Chapter 6: Plant Water Relation
On the foggy northern coast of California stand the tallest living things on Earth — the coast redwoods, Sequoia sempervirens. The tallest of them rise well over 100 metres, taller than a thirty-storey building. Every one of their topmost leaves is alive, and every one of them needs water.
Now think about what that means. The water those leaves use is drawn from the soil, at ground level. It has to travel the entire height of the trunk, straight up, against gravity, all day, every day. A fire engine pumping water to the top of a tall building needs a powerful motor. The redwood has no motor, no muscle, no beating heart, and not a single moving part. Yet it lifts hundreds of litres of water each day from its roots to its crown — silently, and without spending much energy at all.
Even more surprising: most of that water is not “used” by the tree in any obvious way. It simply escapes into the air through tiny pores on the leaves. So the tree spends enormous effort lifting water it then lets go of. That sounds wasteful. It is not.
The redwood is an extreme example, but the same machinery is at work in a mango tree in your street and in a tomato plant in a pot on a balcony.
Diagram
Fig. 6.0 – A tall coast redwood beside a multi-storey building for scale, with an arrow tracing the path of water from soil → root → trunk → branch → leaf → water vapour escaping to air
This chapter will help you answer each of these questions.
6.1 Properties of Water
A plant cannot walk to its food. Everything it needs — water, dissolved minerals, nutrients, and gases such as \(O_2\) and \(CO_2\) — must be taken in from the surroundings in which it is rooted. Of all these, water is the one that decides most. When water is short, growth slows and yield falls; productivity in plants is affected mainly by the non-availability of water.
Water has long been called the elixir of life, and in plants this is close to literal. Water makes up nearly 90 to 95% of most plant cells and tissues. It is the water inside a cell that keeps the cell swollen and firm, which is what gives a soft herb its shape and keeps a leaf held out flat to the sun. This firmness is called turgidity, and you will meet it again and again in this chapter.
What makes water so useful
Water is a liquid at room temperature, and this alone matters — a substance that is gaseous or solid at 25 °C could never flow through a root. It is a chemically inert inorganic compound, and in the pure state it has a neutral pH. Being inert, it can carry substances about without reacting with them and destroying them.
Water is also the best solvent for most solutes. Because it dissolves so much and reacts with so little, it is the ideal transporting medium for dissolved minerals and food molecules — the sap moving through xylem and phloem is essentially water carrying cargo. Inside the cell, water is the aqueous medium in which practically every biochemical reaction takes place.
Water is not only a medium; it is also a raw material. In photosynthesis, water is one of the two starting substances the plant actually consumes:
\[6CO_2 + 12H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2 + 6H_2O\]
Here \(CO_2\) is carbon dioxide, \(H_2O\) is water, \(C_6H_{12}O_6\) is glucose and \(O_2\) is oxygen. Without a steady water supply, this reaction — and therefore the plant’s entire food supply — simply stops.
Water as a thermal buffer
Water has high specific heat, high heat of vaporization and high heat of fusion. These three terms describe how much heat water must gain or lose before its temperature or its physical state changes.
- High specific heat: a large amount of heat must be added to raise the temperature of water even slightly.
- High heat of vaporization: a large amount of heat must be absorbed to convert liquid water into vapour.
- High heat of fusion: a large amount of heat must be removed before water freezes into ice.
Together these mean that a tissue full of water heats up slowly and cools down slowly. Water therefore acts as a thermal buffer, protecting the delicate enzymes and membranes of the cell from sudden swings of temperature between a hot afternoon and a cold night.
Hydrogen bonds — the reason behind the properties
All of these unusual properties trace back to one thing: hydrogen bonds between water molecules. In a water molecule, oxygen pulls the shared electrons towards itself, so the oxygen end carries a slight negative charge and the hydrogen ends a slight positive charge. The slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a neighbouring molecule, and this weak attraction is a hydrogen bond, as shown in Fig. 6.1.1.
Diagram
Fig. 6.1.1 – Two or three water molecules showing partial negative charge on O, partial positive on H, and dotted hydrogen bonds linking H of one molecule to O of the next
Because millions of these bonds must be broken before water molecules can move apart, water resists heating, resists evaporating, and resists freezing. Hydrogen bonding is the single explanation behind the whole list of properties above.
Cohesion, adhesion and the rise of water
Hydrogen bonds also make water molecules cling to one another and to other surfaces. Water molecules show good cohesive and adhesive forces of attraction.
Cohesion also gives water a high surface tension — the surface of water behaves like a stretched skin, because the molecules at the surface are held tightly to one another. Because of high surface tension together with strong adhesive and cohesive forces, water can rise easily in fine capillaries — narrow tubes — without anything pushing it.
Hold on to this last point. A xylem vessel is, in effect, an extremely fine capillary running from the root to the leaf. Water is therefore a molecule that connects the physical world — forces, pressures, surface tension — with biological processes such as absorption, transport and photosynthesis. The rest of this chapter is really the story of how a plant puts these ordinary physical properties of water to extraordinary use.
6.2 Water Absorbing Organ: Root
If water is to be lifted to the top of a plant, it must first get into the plant. The root is the main organ of water and mineral absorption. It is the only part of most plants in permanent, intimate contact with the soil, and it is built for exactly this job.
In terrestrial plants — the ordinary land plants growing in soil — water is absorbed in the liquid form, drawn from the film of water clinging around the soil particles. But not every plant has soil to draw from.
Roots that drink from the air
Epiphytic plants, such as orchids, grow perched on the branches of other trees, with their roots hanging free in the air. They cannot pull liquid water from soil, because there is no soil. Instead, their epiphytic roots absorb water vapour directly from moist air, using a special spongy tissue called velamen that covers the root.
The regions of a root tip
A typical root is divisible into four different regions, and they lie one above the other in a fixed order from the tip upwards, as shown in Fig. 6.1 a.
Diagram
Fig. 6.1 a – Root tip showing, from the tip upwards, root cap, meristematic region, zone of elongation, root hair zone and maturation zone, with root hairs labelled
The order is not accidental: cells are first produced at the tip, then stretched, then matured, and only the matured cells develop root hairs. The sequence is summarised in FC 6.1.
Flowchart
FC 6.1 – Regions of a root tip, from the tip upwards: root cap (protects the tip) → meristematic region (cells divide) → zone of elongation (cells stretch) → zone of maturation / root hair zone, also called the zone of absorption (cells mature and form root hairs)
It is the zone of absorption that concerns us here. In this region, the epidermal cells of the root — called epiblema cells — push out fine, unicellular, hair-like extensions called root hairs. These are the true points of entry for water.
Structure of root hair
A root hair is a cytoplasmic extension, or prolongation, of an epiblema cell — the cell simply stretches a finger of itself out into the soil. Each root hair may be approximately 1 to 10 mm long, and is a tube-like structure. Being a mere extension, it shares the cytoplasm and the nucleus of the parent epiblema cell rather than owning a separate set.
Root hairs are colourless, unbranched and very delicate. They are also short-lived, or ephemeral — an individual root hair survives only a few days before dying, while new ones are constantly formed just behind the growing tip. The plant therefore keeps a fresh absorbing surface always in contact with fresh soil.
Look closely at the internal structure of a root hair in Fig. 6.1 b. It has a large central vacuole, which stores a concentrated cell sap. The vacuole is surrounded by a thin film of cytoplasm, containing the nucleus and mitochondria, and this cytoplasm is bounded on the outside by the plasma membrane.
Diagram
Fig. 6.1 b – Structure of a root hair: root epithelial (epiblemal) cell drawn out into a tube-like root hair, labelled with cell wall, cell membrane, cytoplasm, large central vacuole, nucleus and mitochondria
Outside the plasma membrane lies a thin cell wall, and this wall is two layered. The outer layer is composed of pectin and the inner layer is made up of cellulose. Both pectin and cellulose matter greatly for absorption, as the next section will show.
Now note the single most important structural fact about a root hair: the cell wall is freely permeable, but the plasma membrane is selectively permeable. The wall lets water and dissolved substances pass through without discrimination. The membrane, in contrast, chooses — it allows water through freely but controls which solutes may enter. Every step of water absorption you will study depends on this difference.
6.3 Water Available to Roots for Absorption
A root hair does not absorb water from “the soil” in general. It absorbs from a thin, busy shell of soil that immediately surrounds the root, called the rhizosphere. This is the microenvironment where root, soil particles, soil water and soil microbes all meet, and it is the only region a root ever actually deals with.
Now comes a fact that surprises most students. Soil may be damp to the touch and still leave a plant thirsty. Water is present in the soil in four different forms, and a root can absorb only one of them.
Diagram
Fig. 6.3.1 – A cluster of soil particles showing the four forms of soil water: gravitational water draining down between large pores, capillary water held in the narrow pores between particles, a tightly held hygroscopic film on the particle surface, and combined water locked chemically inside the particle
Gravitational water
When it rains, or when a field is heavily irrigated, a large amount of water enters the soil. Much of it does not stay. It percolates deep down through the wide spaces between the soil particles, pulled by gravity, and drains away below the reach of the roots. This is gravitational water, also called free water.
Because it has already sunk past the root zone, gravitational water is not available to plants for absorption. It is water the plant sees for only a few hours and then loses.
Hygroscopic water
Fine soil particles — especially clay — behave like tiny sponges. They imbibe or adsorb water and hold it as an extremely thin film tightly bound to their surfaces. This tightly held film is hygroscopic water.
The word “tightly” is the whole point. The particle grips this water with a force far greater than the suction a root hair can generate. Roots cannot absorb it. The water is physically there, but it is out of reach.
Combined water
Some water is not even present as water any longer. It is chemically locked into the soil minerals themselves, in the form of hydrated oxides of silicon, aluminium and similar elements. This is combined water.
Since it forms part of the chemical structure of the mineral, it can be released only by chemical change, not by suction. It too is not available to plants for absorption.
Capillary water
That leaves the fourth form. Between neighbouring soil particles there are fine pores, no wider than narrow tubes. Water is held in these pores by capillarity — the same combination of surface tension, cohesion and adhesion you met in Section 6.1. This is capillary water.
Capillary water is held firmly enough not to drain away under gravity, yet loosely enough that a root hair can pull it in. It is the only form of soil water that is available for absorption, and therefore it is, in practice, the plant’s entire water supply.
Having settled which water the root can take, the next question is how it takes it — and that is the subject of the section that follows.
6.4 Absorption of Water by Roots from Soil
We now know that a root hair sits in the rhizosphere, that its wall is freely permeable and its plasma membrane selectively permeable, and that the only water it can use is capillary water clinging in the soil pores. How does that water actually get from the soil pore into the cytoplasm?
The root hair does not use a single trick. It absorbs water by employing three physical processes that occur sequentially — imbibition, diffusion and osmosis. The order matters, because each one delivers the water a little further in, and hands it over to the next.
