitrifying bacteria remove fixed nitrogen — i.e. nitrates — from the ecosystem, and return it to the atmosphere in the inert form.
Denitrifying bacteria include Bacillus spp., Paracoccus spp. and Pseudomonas denitrificans. They transform nitrates to nitrous and nitric oxides, and ultimately to gaseous nitrogen.
\[2NO_3 \longrightarrow 2NO_2 \longrightarrow 2NO \longrightarrow N_2\]
From the plant’s point of view, denitrification is a loss. From the cycle’s point of view, it is essential — it is the return arrow that sends nitrogen back to the atmosphere and closes the loop.
Sedimentation
Nitrates of the soil are washed away to the sea, or leached deep into the earth along with percolating water. This is another route by which nitrogen leaves the reach of plant roots — not converted into gas this time, but simply carried away in solution.
7.14 Summary
Plant growth 1. Growth is a permanent, irreversible increase in the bulk of an organism, accompanied by a change of form. It has a quantitative aspect (increase in length, breadth, size, volume, mass, dry weight, cell number) and a qualitative aspect (development and differentiation). 2. Growth in multicellular vascular plants is indeterminate (open) and is restricted to meristems — apical (tips; growth in length; primary growth), intercalary (node/base of internode; internode length, leaf primordia, lateral buds) and lateral (vascular cambium; increase in girth). 3. Root and stem show indeterminate growth; leaves, flowers and fruits show determinate growth. Growth in unicellular plants is uniform and determinate.
Phases of growth 4. Cell division/formation: thin-walled, non-vacuolated meristematic cells divide mitotically; one daughter remains meristematic. Growth is slow — the lag phase. 5. Cell enlargement/elongation: the new cell becomes vacuolated, osmotically active and turgid; water uptake stretches it lengthwise and breadthwise while new wall material is synthesised. Growth is fastest — the exponential (log) phase. 6. Cell maturation/differentiation: the cell specialises and attains morphological and physiological maturity. Growth slows to a steady state — the stationary phase.
Conditions for growth 7. Water, nutrients, temperature (optimum 25–35 °C), oxygen, carbon/nitrogen ratio, gravitational force, light and growth hormones. Water maintains turgidity and provides the medium for reactions; oxygen supports respiration; light supports germination and photosynthesis; gravity decides the direction of shoot and root growth.
Growth rate and types of growth 8. Growth rate (efficiency index) is growth per unit time. \(\text{AGR} = \dfrac{dn}{dt}\); \(\text{RGR} = \dfrac{\text{AGR}}{n}\). Two leaves may share the same AGR yet differ greatly in RGR. 9. Arithmetic growth: only one daughter cell keeps dividing; rate is constant; \(L_t = L_o + rt\); the curve is linear. 10. Geometric growth: both daughter cells keep dividing; slow at first, then exponential; \(W_1 = W_o e^{rt}\); the curve is exponential. 11. Growth is measured by cell number, leaf surface area, length, fruit volume, girth or dry weight, and linear growth by direct method, horizontal microscope, auxanometer (arc and Pfeffer’s) and crescograph.
Growth curve 12. Three curves: linear (arithmetic), exponential/J-shaped (geometric) and sigmoid/S-shaped (all three phases together — lag, exponential, stationary). 13. The total time required for all phases to occur is the Grand Period of Growth.
Differentiation, de-differentiation and re-differentiation 14. Differentiation: permanent change in structure and function leading to maturation, at the cost of the capacity to divide (e.g. schizogenous interspaces in hydrophyte parenchyma). 15. De-differentiation: a living mature cell regains the capacity to divide (e.g. interfascicular cambium and cork cambium from parenchyma). 16. Re-differentiation: cells from a de-differentiated meristem again lose the capacity to divide and mature (e.g. secondary xylem and secondary phloem from cambium).
Development and plasticity 17. Development is the ordered, progressive change in shape, form and complexity from seed germination to senescence and death; it includes growth, morphogenesis, maturation and senescence. 18. Plasticity is the ability of a plant to form different structures in response to external or internal stimuli. Heterophylly may be intrinsic (juvenile vs mature leaves — coriander, cotton, Delphinium) or environmental (Ranunculus flabellasis).