Flowchart
FC 6.2 – Sequence of water entry into a root hair cell: capillary water in soil → imbibition (water adsorbed by the pectin–cellulose cell wall) → diffusion (water moves through the freely permeable wall, along the concentration gradient, up to the outer surface of the plasma membrane) → osmosis (water crosses the selectively permeable plasma membrane into the cytoplasm) → water inside the root hair cell
Notice that all three are physical processes. None of them needs the cell to spend energy. The plant is simply letting the ordinary properties of water do its work for it.
a. Imbibition
The very first contact between soil water and the plant is with the cell wall, and the cell wall does something rather like a sponge does: it soaks the water up.
Imbibition is the swelling up of hydrophilic colloids due to the adsorption of water. “Hydrophilic” means water-loving, and a colloid is a substance whose particles are large but still dispersed — the sort of material that attracts water molecules onto its surface and holds them there.
Two terms go with it. The substance that adsorbs the water or liquid is called the imbibant, and the water or liquid that gets imbibed is called the imbibate.
Recall the structure of the root hair cell wall from Section 6.2: its outer layer is pectin and its inner layer is cellulose. Pectic compounds and cellulose are both hydrophilic colloids. The wall of a root hair is therefore, by its very chemistry, a natural imbibant sitting in contact with soil water.
During imbibition, the water molecules get tightly adsorbed on to these colloids. This is the key difference from ordinary dissolving — no solution is formed. The water is held on the surface of the imbibant, not mixed into it as a solute in a solvent. The wall simply swells.
Imbibition does not go on forever. It continues till an equilibrium is reached — that is, until the imbibant has taken up as much water as its colloids can hold. In other words, water moves along the concentration gradient, from where water molecules are more crowded to where they are less crowded, and stops when the two sides are balanced.
Imbibition is not a laboratory curiosity — it is going on all around you. It is what happens in the soaking of seeds, the swelling up of dried raisins, and the kneading of flour, where dry flour particles imbibe water and the dough swells into a soft mass.
Wood is largely cellulose — a hydrophilic colloid, and therefore a good imbibant. In the rainy season the air and the wood are damp, the cellulose imbibes water, and the door swells. A door that fitted its frame in summer is now slightly too large for it, and it jams. In the dry season the water is lost again and the door shrinks back.
b. Diffusion
Diffusion means, quite simply, to disperse — to spread out.
What drives this movement? Nothing pushes the molecules. They are already in constant random motion because of their own kinetic energy, and this random motion, on average, carries more of them away from a crowded region than into it. The net result looks purposeful even though every individual molecule is simply wandering.
Diffusion continues till an equilibrium is reached — that is, until the concentration is the same everywhere and there is no longer any net movement in either direction. Molecules keep moving after that, but as many go one way as the other.
Now apply this to the root hair. Water has been imbibed into the wall, and the wall is freely permeable. So water passes into the cell by diffusion through the freely permeable cell wall, moving inward because the concentration of water molecules is higher in the wet wall than deeper inside. Water is now at the interface of the cell wall and the plasma membrane, as shown in Fig. 6.2 — right at the doorstep of the cytoplasm, but not yet inside it.
Diagram
Fig. 6.2 – Diffusion of water into a plant cell across the plasma membrane: water molecules outside the cell, the membrane bilayer, a water-selective pore (aquaporin) with water passing through it, and the cytoplasm within
Diffusion pressure and diffusion pressure deficit
Diffusing particles exert a pressure, called the diffusion pressure (D.P.). It is directly proportional to the number of diffusing particles present — the more crowded the molecules of a substance, the harder they push to spread out.
This has an immediate consequence. Pure water contains nothing but water molecules, so its water molecules are as crowded as they can possibly be. Add a solute, and some of that space is now taken by solute particles. Therefore the diffusion pressure of a pure solvent (pure water) is always more than the diffusion pressure of the solvent in a solution.
The gap between the two has a name. The difference in the diffusion pressures of a pure solvent and of the solvent in a solution is called the Diffusion Pressure Deficit (DPD), also known as Suction Pressure (SP). The term was coined by B. S. Meyer (1938). In modern usage the term water potential has replaced DPD, and we shall come to it in Section 6.5.
In colloquial language, DPD is the thirst of a cell — the eagerness with which it absorbs water from its surroundings. A cell with a high DPD is very thirsty; a cell with zero DPD wants no more water at all. This one idea will explain almost everything about water movement in the rest of the chapter, so it is worth fixing firmly in mind.
Return to our root hair. The water around the cell wall is nearly pure, so it has a more diffusion pressure. The cell sap inside is full of minerals and sugars, so it has a less diffusion pressure. Because of this difference, water moves into the cell by diffusion — down the diffusion pressure gradient, from where the diffusion pressure is higher to where it is lower.
Diffusion is not used by the plant only for absorbing water. It is significant in plants in the absorption of water, in the absorption of minerals, in the conduction of water against gravity, in the exchange of gases, and in the transport and distribution of food. It is one of the workhorse processes of plant life.
c. Osmosis
Water now stands at the interface of the cell wall and the plasma membrane. The wall let it through without objection. The membrane will not.
Osmosis is the process by which water finally enters into the cytoplasm of the root hair cell. It is a special type of diffusion — the diffusion of a solvent through a semipermeable membrane.
Why does osmosis happen in the root hair at all? Because of what is inside it. The cytoplasm of the root hair cell contains minerals, sugars and other dissolved substances. In other words, the solution inside the cell is more concentrated — stronger — than the soil solution outside the cell, which is weaker. So the solvent from the weaker solution outside enters the cytoplasm — the stronger solution — through the semipermeable plasma membrane. This migration of solvent is osmosis.
Thus water sitting at the interface of the cell wall and the plasma membrane enters the cytoplasm of the root hair cell by osmosis. The journey from soil to cytoplasm is complete.
Types of solutions
Whether water enters a cell or leaves it depends entirely on how the outside solution compares with the cell sap. With respect to concentration and osmotic migration, three types of solutions are recognised.
Hypotonic — a weak solution, or a strong solvent, having low osmotic concentration.
Hypertonic — a strong solution, or a weak solvent, having high osmotic concentration.
Isotonic — a solution of such a concentration that there is neither gain nor loss of water in an osmotic system. In other words, the concentration outside and inside the cell is the same.
Endosmosis and exosmosis
Osmosis is of two types, viz., exosmosis and endosmosis, depending on which way the solvent goes.
Exosmosis is the diffusion of solvent from the cell to the outside. It causes flaccidity of the cell — the cell loses water, its contents shrink away from being pressed against the wall, and it goes limp.
Endosmosis is the diffusion of the solvent into the cell. It causes turgidity of the cell, that is, the cytoplasm becomes turgid, swollen and firm. This is what happens in a root hair standing in moist soil.
Turgor pressure and wall pressure
Endosmosis fills the cell with water, and this water has to go somewhere. It pushes outward. Turgidity increases the turgor pressure (T.P.) of the cell.
As the cell becomes more and more turgid, it becomes less and less thirsty. In a fully turgid cell, DPD is zero — it can hold no more water and will absorb none.
The cell wall does not simply give way. Being thick and rigid, the wall exerts a counter pressure on the cell sap, pushing inward exactly as hard as the sap pushes outward. This counter pressure is called wall pressure (W.P.).
In a fully turgid cell, therefore, \(T.P. = W.P.\), but the two operate in opposite directions. They are a balanced pair, like a stretched balloon pressing on the wall of a box and the box pressing back.
Osmotic pressure
There is one more pressure to name, and it belongs to the solution rather than to the cell.
Osmotic pressure (O.P.) is the pressure exerted due to osmosis. Think of it this way: if solvent is entering a solution across a semipermeable membrane, you could stop it by pressing on the solution hard enough from the other side. The pressure you would need is its osmotic pressure.
The more concentrated the solution, the greater its osmotic pressure. A cell with very concentrated sap has a high O.P.
Now we can put the three pressures together. The thirst of a cell is not simply its osmotic pressure, because part of that pull is already being cancelled by the outward turgor pressure. What is left over is the DPD:
\[D.P.D. = O.P. - T.P.\]
And since, in a turgid cell, the turgor pressure and the wall pressure are equal (\(\because T.P. = W.P.\)):
\[\therefore \; D.P.D. = O.P. - W.P.\]
Here D.P.D. is the diffusion pressure deficit, O.P. the osmotic pressure, T.P. the turgor pressure and W.P. the wall pressure — all of them pressures, measured in the SI unit pascal (Pa), though bars and atmospheres are also commonly used in plant physiology.
Two special cases follow directly, and both are favourites in examinations.
In a flaccid cell, the cell has lost its water, so there is no outward push at all: T.P. is zero. \[\therefore \; DPD = OP\]
In a fully turgid cell, the cell is full and cannot take more, so its thirst has vanished: DPD is zero. \[\therefore \; TP = OP\]
Facilitated diffusion
Water crosses the plasma membrane easily. Many solutes do not, and for them the cell provides a shortcut.
The passive absorption of solutes when mediated by a carrier is called facilitated diffusion. Particles that are lipid soluble can easily diffuse straight through the lipoproteinous cell membrane, because the membrane is itself largely lipid — like dissolves in like. But the diffusion of hydrophilic (water-loving) solutes has to be facilitated, because such particles find it very difficult to cross the oily interior of the membrane on their own.
Membrane proteins provide the sites for facilitated diffusion. These proteins are the aquaporins and the ion channels — the water-selective pore you saw labelled in Fig. 6.2 is an aquaporin. They form a hydrophilic passage straight through the membrane, so that the particle never has to enter the lipid layer at all.
Two conditions must be understood clearly, because both are commonly tested. First, these proteins help move substances across the membrane without the expenditure of energy — no ATP is spent. Second, a concentration gradient must be present for the molecules to be diffused through facilitated diffusion; the carrier can speed a particle along a gradient, but it cannot push it uphill against one.
6.5 Water Potential
Everything you learnt in the last section was written in the language of pressures — diffusion pressure, DPD, osmotic pressure, turgor pressure. That is the older language of plant physiology. Modern plant science describes exactly the same events in the language of energy. Both descriptions are correct; you need to be fluent in both.
Free energy and chemical potential
According to the principles of thermodynamics, every component of a system possesses a definite amount of free energy — energy that is available to do work. The osmotic movement of water is, at bottom, a movement driven by free energy: water moves from where its free energy is greater to where it is less, just as a ball rolls from a higher shelf to a lower one.
The free energy per molecule in a chemical system is called its chemical potential. Every substance in a system has one.
Water potential (ψ)
The chemical potential of water is called the water potential. It is represented by the Greek letter psi, written ψ.
Water potential and DPD are two ways of saying the same thing, with the sign reversed. The water potential of the protoplasm is equal in magnitude but opposite in sign to its DPD. A cell whose DPD is high (very thirsty) has a strongly negative ψ.
\[\psi = - \, D.P.D.\]
This immediately tells you something important: water potential has a negative value in living cells.
Why ψ is negative
Start from the reference point. The water potential of pure water is always zero. Pure water has the maximum possible free energy per molecule, and by convention we call that zero.
Now add any solute to it. The solute particles attract and hold water molecules around themselves, which lowers the free energy of the water. So the addition of any solute decreases the ψ value of water. Since we began at zero and went down, any solution must have a ψ below zero — that is, a negative value. The more concentrated the solution, the more negative its ψ.
Diagram
Fig. 6.5.1 – A vertical scale of water potential with pure water at 0 bar at the top, a dilute solution slightly below zero, a concentrated cell sap far below zero, and an arrow showing that water always moves downwards along this scale
The new terminology
The three old pressure terms have been renamed, and the renaming is not merely cosmetic — it also carries a sign.
Because of the sign reversal, the old equation \(D.P.D. = O.P. - T.P.\) becomes, in the new language, a simple sum:
\[\psi_w = \psi_s + \psi_p\]
where \(\psi_w\) is the water potential of the cell, \(\psi_s\) its osmotic (solute) potential — a negative quantity — and \(\psi_p\) its pressure potential — a positive quantity. All three are measured in the SI unit pascal (Pa), though bars and atmospheres remain in common use.
Which way does water move?
Here is the single rule that governs every drop of water in a plant:
Note carefully what this does not say. It does not say water flows from where there is more water to where there is less. It flows down the ψ gradient, and nothing else.
This rule applies from cell to cell as much as it does from soil to root. The difference between the water potentials of two adjacent cells decides the movement of water between them, and this cell-to-cell movement occurs through the plasmodesmata — the fine cytoplasmic threads that pass through the walls and connect the living contents of neighbouring cells.
Factors affecting water absorption
The rate at which a root actually takes up water is not fixed. Five conditions influence it.
The presence of capillary water is essential. As Section 6.3 established, this is the only form of soil water a root can absorb; when the capillary water is exhausted, absorption stops however damp the soil may appear.
The rate of water absorption is maximum at a soil temperature between 20 °C and 30 °C. Below this range, membranes become less permeable and the cell’s metabolic activity slows; well above it, the root’s functioning is disturbed. There is an optimum, not a “hotter is faster” rule.
A high concentration of solutes in the soil water reduces the rate of absorption of water. This follows directly from the ψ rule: a concentrated soil solution has a strongly negative water potential of its own, so the gap between soil and root narrows, and the root can no longer pull water in.
Poorly aerated soil shows a poor absorption rate. Roots respire, and respiration needs oxygen. In waterlogged or compacted soil, the air spaces are filled with water, the root cells are starved of oxygen, their metabolism slows, and absorption suffers — which is why a plant standing in a flooded pot can wilt.
Increased transpiration accelerates the rate of absorption of water in irrigated soil. The faster water is lost from the leaves, the more strongly water is pulled up from below — provided, of course, that soil water is available to replace it. You will see the mechanism behind this in Section 6.9.
6.6 Plasmolysis
So far we have watched water move into a cell. Now reverse the situation. Put a living plant cell into a hypertonic solution — one more concentrated than its own cell sap — and everything runs backwards.
Exo-osmosis occurring in a living cell when it is placed in a hypertonic solution is called plasmolysis.
What happens inside a plasmolysing cell
Because the outside solution is stronger, water leaves the cell sap by exo-osmosis across the selectively permeable plasma membrane. The protoplast — the entire living content of the cell, bounded by its plasma membrane — loses volume, shrinks, and recedes from the cell wall. The cell thus becomes flaccid.
The first point at which the protoplast just begins to pull away from the wall is called incipient plasmolysis. Beyond this point, shrinkage continues and the protoplast collapses towards the centre of the cell, until the cell is fully plasmolysed. Follow the stages in Fig. 6.6.1.
Diagram
Fig. 6.6.1 – Three plant cells side by side: (a) turgid cell with protoplast pressed against the wall, (b) incipient plasmolysis with the protoplast just beginning to pull away at the corners, (c) fully plasmolysed cell with the shrunken protoplast in the centre and the gap between wall and protoplast filled by the outer solution
In a plasmolysed cell, a gap is developed between the cell wall and the protoplast. This gap does not stay empty — remember from Section 6.2 that the cell wall is freely permeable. The outer solution therefore seeps straight through the wall and fills the gap.
Since the protoplast has shrunk away from the wall, it is no longer pressing on it at all. Therefore, in a plasmolysed cell, T.P. is always zero.
Deplasmolysis — putting it back
Plasmolysis is reversible, provided the cell has not been damaged. When a plasmolysed cell is placed in a hypotonic solution, endo-osmosis occurs, water re-enters the cell sap, the protoplast swells out again and presses once more against the wall, making the cell turgid. This recovery is called deplasmolysis.
Flowchart
FC 6.3 – Plasmolysis and deplasmolysis: turgid cell → placed in hypertonic solution → exo-osmosis → protoplast begins to shrink (incipient plasmolysis) → protoplast fully recedes from wall, gap filled by outer solution (plasmolysis, T.P. = 0) → cell transferred to hypotonic solution → endo-osmosis → protoplast expands back against wall (deplasmolysis) → back to turgid cell (DPD = 0, T.P. = O.P.)
Once the cell is fully turgid again, we are back to the condition already met in Section 6.4: in a fully turgid cell \(T.P. = O.P.\), and hence DPD is always zero.
6.7 Path of Water Across the Root
Water is inside the root hair. But the root hair is on the outside of the root, and the xylem — the pipeline that will carry water up the stem — lies at the very centre, in the stele. Between the two lie the cortex, the endodermis and the pericycle. This section follows water across that gap, from the epiblema up to the xylem in the stelar region.
The suction pressure gradient
Recall the sequence from Section 6.4: water is absorbed by the root hair cell through imbibition → diffusion → osmosis, in that order. Consequently the cell becomes turgid. Its turgor pressure increases — and therefore, since \(DPD = OP - TP\), its DPD value decreases. The root hair cell is now full, and no longer thirsty.
But the cortical cell lying immediately adjacent to it, one step further in, is thirsty. That inner cortical cell has a more DPD value, because its O.P. is more — its sap is more concentrated than that of the root hair cell.
Water therefore obeys the rule established in Section 6.5 and moves towards the greater DPD. The cortical cell sucks water out of the turgid root hair cell. The cortical cell now becomes turgid; the root hair cell, having given its water away, becomes flaccid — and a flaccid cell has a high DPD again, so it promptly absorbs a fresh supply of water from the soil.
The same handover is repeated inward, cell by cell. Water from the turgid cortical cell is sucked by the next inner cortical cell, and the process goes on. In this way a gradient of suction pressure (DPD) is developed from the cells of the epiblema right through to the inner cortex of the root — lowest DPD at the outside, highest at the inside.
Consequently, water moves rapidly across the root through the loosely arranged living cells of the cortex — “loosely arranged” matters, because the wide intercellular spaces between cortical cells offer very little resistance. From the cortex, water passes through the passage cells of the endodermis (in monocot roots) and finally into the cells of the pericycle. And the pericycle is exactly where it needs to be, because the protoxylem lies in close proximity with the pericycle.
Flowchart
FC 6.4 – Cell-to-cell route of water across the root, along the DPD gradient: soil water → root hair (epiblema cell) → cortical cells (one after another, through the loosely arranged cortex) → passage cells of endodermis (in monocot roots) → pericycle → protoxylem in the stele → xylem
Root pressure
Absorption of water is a continuous process — water keeps arriving at the root hair, and keeps being pushed inward. Water accumulating in the living cells of the root therefore builds up a sort of hydrostatic pressure in them. This is called root pressure.
It is due to root pressure that water from the pericycle is not only forced into the xylem, but is also conducted upwards against gravity. This is the plant’s first, gentle push upward; you will study it in more detail as the root pressure theory in Section 6.9.
Two pathways across the root
The cell-to-cell account above is not the whole story, because water does not have to pass through the living contents of every cell it meets. The pathway of water across the root essentially occurs in two ways, viz., the apoplast and the symplast, as shown in Fig. 6.3.
Diagram
Fig. 6.3 – Pathways for water uptake by the root: a transverse strip from root hair through cortex, endodermis (with Casparian strip), pericycle to xylem and phloem, with the apoplastic path traced along the cell walls and the symplastic path traced through the cell interiors via plasmodesmata
Flowchart
FC 6.5 – Pathways of water across the root: Pathways → branch into Apoplast and Symplast; Symplast → further branches into Vacuolar and Transmembrane
Apoplast pathway
When some amount of water passes across the root through the cell wall and the intercellular spaces of the cortical cells of the root, it is called the apoplast pathway. This pathway occurs up to the endodermis, and no further.
The apoplast is the “outside lane” of the root — fast, unobstructed, and entirely non-living. The apoplastic (non-living) pathway provides a route towards the vascular stele through the free spaces and the cell walls of the epidermis and the cortex. Because it never crosses a plasma membrane, water in the apoplast meets almost no resistance and moves quickly.
But this fast lane comes to an abrupt end at the endodermis. The endodermis is characterised by the Casparian strip, a suberised layer in its cell walls. Suberin is waxy and waterproof, so the strip acts as a seal that forces all water and solutes to move into the symplast in order to enter the vascular system. Nothing enters the stele through the apoplast; everything must first pass through a living membrane.
There is, however, one leak in the seal. An additional apoplastic route that allows direct access to the xylem and phloem exists along the margins of the secondary roots. Secondary roots develop from the pericycle — a cell layer just inside the endodermis — and as they grow outward they must push their way through the endodermis. Since secondary roots grow through the endodermis, a direct pathway to the xylem and phloem is available at these points that bypasses the Casparian strip and allows entry into the vascular system without moving into the symplast (the living tissue).
Symplast pathway
When water passes across from one living cell to another living cell through the plasmodesmata, it is called the symplast pathway. It is also called the transmembrane pathway.
The symplast is a single continuous mass of living cytoplasm, stitched together by plasmodesmata, running from the epiblema all the way to the stele. Once water is inside the symplast it never has to cross another membrane to keep moving inward — which is precisely why the Casparian strip can afford to insist on it.
6.8 Mechanism of Absorption of Water
We have followed water from the soil pore to the xylem, and we have named the pressures and potentials that move it. One question remains: who does the work? Is the root actively pulling water in, or is the water arriving on its own?
The answer is: both, but not equally. Mainly, there are two ways or modes of absorption of water, viz., passive absorption and active absorption.
Flowchart
FC 6.6 – Modes of water absorption: Absorption of water → branches into Passive absorption (driving force: transpiration pull; no ATP; ~98% of total) and Active absorption (driving force: root pressure; ATP used); Active absorption → further branches into Osmotic absorption and Non-osmotic absorption
a. Passive absorption
This is the main way in which water is absorbed. Note the careful wording used by plant physiologists: in passive absorption, water is absorbed through the roots and not by the roots. The root is behaving like a length of pipe, not like a pump. The pulling is being done somewhere else entirely — up in the leaves.
The driving force of passive absorption is the transpiration pull, and it proceeds along the DPD gradient. Because water is simply moving in accordance with the concentration gradient — from higher water potential to lower — there is no expenditure of energy (ATP). That is precisely what makes it passive.
How much water enters this way? About 98% of the total water absorbed in plants occurs passively. In other words, almost everything a plant drinks is drawn in without the root spending a single molecule of ATP on it.
Passive absorption follows the daily rhythm of transpiration. It occurs during the day time, when transpiration is in progress, and it stops at night, when transpiration stops.