Flowchart
FC 7.14 – The chapter at a glance: Seed germination → meristem (apical / intercalary / lateral) → three phases of growth (division → enlargement → maturation) → sigmoid growth curve → differentiation, de-differentiation, re-differentiation → development and plasticity → controlled internally by growth hormones (auxin, GA, cytokinin, ethylene, ABA) and externally by photoperiod and temperature → flowering → all of it supplied by mineral nutrition and the nitrogen cycle
Growth hormones 19. Growth regulators inhibit, promote or modify growth. Promoters: auxins, gibberellins, cytokinins. Inhibitors: ethylene and abscissic acid. Hormones act in minute amounts at a site remote from where they are produced (Thimann and Pincus, 1948) and are transported through phloem parenchyma. 20. Auxin (IAA; precursor tryptophan): discovered through the coleoptile experiments of Darwin, Boysen-Jensen, Paal and Went. Shows polar (basipetal) transport. Causes cell elongation and apical dominance; induces callus, lateral and adventitious roots, parthenocarpy; prevents fruit drop and abscission layer formation; 2,4-D kills dicot weeds in monocot crops. 21. Gibberellin (GA₃; from mevalonic acid; non-polar transport): discovered from Gibberella fujikuroi (Kurosawa; bakane disease). Causes internode elongation, converts genotypic dwarfs to phenotypic tall plants, promotes bolting, amylase synthesis in germinating cereals, parthenocarpy, and overcomes vernalization. 22. Cytokinin (kinetin from Skoog and Miller; natural cytokinin zeatin): promotes cell division and enlargement, releases lateral buds and reverses apical dominance, delays senescence (Richmond and Lang). The cytokinin : auxin ratio controls morphogenic differentiation — low ratio gives roots, high ratio gives shoots. 23. Ethylene (only gaseous regulator; source compound ethephon): promotes fruit ripening, abscission, epinasty, senescence and degreening; inhibits flowering except in pineapple. 24. ABA (stress hormone; 15-carbon sesquiterpenoid): induces dormancy in buds and seeds, promotes abscission and senescence, inhibits cell division and elongation, and closes stomata by K⁺ efflux from guard cells — hence an antitranspirant.
Photoperiodism 25. Photoperiodism (Garner and Allard, 1920) is the response to the duration of light. Light has three aspects: quality, intensity and duration; duration governs flowering. 26. Short Day Plants (long night plants) need a day shorter than the critical photoperiod and a long uninterrupted night; Long Day Plants (short night plants) need a day longer than it; Day Neutral Plants flower independently of photoperiod. 27. The leaf perceives the photoperiod (Knoff) and produces florigen, which travels through the phloem to the bud. The receptor pigment is phytochrome, existing as P\(_r\) and P\(_{fr}\) (interconverted by 660 nm red and 730 nm far red light; Hendricks and Borthwick). P\(_{fr}\) accumulates by day and inhibits flowering in SDP but initiates it in LDP; P\(_r\) accumulates in darkness and does the reverse. Control of morphogenesis by light and phytochrome is photomorphogenesis.
Vernalization 28. Vernalization is the acceleration of the ability to flower by chilling treatment (1–6 °C for one to one and a half months), perceived at the shoot apical meristem, with vernalin as the chemical stimulus, transferable by grafting. Its reversal by high temperature is devernalization. It shortens the juvenile phase and lets crops be grown where they do not naturally flower.
Mineral nutrition 29. Plants absorb about 36 to 40 elements in ionic form, mainly through the root’s regions of elongation and growth. Carbon comes from atmospheric CO₂, hydrogen from water, oxygen from air and water — these three are not mineral in origin. 30. Arnon and Stout’s three criteria define an essential element; Liebig’s Law of the Minimum states that yield is set by the essential element available in minimum quantity. 31. Macroelements (C, H, O, P, Mg, N, K, S, Ca, Fe) play nutritive and structural roles; microelements (Zn, Cu, Al, Si, Mn, B, Mo, Cl) act catalytically as co-factors. Minerals are now classified by their biochemical functions. 32. Deficiency below the critical concentration produces symptoms — stunting, chlorosis, necrosis, mottling and abscission. Mobile elements (N, Mg, K) show symptoms first in older leaves; immobile elements (S, Ca) show them first in young tissues. 33. A concentration that reduces tissue dry weight by 10% is toxic; excess Mn induces Ca, Mg and Fe deficiency. Antagonism is the counterbalancing of one ion’s toxic effect by another. 34. Absorption is passive (diffusion, ion-exchange, mass flow, Donnan equilibrium — no energy) or active (against the gradient, using ATP from respiration; carrier hypothesis of Hoagland and Davis). Ions pass from root hair → cortex → xylem → up the transpiration stream → assimilated → redistributed through phloem.