How the pull is generated
Rapid transpiration removes water from the leaf. This creates a tension in the xylem vessels, because the water potential there becomes strongly negative. Water under tension is water being stretched, not pushed.
This tension is transmitted downwards, from the xylem of the leaf, through the xylem of the stem, all the way to the xylem in the roots. Consequently water is pulled upwards — passively. The root does not push it; the leaf pulls it, and the root merely supplies.
Since no ATP is utilised during passive absorption, the rate of respiration of the root is obviously not affected by it. A root can be respiring slowly and still be delivering large volumes of water, so long as the leaves above are transpiring. In plants, water is mainly absorbed passively.
b. Active absorption
Here, water is absorbed due to the activity of the roots themselves. The root cells play an active role, and it is a genuine effort on their part.
The driving force is the root pressure developed in the living cells of the root, the hydrostatic pressure you met at the end of Section 6.7. Active absorption occurs usually at night, when transpiration stops due to the closure of stomata and the transpiration pull is no longer available.
Since water absorption here occurs against the DPD gradient, it cannot happen for free. There is an expenditure of ATP (energy), generated through the respiratory activity of the root cells.
Active absorption may be of two kinds, viz., osmotic and non-osmotic.
1. Osmotic absorption
Atkins and Priestly (1922) proposed that water is absorbed from the soil into the xylem of the root according to the osmotic gradient.
There is a subtlety here that students often miss. To create the osmotic conditions in the first place — that is, to accumulate salts and sugars in the root cells so that their sap stays concentrated — there is an expenditure of energy. But once those conditions exist, the water itself flows in osmotically. So such absorption does not directly require an expenditure of energy. The ATP is spent on setting up the gradient, not on moving the water.
Because of the activity of the living cells of the root, a gradient of DPD develops from the cell of the epiblema right through to the pericycle. As the process is continuous, a hydrostatic pressure — the root pressure — is developed in the root cells. This root pressure forces water from the pericycle into the xylem, and then upwards into the stem.
2. Non-osmotic absorption
Kramer and Thimann (1959) proposed this theory. Sometimes water is absorbed from the soil against the concentration gradient — that is, the water moves into the root even though the osmotic gradient does not favour it.
Such absorption requires an expenditure of the energy released during respiration, directly. Here the ATP is not merely setting the stage; it is doing the lifting itself.
This direct dependence on respiration gives the theory its strongest evidence — three separate lines of it, all pointing the same way:
- A poor supply of oxygen retards water absorption. Without oxygen there is little respiration, and without respiration there is no ATP.
- Low temperature retards water uptake, because of the decrease in the rate of respiration at low temperatures.
- The use of metabolic inhibitors also retards the rate of respiration, and thus the water uptake, in step with it.
In each case, anything that damages respiration damages water uptake — which would be inexplicable if no energy were being spent.
6.9 Translocation of Water
Water has reached the xylem in the root. Everything up to this point has been horizontal — a journey of a few millimetres across the root. Now the water must go up, sometimes for tens of metres, against gravity, and it must keep going up for as long as the plant lives.
The transport of water, along with dissolved minerals, from the root to the other aerial parts such as the stem and the leaves, against the gravity, is called translocation of water or ascent of sap.
The route: xylem
Translocation of water occurs through the lumen — the hollow central cavity — of the conducting elements of the xylem, namely the tracheids and the vessels, in all vascular plants. These are dead, hollow, lignified cells stacked end to end, forming continuous pipes running from root to leaf.
That the xylem, and not the phloem, is the path of the ascent of sap has been proved by the ringing experiment. In this experiment a ring of bark — which carries the phloem — is stripped away from all round a stem, leaving the xylem intact. The plant continues to conduct water and stays fresh above the ring, showing that the removed tissue was not the water pipeline. The xylem is.
The theories
Several mechanisms or theories have been put forth to explain how water is lifted. These include the vital force theory, the relay pump theory, the physical force theory, the root pressure theory, and others. We shall consider three of them in detail — and, importantly, we shall look at what is wrong with each, because no single theory explains everything.
a. Root Pressure Theory (Vital Theory)
According to this theory, the activity of the living cells of the root is responsible for the translocation of water. Because it depends on living activity, it is classed as a vital theory. J. Pristley proposed this theory.
The evidence is simple and can be seen with the naked eye. If the stem of a potted plant is cut a few inches above the soil with a sharp knife, xylem sap is seen flowing out — oozing out — through the cut end. Nobody is pulling the sap from above; the leaves are gone. Something below must be pushing it. This exudation at the cut end of the stem is good proof for the existence of root pressure.
Where does the push come from? As the absorption of water by the roots is a constant and continuous process, a hydrostatic pressure is developed in the living cells of the cortex of the root. This is termed root pressure by S. Hales. Due to root pressure, water along with the dissolved minerals is not only forced into the xylem, but is also conducted upwards against gravity.
Diagram
Fig. 6.4 – Experiment to demonstrate root pressure: a potted plant cut down to a stump, with a manometer attached to the stump to measure exudation pressure, showing the initial level and the raised final level of liquid in the manometer
Root pressure seems to be largely an osmotic phenomenon, and its development is an active process — it depends on the living, respiring cells of the root, exactly as active absorption did in Section 6.8.
How strong is it? The value of root pressure is +1 to +2 bars, which is enough to pump water to a height of 10 to 20 metres. Note the plus sign: root pressure is a genuine positive push. The factors that influence root pressure are oxygen, moisture, temperature of the soil, and salt contents — the same conditions that govern the respiratory activity of the root.
Objections and limitations of root pressure theory
Although the ascent of sap does take place due to root pressure, certain objections are raised, such as:
It is not applicable to plants taller than 20 metres. A pressure of +1 to +2 bars simply cannot lift water higher than that, and many trees are far taller.
The ascent of sap can also occur even in the absence of the root system. A cut shoot standing in water still conducts water to its leaves — yet it has no roots at all, and therefore no root pressure.
The root pressure value is almost nearly zero in taller gymnosperm trees — precisely the plants that would need it most.
In actively transpiring plants, no root pressure is developed. This is the most awkward objection of all: the theory fails exactly when the plant is moving the most water.
Xylem sap under normal conditions is under tension — that is, it shows a negative hydrostatic pressure, or a high osmotic pressure. It is being pulled, not pushed. A positive pressure at the base cannot be the main explanation for sap that is under negative pressure.
To sum up, therefore, root pressure is not the sole mechanism explaining the ascent of sap in all plants of varying heights.
b. Capillarity Theory (Physical Force Theory)
According to this theory, physical forces and dead cells are responsible for the ascent of sap. No living activity is invoked at all. This theory was put forth by Bohem in 1863.
The everyday evidence is familiar. A wick dipped in an oil lamp shows capillarity, and the oil is raised upwards along the wick without anything pushing it. In the same way, water in a straw dipped in water rises to a certain height because of capillarity. And the height to which the water is raised depends on the diameter of the straw — the narrower the tube, the higher the rise.
Capillarity is a consequence of properties you already know from Section 6.1: surface tension, together with the forces of cohesion (attraction between like molecules) and adhesion (attraction between unlike molecules).
Now make the comparison the theory rests on. A xylem vessel or tracheid, with its narrow lumen, is comparable to a straw. A continuous water column exists inside it because of the combined cohesive and adhesive forces between the water molecules themselves and between the water and the xylem wall — that is, because of capillarity. It is because of capillarity that water is raised, or conducted upwards against gravity, in the xylem.
But note the last words carefully: to a few centimetres only. Capillarity is real, but it is small.
Objections and limitations of capillarity theory
A few important objections are:
A capillary tube must be continuously and completely hollow from one end to the other for capillarity to lift a column. But the tracheids in the xylem show closed end-walls, which interrupt the tube.
The lower end of a capillary tube — here, the xylem — must be in direct contact with the soil water. However, there exists a barrier of the root cortex between the xylem and the soil water, as Section 6.7 showed in detail. The xylem never touches the soil.
The narrower the capillary tube, the greater is the height to which the water column is raised. Thus, taller trees ought to have xylem vessels with a very narrow bore. However, in nature, tall trees show xylem vessels having a wider bore — the exact opposite of what the theory predicts.
Hence, to sum up, capillarity cannot be the sole mechanism to explain the ascent of sap in all the plants of varying heights.
c. Cohesion–Tension Theory (Transpiration Pull Theory)
This is presently the widely accepted theory explaining the ascent of sap in plants. It was put forth by Dixon and Jolly in 1894. The theory is based on two principles.
Cohesion and adhesion — the unbroken water column
A strong force of attraction between water molecules is called the cohesive force. A strong force of attraction between water molecules and the lignified wall of the lumen of the xylem vessel is called the adhesive force.
Due to the combined cohesive and adhesive forces, a continuous water column is developed in the xylem, running right from the root up to the tip of the topmost leaf in the plant. This is the first principle, and it is a remarkable one: the water in a tree is not a series of separate drops being passed along, but a single, unbroken thread of liquid, kilometres long in total, held together by hydrogen bonds.
Diagram
Fig. 6.9.1 – A whole plant in outline, with an unbroken column of water shown inside the xylem from root through stem to the topmost leaf, cohesive forces marked between water molecules and adhesive forces marked between water and the lignified xylem wall
Transpiration pull — the force at the top
The second principle supplies the pull. The transpiration pull developed in the leaf vessel is transmitted down to the root, and thus accounts for the ascent of sap.
Follow the chain of events in the leaf. Excess water is lost in the form of vapour, mainly through the stomata found on the leaf. This water loss increases the DPD of the mesophyll cells — they have lost water, so they have become thirsty. These mesophyll cells then withdraw water from the neighbouring cells, and ultimately from the xylem in the leaf.
In other words, due to continuous transpiration, a gradient of suction pressure (that is, of DPD) is developed right from the guard cells up to the xylem in the leaf. Every cell in the chain is thirstier than the one below it.
This creates a tension in the xylem — called the negative pull, or the transpiration pull. Consequently, the water column is pulled out of the xylem at the top. And because the column is continuous and cohesive, pulling at the top pulls the entire column. Thus water is pulled upwards passively against gravity, leading to the ascent of sap.
Flowchart
FC 6.7 – Transpiration pull: water evaporates from mesophyll cell surfaces → escapes through stomata → DPD of mesophyll cells increases → mesophyll cells withdraw water from the leaf xylem → tension (negative pressure) develops in the xylem of the leaf → tension is transmitted down the continuous, cohesive water column through stem xylem → water column is pulled up from the root xylem → root xylem draws water from pericycle and cortex → root hair absorbs fresh water from soil → ascent of sap
Objections and limitations of transpiration pull theory
Even the widely accepted theory is not free of difficulties:
For the transpiration pull to operate, the water column should be unbroken and continuous. However, due to temperature fluctuations during day and night, gas bubbles may enter the water column, breaking its continuity.
The mechanism assumes that tracheids are more efficient than vessels, since their end walls support the water column. However, vessels are more evolved than tracheids and are, in fact, more efficient.
If the plant leaves are smeared with vaseline in order to stop transpiration, even then the ascent of sap occurs — although, by the theory, there should be no pull at all.
The ascent of sap also occurs in deciduous plants that have shed all of their leaves — plants with no transpiring surface whatever.
These last two observations point to an important fact: besides the physical forces, the activity of living cells seems to be necessary for lifting the water column up. No purely physical theory is complete on its own.
6.10 Transport of Mineral Ions
Water never travels alone. The sap climbing the xylem carries dissolved substances with it, and the most important of these are the mineral ions. A plant that had water but no minerals could not build a single protein, a single chlorophyll molecule, or a single nucleic acid.
The soil serves as the main source of minerals for the plant. Minerals are the most commonly occurring solid, crystalline inorganic materials obtained from the earth’s crust — the weathered rock fragments that make up the mineral part of any soil. They play an important role in the day-to-day life of the plant.
A crucial point about how they enter: minerals are absorbed by plants in the ionic (dissolved) form, mainly through the roots, and are then transported. A crystal of a salt lying in the soil is of no use to a root; it must first dissolve, so that its ions are free in the soil solution.
Essential elements: macro and micro
The words “large” and “small” here refer only to quantity, never to importance. A plant deprived of boron, which it needs in traces, dies just as surely as one deprived of nitrogen, which it needs in bulk.
How do we know minerals come from the soil?
The evidence is old and elegant: the analysis of plant ash. Burn a plant and the carbon, hydrogen, oxygen and nitrogen escape as gases, but the mineral elements remain behind in the ash. Analysing that ash demonstrates that minerals are absorbed by plants from the soil and the surroundings, and it tells us which elements, and roughly how much of each.
There is one fact here that surprises most students. The absorption of minerals is independent of that of water. The two happen in the same root, often at the same moment, but they are not linked: a root may absorb a great deal of water and very few ions, or take up ions vigorously while absorbing little water. Water largely enters passively, down a gradient; ions are frequently taken in actively, against one.
The upward journey
Once inside the xylem, the absorbed mineral ions are pulled in the upward direction along with the xylem sap, because of transpiration pull — the same pull, described in Section 6.9, that lifts the water. The ions are simply passengers on a stream that is going up anyway. This can be verified directly: when the ascending sap is analysed, the mineral ions are found in it.
Where are they needed? Mineral ions are required in the actively growing and metabolically busy regions of the plant — the apical and lateral growing points, the young leaves, the developing flowers, fruits and seeds, and the storage organs. All of these are places where new tissue is being built or reserves are being laid down.
So, from the source — the root — the ions are pulled and transported ascendingly through the sap, and they get unloaded by the fine veins through the process of diffusion in the vicinity of the cells that need them. Once outside the vein, the cells take them up actively, spending energy to pull them in.
Flowchart
FC 6.8 – Journey of a mineral ion: mineral salt dissolves in soil water → ion enters root hair (largely by active uptake) → crosses cortex and endodermis → loaded into xylem → carried upward in the xylem sap by transpiration pull → unloaded from the fine veins by diffusion in the vicinity of the target cells → actively taken up by cells of the apex, young leaves, developing flowers, fruits, seeds and storage organs
Remobilisation: the plant’s own recycling
The soil would not be the only source for mineral uptake. Mineral ions can be remobilised within the parts of the plant — moved out of one organ and sent to another.
The most striking example is a deciduous tree in autumn. Before shedding its leaves, the plant does not simply throw them away with everything in them. Older parts, like the leaves of deciduous plants, export their ions to the younger leaves before the fall. What drops to the ground is, in a sense, a leaf that has already been emptied of its most valuable contents.
Not everything can be recovered. The most readily mobilised ions are those of phosphorus, sulphur, nitrogen and potassium — ions that remain free or loosely held in the cell. But the ions locked into structural components, like calcium, are not remobilised. Calcium has been built into the cell wall itself; it cannot be pulled back out.
Diagram
Fig. 6.10.1 – A plant outline with arrows showing mobile ions (P, S, N, K) being withdrawn from older lower leaves and sent up to the young growing leaves, while calcium is shown fixed in place in the walls of the older leaves with no arrow leaving them
What the xylem actually carries
One long-standing belief needs correcting. Analysis of the xylem exudate shows that some nitrogen travels as inorganic ions, whereas much of it is carried in the organic form, as amino acids and related compounds. Small amounts of inorganic molecules of phosphorus and sulphur are also carried.
It was a belief earlier that the xylem transports only inorganic substances and the phloem only organic ones. However, this is not correct, because some exchange of materials also occurs between the xylem and the phloem. The two tissues run side by side in every vascular bundle, and they trade with one another. It is more accurate to say that the xylem is mainly an upward stream and the phloem mainly a distributing one, rather than to divide them by chemistry.
6.11 Transport of Food
Water moves up. Food moves out. This section deals with the second stream — the traffic of sugar through the plant — and it runs in a different tissue, in a different direction, and by a different mechanism.
Source and sink
All the plant parts require a continuous supply of food (photosynthate) for their nutrition and development. Every living cell, whether in a root tip, a flower or a seed, must respire, grow and build.
But in higher plants there is a great differentiation and division of labour. Chloroplasts are confined to the green cells of the leaves, and it is there that food is synthesised. The non-green parts, such as the root and the stem, cannot make any food of their own; they must receive it from the leaves.
The part where food is synthesised is called the source, and the part where it is utilised is called the sink. Food therefore has to travel from source to sink, and this movement of food from one part of the plant to another is called the translocation of food.
Path of translocation
Food has to be translocated over long distances in higher plants — from the topmost leaf to the deepest root tip. Hence plants must have adequate channels for the transport of food. Sieve tubes and vessels are structurally ideally suited for longitudinal (vertical) translocation, being elongated cells joined end to end into continuous tubes.
Four independent lines of evidence — the ringing experiment, the structure and distribution of phloem, the chemical analysis of phloem sap, and the use of the isotope \(^{14}C\) — clearly point out that the phloem tissue is primarily responsible for the flow of food in the longitudinal, downward direction. When a plant is allowed to photosynthesise in air containing radioactive \(^{14}CO_2\), the labelled sugar that appears later can be traced, and it is found travelling in the phloem.
The horizontal or lateral translocation occurs from the phloem to the pith, or from the phloem to the cortex, via the medullary rays in the stem. The medullary rays are the radial strips of living tissue that run like spokes across a stem, and they are the plant’s side-roads.
What travels, and in which direction
Food is always translocated in the form of sucrose — the soluble form — and always along the concentration gradient, from source to sink. Sucrose is a disaccharide, \(C_{12}H_{22}O_{11}\), and it is highly soluble in water, so a great deal of it can be carried in a small volume of sap.
The transport of food occurs in two directions, vertical and lateral.
Vertical translocation
In vertical (longitudinal) transport, food is translocated in the downward direction from the leaves (source) to the stem and the root (sink). This is the everyday direction of flow in a mature, leafy plant.
But it is not the only one. Vertical translocation also occurs in the upward direction during the germination of a seed, and from bulbils, corms and similar storage organs — here the store is the source, and the growing shoot is the sink. Upward translocation also occurs from the leaves to the growing point of the stem, and to the developing flowers and fruits situated near the ends of the branches of the stem. In every case, food is going towards the sink, whichever way that happens to lie.
Lateral translocation
Lateral translocation occurs horizontally, or laterally, across the root and the stem. It has two named forms. When food is translocated from the phloem to the pith, it is called radial translocation. When it is translocated from the phloem to the cortex, it is called tangential translocation.
The phloem and its contents
Because food can travel both up and down, the transport of food through the phloem is bidirectional — and this single fact separates the phloem sharply from the xylem, in which the sap moves only upward.
The phloem sap contains mainly water and food in the form of sucrose. But sugars, amino acids and hormones are also transported through the phloem. The phloem is therefore not only a food line but also a signalling line, carrying chemical messages between the parts of the plant.
The conducting element itself is the sieve tube, a file of elongated living cells — the sieve tube elements — placed end to end. Their end walls are perforated into sieve plates, so that the cytoplasm of one element is continuous with that of the next. Alongside each sieve tube element lies a companion cell, as seen in Fig. 6.5.
Diagram
Fig. 6.5 – L. S. of a sieve tube, labelled with sieve tube element, sieve plate, companion cell and cytoplasm, with a surface view of a sieve plate showing its pores
Mechanism of sugar transport through phloem
Several mechanisms or theories have been put forth to explain how sugar actually moves through the sieve tubes — diffusion, activated diffusion, protoplasmic streaming, electro-osmosis, pressure-flow, and others. Of these, the most convincing theory is Munch’s pressure flow theory, also called the mass flow hypothesis.
Ernst Munch proposed the theory, and its logic is beautifully simple: make one end of a tube turgid and the other end flaccid, and the contents will flow from the first to the second, all by themselves.
At the source end — loading of the vein
A photosynthetic cell in the leaf synthesises glucose. Hence its osmotic concentration increases — the cell sap is now crowded with sugar.
Because of this, water is absorbed into it by endo-osmosis from the surrounding cells and from the xylem. The cell becomes turgid, and its turgor pressure rises.
Due to this increase in turgor pressure, sugar from the photosynthetic cell is forced, ultimately, into the sieve tube of the vein. This is called the loading of the vein.
At the sink end — unloading of the vein
At the sink end, the root cell utilises the sugar for its own respiration and growth, and also polymerises the excess sugar into starch — an insoluble store that is osmotically inactive and so no longer counts as dissolved sugar at all.
Consequently the osmotic concentration of the root cell is lowered. Exo-osmosis occurs: water in the root cell is lost to the surrounding cells, thereby decreasing the turgidity of the cell, and its turgor pressure is lowered.
Hence a turgor pressure gradient is developed, running from the sieve tube in the leaf — high pressure — down to the root cell — low pressure. Consequently, food is translocated along the concentration gradient, passively, pushed by that pressure difference. This is vein unloading. At the sink end the sugar is used, and the excess water exudes into the xylem — where it is free to be carried up to the leaves once again.
Flowchart
FC 6.9 – Munch’s pressure flow (mass flow) hypothesis: photosynthetic cell makes glucose → osmotic concentration of the cell rises → endo-osmosis of water from surrounding cells and xylem → cell becomes turgid, turgor pressure rises → sugar forced into the sieve tube of the vein (vein loading) → high turgor pressure at the source end → mass flow of sugary sap through the sieve tubes down the pressure gradient → at the sink, sugar is used up and excess sugar polymerised into starch → osmotic concentration of sink cell falls → exo-osmosis, water leaves the cell → turgor pressure at the sink end falls (vein unloading) → pressure gradient from source to sink is maintained → excess water exudes into the xylem and returns to the leaf
Objection to the theory
The main objection to this theory is that the mechanism does not explain the bidirectional transport of food. A single pressure gradient can drive flow in one direction only, yet phloem transport is known to be bidirectional. Moreover, according to Munch, the pressure flow is purely a physical process, with no active role for the living sieve tube — which is difficult to reconcile with the fact that the sieve tubes must remain alive for translocation to occur at all.
6.12 Transpiration
Plants absorb water constantly and continuously. It is fair to ask what becomes of it all — and the answer is startling.