Nitrogen cycle 35. Nitrogen is a limiting nutrient because atmospheric N₂ carries a strong triple bond (\(N \equiv N\)) and is inert. 36. Fixation may be physical (lightning; and industrial Haber–Bosch at 450 °C and 200 atm) or biological (diazotrophs; ~70% of natural fixation; 16 ATP per molecule of N₂). Cyanobacteria fix nitrogen in heterocysts; Rhizobium fixes it symbiotically in root nodules of Fabaceae. 37. Ammonification returns organic nitrogen to the soil as ammonia; nitrification (Nitrosomonas, Nitrosococcus, then Nitrobacter) converts ammonia to nitrate; assimilation builds nitrate into amino acids and proteins via reductive amination and transamination, with amides (glutamine, asparagine) transporting extra nitrogen through the xylem. 38. Denitrification (Bacillus spp., Paracoccus spp., Pseudomonas denitrificans) returns nitrate to atmospheric N₂, and sedimentation carries nitrates to the sea or deep into the earth.
7.15 Points to Ponder
These questions have no single tidy answer. They are here to be thought about, argued over, and returned to after you have finished the chapter.
Growth was defined as a permanent, irreversible increase in bulk. A wilted plant that swells again after watering has changed in size, but has not grown. Where exactly would you draw the line between growth and a merely reversible change — and would a fruit that swells with water as it ripens fall on the same side of that line?
An animal reaches a fixed adult size and stops; a tree does not. What advantages does a rooted, immobile organism gain from keeping its growth open and indeterminate throughout life — and what costs does it pay for never being “finished”?
In arithmetic growth, one daughter cell stays meristematic while the other matures. If both daughters could always keep dividing, a plant would grow geometrically for ever. Why might a plant be better off for deliberately taking half of its new cells out of the dividing pool?
A mature plant cell can de-differentiate, become meristematic again, and build an entire new organ. An adult nerve cell in an animal cannot. What might it be about the life a plant leads — rooted, exposed, grazed, broken by wind — that makes such reversibility worth having?
The same auxin promotes root elongation at a low concentration and shooting at a higher one. If a single molecule can produce opposite outcomes at different doses, how useful is it to label any hormone simply a “promoter” or an “inhibitor”?
Ethylene, classified as a growth inhibitor, is also what makes a mango sweet and a banana yellow. Is ripening better understood as the beginning of a fruit’s usefulness or as the beginning of its death — and does the answer depend on whether you take the plant’s point of view or the seed’s?
Cytokinin and auxin control morphogenic differentiation not by their individual amounts but by their ratio. Why might a plant have evolved to read a ratio rather than an absolute concentration? What would go wrong if it read only one hormone at a time?
A short day plant is really a long night plant: it measures darkness, not light. Why should the plant have evolved to time itself by the length of the night rather than the length of the day, when both carry the same information about the season?
Vernalization gives a plant the ability to flower, and can be undone by a subsequent spell of heat. Why might it be an advantage for a plant to be able to “forget” a winter it has already experienced?
Liebig’s Law of the Minimum says that yield is capped by the element in shortest supply. If that is true, what does it imply about the common practice of adding more and more nitrogen fertiliser to a field whose yield has stopped rising?
Deficiency of nitrogen appears first in old leaves; deficiency of calcium appears first in young ones. The plant is, in effect, choosing which of its own parts to sacrifice. What does this tell you about how a plant “values” its tissues, and is there any parallel in the way an animal body responds to starvation?
Nitrogen fixation costs 16 molecules of ATP for every molecule of nitrogen fixed, while denitrification hands that hard-won nitrogen straight back to the atmosphere. From the point of view of the ecosystem as a whole, is denitrification a wasteful loss or a necessary part of the cycle?
Nearly 80% of the nitrogen in human tissue is thought to originate from the Haber–Bosch process. What does it mean for humanity that so much of what our bodies are made of now depends on a single industrial reaction — and what should we be asking about the consequences of the nitrate that runs off our fields?
7.16 Objective Questions
- Which of the hormones can replace vernalization?