Hardly 5% of the total water absorbed by the roots is actually utilised for cell expansion and plant growth. The remaining 95% becomes surplus, and is lost into the atmosphere through the aerial parts of the plant. A plant, in other words, throws away nineteen-twentieths of everything it so carefully absorbs.
That surplus leaves the plant in two different forms. Hardly 1% of the surplus water is lost in the form of liquid, while 99% of the surplus water is lost in the form of vapour.
Flowchart
FC 6.10 – Fate of the water absorbed by a plant: total water absorbed → 5% used for cell expansion and growth; 95% surplus → of the surplus, 1% lost as liquid (guttation, through hydathodes) and 99% lost as vapour (transpiration) → transpiration occurs at three sites: cuticle (cuticular, 8–10%), stomata (stomatal, 90–93%), lenticels (lenticular, 0.1–1.0%)
Guttation — losing water as liquid
The loss of water in the form of liquid is called guttation. It occurs through special structures called water stomata, or hydathodes, which are usually found at the margins and tips of leaves.
Transpiration — losing water as vapour
The loss of water in the form of vapour is called transpiration. It occurs through the leaves, the stem, the flowers and the fruits — through any aerial surface at all. But most of the transpiration occurs through the leaves, and this is given a special name: foliar transpiration, as shown in Fig. 6.6.
Diagram
Fig. 6.6 – Transpiration in a whole plant: roots taking up water from the soil, water drawn up the stem, veins carrying water into the leaves, and water evaporating from the leaves into the air
The three sites of transpiration
The actual water loss during transpiration occurs through three main sites — the cuticle, the stomata and the lenticels. Accordingly, three types of transpiration are recognised: cuticular, stomatal and lenticular.
i. Cuticular transpiration
The cuticle is a layer of a waxy substance called cutin, present on the outer surface of the epidermal cells of the leaves and the stem. It is a raincoat worn on the outside of the plant — but no raincoat is perfect, and a small amount of water escapes straight through it.
Cuticular transpiration occurs by simple diffusion, and it contributes 8–10% of the total transpiration. Since it needs no pore to open and no cell to act, it occurs throughout the day — and its rate is inversely proportional to the thickness of the cuticle. The thicker the waxy layer, the less water gets through.
ii. Lenticular transpiration
Lenticels are small raised structures composed of loosely arranged complementary cells. Each lenticel is a porous tissue consisting of cells with large intercellular spaces, present in the periderm of the secondarily thickened organs, and in the bark of the woody stems and roots of dicotyledonous flowering plants. Look at their structure in Fig. 6.7.
Diagram
Fig. 6.7 – Structure of a lenticel: a raised break in the periderm, labelled with the lenticular pore and the loose complementary cells beneath it, with the underlying cortex
Where are they found, and where not? Lenticels are present in the bark of an old stem and in the pericarp of woody fruits, but they are absent in leaves. A leaf has no periderm, and therefore no lenticels.
Lenticular transpiration contributes only about 0.1–1.0% of the total transpiration. The rate of lenticular transpiration is very slow, and, like cuticular transpiration, it also occurs throughout the day — the lenticel has no mechanism for closing itself.
iii. Stomatal transpiration
Stomata are minute apertures formed of two guard cells and accessory cells. They are located in the epidermis of the young stem and the leaves. Leaves generally show a greater number of stomata on the lower surface.
Depending upon the distribution of stomata on the leaves, leaves are categorised into three types:
- Epistomatic — stomata on the upper epidermis. Found in hydrophytes, e.g. Lotus.
- Hypostomatic — stomata on the lower epidermis. Found in xerophytes, e.g. Nerium.
- Amphistomatic — stomata on both surfaces. Found in mesophytes, e.g. Grass.
Notice how neatly the arrangement matches the habitat. This is a favourite question pattern: given a habitat, predict the stomatal distribution — or given the distribution, deduce the habitat.
Stomatal transpiration occurs only during the daytime — the stomata being closed at night — with the exception of desert plants, which open their stomata at night instead.
And it is by far the largest of the three. 90 to 93% of the total transpiration occurs through the stomata, and that too during the day time only.
6.13 Structure of Stomatal Apparatus
Since 90 to 93% of all transpiration passes through the stomata, the stoma is the one place where the plant can genuinely control its water loss. It is worth looking at it closely.
A typical stomatal apparatus consists of three parts: two guard cells, the stoma itself, and the accessory cells.
Diagram
Fig. 6.8 (a) – Structure of the guard cell in surface view: two kidney-shaped guard cells enclosing the stoma pore, labelled with inner thick wall, nucleus, chloroplasts, guard cell, stoma pore, subsidiary cell and the surrounding epidermal cells
Guard cells
Stomata are minute, elliptical pores bounded by two kidney-shaped or dumb-bell-shaped guard cells. A guard cell is a type of epidermal tissue, and may be called a modified epidermal parenchyma cell — an ordinary epidermal cell that has been rebuilt for a special job.
Their shape depends on the group of plants. Guard cells are kidney-shaped in dicotyledons and dumb-bell-shaped in grasses (monocotyledons, or monocots). In Cyperus, unusually, both kidney-shaped and dumb-bell-shaped guard cells are present in the same plant.
Diagram
Fig. 6.9 – Types of guard cells: (A) kidney-shaped guard cells in surface view with epidermal cells, subsidiary cells, chloroplasts and the stomatal aperture labelled; (B) dumb-bell-shaped guard cells of a grass, similarly labelled
Now the details that make the mechanism work. Guard cells are living, nucleated cells with unevenly thick walls. The inner wall — the wall facing the stoma — is thick and inelastic. The lateral, or outer, wall is thin and elastic. Remember this asymmetry; it is the whole secret of stomatal movement.
Guard cells also contain a few chloroplasts, which are capable of only poor photosynthesis. They are not there to feed the plant; their significance lies elsewhere, as the theories below will show.
Above all, guard cells have the ability to change their size and form, and because of this the stoma opens (widens) or closes (narrows).
The stoma
The stoma is the elliptical pore formed by the specific arrangement of the two guard cells. It is through this pore that excess water is lost in the form of vapour — and, equally importantly, through which \(CO_2\) enters and \(O_2\) leaves. Fig. 6.8 (b) shows the same stoma in its open and closed states.
Diagram
Fig. 6.8 (b) – Open and closed stoma: two drawings side by side, one with turgid, bulging guard cells and a wide pore, the other with flaccid guard cells and the pore almost closed; chloroplasts labelled in both
Accessory cells
Accessory cells are specialised epidermal cells surrounding the guard cells. Their number is variable from plant to plant, and they are also called subsidiary cells.
Their most important property, for our purposes, is chemical rather than structural: they are the reservoirs of \(K^{+}\) ions. The guard cells will borrow potassium from them to open, and return it to them to close.
Opening and closing of stoma
The opening and closing of the stoma is controlled by the turgor of the guard cells — by nothing else.
During the day time, the guard cells become turgid due to endosmosis. Turgor pressure is then exerted on the thin walls of the guard cells. Being elastic and thin, the lateral walls are stretched out. And because of the kidney-like or dumb-bell-like shape of the cells, this stretching pulls the inner thick walls apart from one another, so that the stoma opens and widens.
During the night time, the guard cells become flaccid due to exosmosis. This flaccidity closes the stoma almost completely — the guard cells relax back against each other and the pore all but disappears.
Flowchart
FC 6.11 – Opening of the stoma: osmotic potential of guard cells changes → endosmosis → guard cells become turgid → turgor pressure rises → thin, elastic outer walls stretch → curved shape pulls the thick, inelastic inner walls apart → stoma opens. Closing: exosmosis → guard cells become flaccid → turgor pressure falls → walls relax → inner walls come together → stoma closes almost completely
But this only pushes the question one step back. Endosmosis and exosmosis occur due to diurnal changes in the osmotic potential of the guard cells — that is, the guard cells become osmotically stronger by day and weaker by night. Why does their osmotic potential change with the time of day? Different theories are proposed to explain it, and we shall look at two.
The starch–sugar interconversion theory
This theory was proposed by Steward (1964).
During the day time, the enzyme phosphorylase converts starch into sugar in the guard cells. Starch is insoluble and osmotically inactive; sugar is soluble and osmotically very active. The osmotic concentration of the guard cells therefore rises sharply. Water enters by endosmosis, the guard cells are stretched, and the stoma widens.
The reverse reaction occurs at night: sugar is converted back into starch, the osmotic concentration falls, water leaves, and the stoma closes.
\[\underset{\text{(Stoma closes)}}{\text{Starch}} \;\; \xrightleftharpoons[\;\text{Night}\;]{\;\text{Phosphorylase (Day)}\;} \;\; \underset{\text{(Stoma opens)}}{\text{Sugar}}\]
The theory of proton transport
According to the theory of proton transport (Levitt, 1974), stomatal movement occurs due to the transport of protons (\(H^{+}\)) and \(K^{+}\) ions.
During the day time, starch is converted into malic acid. The malic acid then dissociates to form malate and protons:
\[\text{Malic acid} \rightarrow \text{Malate} + H^{+}\]
The protons are transported out of the guard cells into the subsidiary cells, and in exchange, \(K^{+}\) ions are imported from them — remember that the subsidiary cells are precisely the reservoirs of \(K^{+}\). Inside the guard cell, the imported potassium joins the malate left behind, and potassium malate is formed.
Potassium malate is a soluble salt, so it increases the osmolarity of the guard cell sap, and this causes endosmosis. Water floods in, the guard cells swell, and the stoma opens. The uptake of \(K^{+}\) ions is always accompanied by \(Cl^{-}\) ions, which enter alongside to balance the charge.
At night, the uptake of \(K^{+}\) and \(Cl^{-}\) ions is prevented by abscisic acid, which changes the permeability of the guard cells. Deprived of the ions that made them osmotically strong, the guard cells become hypotonic with respect to their surroundings, water leaves them, and they become flaccid — closing the stoma.
Flowchart
FC 6.12 – Proton transport theory (day): starch → malic acid → dissociates into malate + \(H^{+}\) → \(H^{+}\) pumped out into subsidiary cells → \(K^{+}\) imported from subsidiary cells (with \(Cl^{-}\) following) → potassium malate formed in guard cell → osmolarity rises → endosmosis → guard cells turgid → stoma opens. At night: abscisic acid alters guard cell permeability → uptake of \(K^{+}\) and \(Cl^{-}\) prevented → guard cells become hypotonic → exosmosis → guard cells flaccid → stoma closes
Significance of transpiration
We have called transpiration a loss. But a process that removes 95% of everything the plant absorbs, and has been retained through hundreds of millions of years of evolution, is unlikely to be a simple accident. What does the plant get in return?
Advantages
It removes the excess of water that the plant has absorbed but does not need.
It helps in the passive absorption of water and minerals from the soil — recall from Section 6.8 that transpiration pull is the driving force behind roughly 98% of all water absorption.
It helps in the ascent of sap, as the cohesion–tension theory of Section 6.9 explained.
Since the stomata are open, the gaseous exchange required for photosynthesis and respiration is facilitated — \(CO_2\) can enter and \(O_2\) can leave.
It maintains the turgor (turgidity) of the cells, by keeping water moving steadily through the plant.
Transpiration helps in reducing the temperature of the leaf, imparting a cooling effect. Evaporating water carries away a great deal of heat — this is water’s high heat of vaporization, met in Section 6.1, put to work on a hot afternoon.
Disadvantages
Excessive transpiration leads to wilting and injury in the plant. If the leaves lose water faster than the roots can replace it, the cells go flaccid, the plant droops, and — if the imbalance continues — it may even lead to the death of the plant.
Transpiration is ‘a necessary evil’
Here, then, is the plant’s dilemma, and it cannot be escaped.
For stomatal transpiration to occur, the stoma must remain open during the day time. But the stoma is not a water valve only — it is also the only doorway for gases. When the stomata are open, then and only then will the gaseous exchange needed for respiration and photosynthesis take place.
If stomatal transpiration were to stop — that is, if the plant kept its stomata firmly shut — it would directly affect the productivity of the plant, through the loss of photosynthetic and respiratory activity. No open stoma means no \(CO_2\); no \(CO_2\) means no food.
Hence, for productivity, the stomata must remain open. And consequently, transpiration cannot be avoided. The plant loses water not because it is careless, but because the same door it must open to eat is the door through which water escapes.
For this reason, Curtis (1926) regarded transpiration as ‘a necessary evil’.
6.14 Summary
Plants take in water, minerals, nutrients and gases such as \(O_2\) and \(CO_2\) from their surroundings. Productivity is affected mainly by the non-availability of water. Water, the ‘elixir of life’, makes up 90 to 95% of most plant cells and tissues and keeps them turgid.
Water is liquid at room temperature, is an inert inorganic compound of neutral pH when pure, is the best solvent for most solutes, is the medium for all biochemical reactions, and is a raw material of photosynthesis. Its high specific heat, high heat of vaporization and high heat of fusion make it a thermal buffer. All these properties, along with cohesion, adhesion and high surface tension, arise from hydrogen bonding between water molecules.
The root is the main organ of water absorption. Terrestrial plants absorb liquid water from soil; epiphytes such as orchids absorb water vapour through epiphytic roots bearing velamen. A typical root has four regions, and in the zone of absorption the epidermal (epiblema) cells form unicellular root hairs.
A root hair is a cytoplasmic extension of an epiblema cell, 1 to 10 mm long, colourless, unbranched, delicate and ephemeral. Its two-layered wall (outer pectin, inner cellulose) is freely permeable; its plasma membrane is selectively permeable.
Soil water occurs in four forms — gravitational, hygroscopic, combined and capillary. Only capillary water, held in the pores between soil particles, is available for absorption. Plants absorb it from the rhizosphere.
Root hairs absorb water by three physical processes occurring in sequence: imbibition → diffusion → osmosis.
Flowchart
FC 6.13 – The whole chapter in one line: capillary water in soil → imbibition into the pectin–cellulose wall → diffusion through the freely permeable wall → osmosis across the selectively permeable plasma membrane → cortex (apoplast and symplast pathways) → endodermis with Casparian strip → pericycle → xylem → ascent of sap by cohesion–tension (transpiration pull) → mesophyll of leaf → water vapour lost through stomata (transpiration)
Imbibition is the swelling of hydrophilic colloids by adsorption of water; the solid is the imbibant and the liquid the imbibate. Seen in soaking of seeds, swelling of raisins and kneading of flour.
Diffusion is the movement of ions, atoms or molecules from a region of higher concentration to one of lower concentration, due to their kinetic energy, continuing till equilibrium. Diffusion pressure is proportional to the number of diffusing particles, and is always greater in pure solvent than in a solution. The difference between them is the Diffusion Pressure Deficit (DPD) or Suction Pressure, coined by B. S. Meyer (1938) — colloquially, the “thirst” of a cell.
Facilitated diffusion is the passive, carrier-mediated absorption of solutes through membrane proteins such as aquaporins and ion channels. It needs no ATP, but requires a concentration gradient.
Osmosis is the diffusion of solvent from a weaker to a stronger solution through a semipermeable membrane. Solutions are hypotonic, hypertonic or isotonic with respect to the cell. Endosmosis makes a cell turgid; exosmosis makes it flaccid.
Turgidity raises the turgor pressure (T.P.), against which the rigid wall exerts an equal and opposite wall pressure (W.P.). Osmotic pressure (O.P.) is the pressure that must be applied to a solution to prevent solvent from entering it. These are related as: \[D.P.D. = O.P. - T.P. \quad ; \quad \therefore \; D.P.D. = O.P. - W.P.\] In a flaccid cell T.P. is zero, so \(DPD = OP\). In a fully turgid cell DPD is zero, so \(TP = OP\).
Water potential (ψ) is the chemical potential (free energy per molecule) of water. It is equal but opposite in sign to DPD, and is measured in bars, pascals or atmospheres. Pure water has ψ = 0; adding solute makes ψ negative. DPD is now called water potential, O.P. is called osmotic potential, and T.P. is called pressure potential (always positive). Water always flows from a less negative to a more negative water potential, and the difference between adjacent cells decides movement through plasmodesmata.
Water absorption is favoured by capillary water, a soil temperature of 20 °C to 30 °C, low solute concentration in soil water, good soil aeration, and increased transpiration in irrigated soil.
Plasmolysis is exo-osmosis in a living cell placed in a hypertonic solution: the protoplast shrinks and recedes from the wall, the gap filling with the outer solution, and T.P. becomes zero. In a hypotonic solution the cell recovers by endo-osmosis — deplasmolysis.
Across the root, a DPD gradient runs from epiblema to pericycle, so water passes from root hair → cortical cells → passage cells of endodermis (in monocots) → pericycle → protoxylem. Continuous absorption builds a hydrostatic root pressure.
Water crosses the root by two pathways: the apoplast (non-living — cell walls and intercellular spaces, only up to the endodermis, where the suberised Casparian strip blocks it) and the symplast (living — cytoplasm to cytoplasm through plasmodesmata; also called the transmembrane pathway). Symplast is further divided into vacuolar and transmembrane routes. Secondary roots growing out through the endodermis provide an apoplastic bypass to the stele.
Absorption is of two modes. Passive absorption is driven by transpiration pull, follows the DPD gradient, uses no ATP, occurs by day, and accounts for about 98% of the water absorbed. Active absorption is driven by root pressure, occurs against the DPD gradient at night, and needs ATP from respiration. It is osmotic (Atkins and Priestly, 1922 — energy sets up the gradient but is not used directly) or non-osmotic (Kramer and Thimann, 1959 — respiratory energy used directly; hence poor oxygen supply, low temperature and metabolic inhibitors all reduce uptake).
Ascent of sap is the upward transport of water with minerals against gravity, through the lumen of tracheids and vessels. The ringing experiment proves the xylem is the path. Three theories:
- Root pressure theory (vital theory) — J. Pristley; root pressure termed by S. Hales; value +1 to +2 bars, enough for 10 to 20 metres. Fails for taller plants, for plants without roots, for tall gymnosperms, for actively transpiring plants, and because xylem sap is normally under tension.
- Capillarity theory (physical force theory) — Bohem (1863); based on surface tension, cohesion and adhesion; raises water a few centimetres only. Fails because tracheids have closed end-walls, xylem is not in direct contact with soil water, and tall trees have wider (not narrower) vessels.
- Cohesion–tension theory (transpiration pull theory) — Dixon and Jolly (1894); the presently accepted theory. Cohesion between water molecules and adhesion to the lignified xylem wall maintain a continuous water column from root to topmost leaf; transpiration raises the DPD of mesophyll cells, creating a tension that is transmitted down and pulls the column up. Objections: bubbles may break the column; vessels are in fact more efficient than tracheids; ascent occurs even when leaves are vaselined or shed — so the activity of living cells also seems necessary.
Soil is the main source of minerals, absorbed in the ionic (dissolved) form, independently of water absorption, as shown by plant ash analysis. Essential elements number about 36 to 40; macro elements (C, H, O, P, N, S, Mg) are needed in large quantity and micro elements (Cu, Co, Mn, B, Zn) in small quantity. Ions are carried up in the xylem sap by transpiration pull, unloaded from fine veins by diffusion, and taken up actively by cells. Ions such as P, S, N and K are readily remobilised — older leaves export them to younger leaves before falling — but structural ions like calcium are not. Xylem also carries nitrogen in organic forms, and materials are exchanged between xylem and phloem.
Translocation of food runs from source (where food is made) to sink (where it is used). The phloem is the path — proved by ringing, phloem structure and distribution, analysis of phloem sap, and the use of \(^{14}C\). Food travels as sucrose, along the concentration gradient. Transport is vertical (downward to stem and root; upward during germination of seeds, bulbils and corms, and to growing points, flowers and fruits) and lateral (radial, phloem to pith; tangential, phloem to cortex, via medullary rays). Phloem transport is bidirectional, and the sap also carries amino acids and hormones.
Munch’s pressure flow (mass flow) hypothesis is the most convincing mechanism: sugar made in the photosynthetic cell raises its osmotic concentration → endo-osmosis → turgidity → sugar forced into the sieve tube (vein loading); at the sink, sugar is used or polymerised to starch → osmotic concentration falls → exo-osmosis → turgor falls (vein unloading). The resulting turgor pressure gradient drives food passively from source to sink. Its main objection is that it does not explain bidirectional transport.
Of the water absorbed, hardly 5% is used in growth; 95% is surplus. Of that surplus, 1% is lost as liquid — guttation, through hydathodes (water stomata) — and 99% as vapour — transpiration.
Transpiration occurs at three sites: the cuticle (cuticular, 8–10%, all day, rate inversely proportional to cuticle thickness), the lenticels (lenticular, 0.1–1.0%, very slow, all day, absent in leaves) and the stomata (stomatal, 90–93%, day time only, except in desert plants).
Leaves are epistomatic (upper surface; hydrophytes, e.g. Lotus), hypostomatic (lower surface; xerophytes, e.g. Nerium) or amphistomatic (both surfaces; mesophytes, e.g. Grass). Stomatal frequency is the number of stomata per unit area; stomatal index (I) relates stomatal number to epidermal cell number.
The stomatal apparatus consists of two guard cells, the stoma and the accessory (subsidiary) cells. Guard cells are living, nucleated, modified epidermal parenchyma cells with a thick inelastic inner wall and a thin elastic outer wall, and a few chloroplasts. They are kidney-shaped in dicots and dumb-bell-shaped in grasses; both types occur in Cyperus. Accessory cells are reservoirs of \(K^{+}\) ions.
Stomatal movement is controlled by the turgor of guard cells: endosmosis by day makes them turgid and opens the pore; exosmosis by night makes them flaccid and closes it. Two theories explain the diurnal change in osmotic potential:
- Starch–sugar interconversion theory (Steward, 1964): by day, phosphorylase converts starch to sugar, raising osmotic concentration, so water enters and the stoma opens; the reverse occurs at night. \[\underset{\text{(Stoma closes)}}{\text{Starch}} \;\; \xrightleftharpoons[\;\text{Night}\;]{\;\text{Phosphorylase (Day)}\;} \;\; \underset{\text{(Stoma opens)}}{\text{Sugar}}\]
- Proton transport theory (Levitt, 1974): by day, starch → malic acid → malate + \(H^{+}\); protons pass to the subsidiary cells and \(K^{+}\) (with \(Cl^{-}\)) is imported; potassium malate raises osmolarity, causing endosmosis and opening. At night, abscisic acid prevents \(K^{+}\) and \(Cl^{-}\) uptake, the guard cells become hypotonic and flaccid, and the stoma closes.