- Auxin
- Cytokinin
- Gibberellins
- Ethylene
- The principal pathway of water translocation in angiosperms is ……….
- Sieve cells
- Sieve tube elements
- Xylem
- Xylem and phloem
- Abscisic acid controls ………………
- cell division
- leaf fall and dormancy
- shoot elongation
- cell elongation and wall formation
- Which is employed for artificial ripening of banana fruits?
- Auxin
- Ethylene
- Cytokinin
- Gibberellin
- Which of the following is required for stimulation of flowering in plants?
- Adequate oxygen
- Definite photoperiod
- Adequate water
- Water and minerals
- For short day plants, the critical period is ……………..
- light
- dark/night
- uv rays
- both a and c
- Which of the following is a day neutral plant?
- Tomato
- Cotton
- Sunflower
- Soybean
- Essential macroelements are …………
- manufactured during photosynthesis
- produced by enzymes
- absorbed from soil
- produced by growth hormones
- The function of zinc is …………….
- closing of stomata
- biosynthesis of 3-IAA
- synthesis of chlorophyll
- oxidation of carbohydrates
- Necrosis means ……………
- yellow spots on the leaves
- death of tissue
- darkening of green colour in leaves
- wilting of leaves
- Conversion of nitrates to nitrogen is called …………………
- ammonification
- nitrification
- nitrogen fixation
- denitrification
- How many molecules of ATP are required to fix one molecule of nitrogen?
- 12
- 20
- 6
- 16
7.17 Check Your Concepts
A plant that has wilted in the afternoon becomes firm again after watering in the evening. Has it grown? Justify your answer using the definition of growth.
Assertion: A meristem never runs out of cells, even though it goes on supplying new cells to the plant throughout its life. Reason: After each mitotic division in the meristem, one daughter cell remains meristematic while the other undergoes enlargement and differentiation. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Grasses continue to grow back after being repeatedly grazed or mown, whereas many other plants do not. Which meristem is responsible for this, and why does its position make this possible?
Arrange the following in the correct sequence, moving upward from the tip of a growing root: zone of cell differentiation, zone of cell formation, zone of mature cells, zone of cell elongation.
Explain why the phase of cell enlargement is described as an inexpensive way for a plant to gain size, compared with the phase of cell division.
A root tip is placed in a solution that blocks the entry of water into cells, but does not affect mitosis. Which phase of growth will be affected first, and what will happen to the length of the root?
Roots grow underground in complete darkness and carry out no photosynthesis. Why, then, does a waterlogged soil damage them so severely? Explain with reference to respiration and active absorption.
Two leaves grow by exactly the same area in one day, yet a scientist reports that one of them is growing far more vigorously. Explain how both statements can be correct.
Why is dry weight preferred over fresh weight as a parameter for measuring growth?
Match the following:
| Column A | Column B |
|---|---|
| a. Auxanometer | i. Records primary growth, magnifies up to 10,000 times |
| b. Crescograph | ii. Used to measure growth in fields |
| c. Horizontal microscope | iii. Precise measurement of linear growth of shoot |
| d. Direct method | iv. Ordinary measuring scale |
A sigmoid curve and an exponential curve both begin slowly and then rise steeply. What single feature distinguishes them, and what does that feature tell you about the plant?
A mature xylem vessel element cannot de-differentiate, but a mature cortical parenchyma cell can. Why?
Secondary xylem is formed from the cambium present in the vascular bundle. Is this an example of differentiation, de-differentiation or re-differentiation? Justify your choice, and state which process must have occurred immediately before it.
Grafting a mango scion onto a rootstock works only if the cut surfaces are alive and in contact. Explain the role of de-differentiation and re-differentiation in a successful graft.
A single buttercup plant bears finely dissected leaves below the water surface and broad, undivided leaves above it, although all these leaves carry identical genes. Explain this observation, and name the phenomenon.
Distinguish between intrinsic plasticity and environmental heterophylly, giving one example of each.
In Went’s experiment, an agar block that had merely been in contact with a coleoptile tip could, by itself, cause a decapitated coleoptile to bend. What exactly does this experiment prove that Boysen-Jensen’s gelatin experiment does not?
A gardener wants a hedge to grow dense and bushy rather than tall. What should the gardener do, and explain the hormonal basis of the result.
2,4-D is sprayed over a wheat field to kill broad-leaved weeds, yet the wheat plants are unharmed. Explain.
Assertion: Gibberellin treatment can make a genetically dwarf maize plant as tall as a normal maize plant. Reason: Gibberellin changes the genotype of the dwarf plant into that of the tall variety. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Explain how gibberellin promotes germination in a barley grain. In your answer, state clearly why the seed’s stored starch is useless to the embryo until this hormone acts.
A tissue culture laboratory has raised a callus from a tobacco explant, and now needs it to produce shoots first and roots afterwards. Explain, in terms of the cytokinin : auxin ratio, how the medium should be altered at each stage.
Auxin causes apical dominance, while cytokinin reverses it. What would happen to the shape of a plant in which cytokinin was consistently dominant over auxin? What would happen if auxin were consistently dominant?
Ethylene prevents the formation of the abscission layer — true or false? Correct the statement if it is wrong, and name the hormone that has the opposite effect.
A raw mango placed in a closed container along with a ripe banana ripens much faster than one kept in the open. Explain, naming the hormone and stating the property of that hormone which makes the closed container important.
ABA is called both a stress hormone and an antitranspirant. Trace the chain of events, beginning with drought and ending with reduced water loss, that justifies both names.
Match each hormone with its standard bioassay:
| Column A | Column B |
|---|---|
| a. Auxin | i. Chlorophyll retention test |
| b. Gibberellin | ii. Triple pea test |
| c. Cytokinin | iii. Avena curvature test |
| d. Ethylene | iv. Alpha amylase bioassay |
A short day plant is kept in a photoperiod that suits it perfectly, but a brief flash of light is given in the middle of every night. It fails to flower. What does this experiment prove about what the plant is actually measuring? Why does the same treatment help a long day plant?
All the leaves are removed from a plant, which is then given the exact photoperiod it needs to flower. It does not flower. Explain, naming the organ of perception and the chemical stimulus involved.
A short day plant is given a flash of red light (660 nm) in the middle of its dark period, immediately followed by a flash of far red light (730 nm). Predict whether it will flower, and explain your answer in terms of P\(_r\) and P\(_{fr}\).
Why are Chrysanthemum growers able to sell flowers all year round, even though the plant is a short day plant? Describe the treatment used in summer and in winter.
Winter wheat sown in spring fails to flower that summer. Explain why, and describe the treatment that would allow it to flower in the same season.
Vernalization is said to be transferable through grafting. What conclusion did this observation allow scientists to draw, and what name was given to the substance involved?
Distinguish between photoperiodism and vernalization with respect to: the environmental factor involved, the site of perception, and the chemical stimulus produced.
An element is found in the ash of a plant. Is it therefore an essential element? Answer using Arnon and Stout’s criteria.
A farmer has been adding large quantities of nitrogen fertiliser to a field, but the yield has stopped rising. Using Liebig’s Law of the Minimum, suggest what may be wrong and what should be done next.
Deficiency of nitrogen shows up first in the older leaves, while deficiency of calcium shows up first in the young leaves. Explain the reason for this difference.
Chlorosis and necrosis are both deficiency symptoms seen on leaves. Distinguish between them, and state which of the two can be reversed by correcting the deficiency.
A plant shows the classic symptoms of iron deficiency, but a soil test reveals plenty of available iron. Suggest an explanation, and name the phenomenon involved.
In Donnan equilibrium, ions move against their own concentration gradient — yet the process is classified as passive absorption. Explain why this is not a contradiction.
Arrange the following in the correct sequence of a mineral ion’s journey: xylem, root hair, transpiration stream, soil solution, cortex, assimilation in the leaf, redistribution through phloem.
Nitrogen makes up nearly 78% of the atmosphere, and yet it is described as a limiting nutrient in agricultural ecosystems. Resolve this apparent contradiction.
Assertion: Nitrogen fixation is an energy-expensive process, but nitrification actually yields energy to the bacteria that carry it out. Reason: Fixation must break the triple covalent bond of N₂, whereas nitrification only oxidises ammonia that is already available in the soil. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Name the specialised cell in which cyanobacteria fix nitrogen, and explain how its structure protects the process.
Arrange the following processes of the nitrogen cycle in the sequence in which a single nitrogen atom would pass through them, beginning with atmospheric nitrogen: nitrification, denitrification, ammonification, nitrogen fixation, assimilation.
Why does a plant convert nitrate into ammonia and then immediately use the ammonia to build amino acids, instead of storing the ammonia?
Distinguish between reductive amination and transamination. Which of the two brings new nitrogen into the organic world, and why is the other still necessary?
Amides such as glutamine and asparagine are transported through the xylem. Give two reasons why a plant benefits from moving nitrogen in the form of an amide rather than as an ordinary amino acid.
A waterlogged field loses much of its applied nitrogen fertiliser. Explain this loss with reference to denitrifying bacteria, and name two genera responsible.
A field is sown with gram (a legume) one season and with wheat the next. Explain the benefit to the wheat crop, naming the bacterium and the structure involved.
Label the following on a diagram of a growing root and briefly state what happens in each: zone of cell formation, zone of cell elongation, zone of cell differentiation, zone of mature cells.
Draw a sigmoid growth curve. Mark the lag phase, the exponential (log) phase and the stationary phase on it, and mark the grand period of growth.
Enlist the phases of growth in plants. (30–40 words)
Give the full form of IAA. (30–40 words)
What is meant by ‘open growth’? (30–40 words)
Which is the plant stress hormone? (30–40 words)
What is denitrification? (30–40 words)
Name the bacteria responsible for the conversion of nitrite to nitrate. (30–40 words)
What is the role of gibberellin in rosette plants? (30–40 words)
Define vernalization. (30–40 words)
Define photoperiodism. (30–40 words)
What is the grand period of growth? (30–40 words)
Write a short note on: (80–100 words)
- Differentiation
- Redifferentiation
Differentiate between arithmetic and geometric growth. (80–100 words)
Enlist the role and deficiency symptoms of: (80–100 words)
- Nitrogen
- Phosphorus
- Potassium
What is a short day plant? Give any two examples. (80–100 words)
What is vernalization? Give its significance. (80–100 words)
Explain the sigmoid growth curve with the help of a diagram. (150–200 words)
Describe the types of plants on the basis of the photoperiod required, with the help of suitable examples. (150–200 words)
Explain biological nitrogen fixation with an example. (150–200 words)
Write on the macronutrients and micronutrients required for plant growth. (150–200 words)
How are minerals absorbed by plants? (150–200 words)
7.18 Practice With Numericals
A leaf measures 6 cm² in area at the start of a day and 9 cm² at the end of it. On the same plant, a second leaf grows from 60 cm² to 63 cm² over the same day. Calculate the absolute growth rate and the relative growth rate of each leaf, and state which leaf is growing more vigorously relative to its own size.
A root is 3 cm long at time zero and shows arithmetic growth at a constant rate of 0.4 cm per day. Using \(L_t = L_o + rt\), calculate its length after 5 days and after 10 days. Sketch the shape of the curve you would obtain by plotting length against time.
A seedling is 12 cm tall after 4 days of arithmetic growth and 18 cm tall after 10 days. Using \(L_t = L_o + rt\), calculate the growth rate \(r\) and the length of the seedling at time zero.
A tissue culture begins with a single cell that undergoes geometric growth, with every cell dividing at each round. How many cells are present after 4, 8 and 12 rounds of division? Explain why the growth appears slow at first and then extremely rapid, and write the equation that describes this type of growth, defining every symbol in it.
A single maize root apical meristem can give rise to more than 17,500 new cells per hour. Calculate the minimum number of new cells produced by one such meristem in a full day of 24 hours. If a plant has 40 such actively growing root tips, estimate the minimum number of new root cells the plant produces in a day.
Nitrogen fixers use 16 molecules of ATP to fix each molecule of nitrogen. Calculate the number of ATP molecules required to fix 25 molecules of \(N_2\), and the number of molecules of ammonia produced in the process. Using the equation \(N_2 + 8H^+ + 8e^- + 16\,ATP \longrightarrow 2NH_3 + H_2 + 16\,ADP + 16\,P_i\), also state how many ADP molecules are released.
In the Haber–Bosch process, \(N_2 + 3H_2 \longrightarrow 2NH_3\) is carried out at 450 °C and 200 atm. Calculate the number of molecules of hydrogen required to convert 15 molecules of nitrogen completely into ammonia, and the number of ammonia molecules formed.
A mineral ion concentration that reduces the dry weight of tissues by 10% is considered toxic. A leaf sample has a normal dry weight of 0.80 g. What dry weight would indicate that the concentration of a micronutrient supplied to it has reached the toxic level?