Significance of transpiration. Advantages: removes excess water; helps passive absorption of water and minerals; helps the ascent of sap; permits gaseous exchange for photosynthesis and respiration through the open stomata; maintains cell turgidity; cools the leaf. Disadvantage: excessive transpiration causes wilting, injury and even death of the plant.
Because the stomata must stay open by day for the gaseous exchange that photosynthesis and respiration require, transpiration cannot be avoided without losing productivity. Hence Curtis (1926) called transpiration ‘a necessary evil’.
6.15 Points to Ponder
Water makes up 90 to 95% of a plant’s cells, yet a plant keeps hardly 5% of what it absorbs. Is it fair to call the rest “wasted”? What would change in a plant that had found a way to keep it all?
Nearly every remarkable property of water — its high specific heat, its surface tension, its ability to be pulled up a hundred metres without snapping — traces back to a single weak attraction between molecules. What does this tell you about how large biological effects can arise from small physical causes?
The cell wall lets everything through; the plasma membrane chooses. What would happen to a root hair if this were reversed?
A soil can be visibly damp and still leave a plant dying of thirst. What does this suggest about the difference between what is present in an environment and what is available in it? Can you think of the same distinction elsewhere in biology?
Root hairs are delicate, unbranched and live only a few days. Why might a short-lived, disposable structure be a better design here than a strong, permanent one?
Root pressure can lift water only 10 to 20 metres, and capillarity only a few centimetres — yet trees far taller than this existed long before either theory was proposed. What does the history of these three theories tell you about how science replaces an explanation that is partly right?
The cohesion–tension theory is the accepted explanation, and yet sap still ascends in a plant whose leaves have been sealed with vaseline or shed entirely. Should a theory be abandoned when it cannot explain every observation, or held while the exceptions are investigated?
The Casparian strip forces everything entering the stele to pass through a living membrane. What is the plant gaining by insisting on this, and what is it paying for it?
Water absorption is passive and needs no ATP, while mineral absorption is often active and expensive. Why should a plant be willing to pay for one and not the other?
A potato tuber is a sink for most of the year and a source when it sprouts. What other structures in a plant do you think change roles like this, and what decides which role an organ plays at a given moment?
Deficiency of nitrogen shows first in the old leaves, deficiency of calcium first in the young ones. What does this pattern reveal about the plant’s priorities when a resource runs short?
Transpiration is called ‘a necessary evil’. If you could engineer a plant that lost far less water, which of the six advantages of transpiration would you have to replace, and how would you replace them? Would such a plant still be a plant as we know it?
6.16 Objective Questions
- In soil, the water available for absorption by the root is ……………….
- gravitational water
- capillary water
- hygroscopic water
- combined water
- The most widely accepted theory for the ascent of sap is ……………
- capillarity theory
- root pressure theory
- diffusion
- transpiration pull theory
- Water movement between the cells is due to ………….
- T. P.
- W. P.
- D. P. D.
- incipient plasmolysis
- In guard cells, when sugar is converted into starch, the stomatal pore ………….
- closes almost completely
- opens partially
- opens fully
- remains unchanged
- Surface tension is due to …………..
- diffusion
- osmosis
- gravitational force
- cohesion
- Which of the following types of solution has a lower level of solutes than the solution?
- Isotonic
- Hypotonic
- Hypertonic
- Anisotonic
- During the rainy season, wooden doors warp and become difficult to open or to close because of ……………
- plasmolysis
- imbibition
- osmosis
- diffusion
- Water absorption takes place through ………….
- lateral roots
- root cap
- root hair
- primary root
- Due to low atmospheric pressure, the rate of transpiration will ………….
- increase
- decrease rapidly
- decrease slowly
- remain unaffected
- Osmosis is a property of ………………
- solute
- solvent
- solution
- membrane
6.17 Check Your Concepts
A field is irrigated heavily and the soil is visibly moist, yet the crop wilts within two days. Give two possible reasons, based on what you have learnt about the forms of soil water and about soil aeration.
Assertion (A): Water present in the soil as hygroscopic water is not available to the plant. Reason (R): Hygroscopic water is chemically bound inside the hydrated oxides of silicon and aluminium. Choose whether both A and R are true and R explains A; both are true but R does not explain A; A is true and R is false; or A is false and R is true. Justify your choice.
Why is the cell wall of the root hair freely permeable while its plasma membrane is selectively permeable? What would happen to absorption if the plasma membrane also became freely permeable?
Arrange in the correct sequence the events by which water enters a root hair cell from the soil: osmosis, imbibition, diffusion. Explain in one line what each step achieves that the previous one could not.
Distinguish between an imbibant and an imbibate, giving one example of each from everyday life.
A fully turgid cell has a very concentrated cell sap and therefore a high osmotic pressure, yet it absorbs no more water. Explain why, using the relation between DPD, O. P. and T. P.
What happens to the DPD, T. P. and O. P. of a root hair cell (a) as it absorbs water from the soil, and (b) as the adjacent cortical cell draws water away from it?
Why is the water potential of pure water taken as zero, and why can the water potential of a cell never be positive? What is the sign of pressure potential, and why?
Assertion (A): Water flows from a cell with a water potential of \(-5\) bars to a cell with a water potential of \(-12\) bars. Reason (R): Water always flows from a less negative to a more negative water potential. State whether A and R are true, and whether R correctly explains A.
Why does adding a heavy dose of soluble fertilizer to a field sometimes cause the crop to wilt, even though the field has just been watered?
Why is plasmolysis used as a test of whether a plant cell is living? What result would you expect with a dead cell, and why?
A plasmolysed cell is transferred to distilled water. Describe, step by step, what happens to its protoplast, its turgor pressure and its DPD.
Explain how a gradient of suction pressure develops from the epiblema to the pericycle, and why this gradient is essential for the continuous absorption of water by the root hair.
What is the Casparian strip made of, and what does it force water and dissolved ions to do? Suggest one advantage the plant gains from this arrangement.
Draw a simple diagram of a transverse section of a root from the root hair to the xylem, and label: epiblema, cortex, endodermis, Casparian strip, pericycle, xylem. On the same diagram, mark the apoplast and symplast pathways with arrows of different styles.
“In passive absorption, water is absorbed through the roots and not by the roots.” Explain what this statement means, and name the force actually responsible.
Both osmotic and non-osmotic active absorption ultimately depend on respiration, yet only one of them uses energy directly. Explain the difference. Also state why a poorly aerated soil reduces water uptake.
Root pressure can pump water to a height of 10 to 20 metres. Give three separate observations that show root pressure cannot be the sole mechanism for the ascent of sap.
Capillarity predicts that the narrower the tube, the higher the water rises. What do we actually observe in the xylem vessels of tall trees, and why is this a problem for the capillarity theory?
Arrange the following events of the transpiration pull in their correct sequence: tension is transmitted down the xylem; water vapour escapes through the stomata; the water column is pulled up from the root; the DPD of the mesophyll cells increases; the mesophyll cells withdraw water from the leaf xylem.
Why must cohesion and adhesion both be present for the transpiration pull theory to work? What happens to the pull if a gas bubble enters the xylem?
Match the following:
| Column A | Column B |
|---|---|
| a. J. Pristley | i. Diffusion Pressure Deficit (1938) |
| b. Bohem | ii. Root pressure theory |
| c. Dixon and Jolly | iii. Capillarity theory (1863) |
| d. B. S. Meyer | iv. Starch–sugar interconversion theory (1964) |
| e. Steward | v. Cohesion–tension theory (1894) |
| f. Levitt | vi. Theory of proton transport (1974) |
| g. Ernst Munch | vii. Pressure flow hypothesis |
| h. Curtis | viii. Transpiration as ‘a necessary evil’ |
Why does a deficiency of nitrogen show first in the older leaves of a plant, while a deficiency of calcium shows first in the young leaves and growing tips?
Assertion (A): The absorption of minerals by a root is independent of its absorption of water. Reason (R): Minerals are absorbed in the ionic form and are often taken in actively, while most water enters passively along a gradient. Evaluate A and R.
Why is food translocated in the phloem as sucrose rather than as glucose? Give two reasons.
A potato tuber acts as a sink during the growing season but as a source when it sprouts. Explain, and give one more example of an organ that changes its role in this way.
In Munch’s pressure flow hypothesis, why is it essential that the sink cell converts the arriving sugar into starch? What would happen to the flow if it did not?
State the main objection to Munch’s pressure flow hypothesis, and explain why the bidirectional nature of phloem transport creates a difficulty for it.
A complete ring of bark is removed from around the trunk of a young tree. The leaves stay green for the whole season, but the tree eventually dies. Explain both observations.
Distinguish between guttation and transpiration with respect to the form in which water is lost, the structure involved, the time of occurrence and the driving force.
What happens if a plant’s stomata are kept permanently closed? Discuss the consequences for water balance, gaseous exchange and productivity, and relate your answer to the phrase ‘a necessary evil’.
A lotus leaf floats on water and a Nerium leaf grows in a dry, exposed habitat. Predict the stomatal distribution of each, name the category it belongs to, and justify the arrangement in terms of the plant’s habitat.
Why is the rate of cuticular transpiration inversely proportional to the thickness of the cuticle? Which category of plants would you expect to have the thickest cuticle, and why?
Lenticular transpiration accounts for only about 0.1–1.0% of the total, yet it goes on all day and all night. Why can the plant not control it, and why is this acceptable to the plant?
Draw a labelled diagram of the stomatal apparatus of a dicot leaf in surface view. Label: guard cells, stoma pore, inner thick wall, chloroplast, nucleus, subsidiary (accessory) cells and epidermal cells.
Explain why the uneven thickening of the guard cell wall is essential for the opening of the stoma. What would happen if both walls of the guard cell were equally thick and elastic?
Compare the starch–sugar interconversion theory and the theory of proton transport with respect to: the substance formed by day, the ions involved, the role of the subsidiary cells, and the cause of closure at night.
Arrange in sequence the events of stomatal opening according to the proton transport theory: potassium malate is formed; starch is converted into malic acid; the guard cells become turgid and the stoma opens; \(H^{+}\) ions are transported to the subsidiary cells; endosmosis occurs; \(K^{+}\) ions are imported along with \(Cl^{-}\); malic acid dissociates into malate and protons.
What is the role of abscisic acid in the closing of stomata at night, and why does this cause the guard cells to become hypotonic?
List the six advantages of transpiration, and then explain why, despite all of them, transpiration is still described as an “evil”.
What is osmotic pressure? (30–40 words)
Name the condition in which the protoplast of the plant cell shrinks. (30–40 words)
What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution? (30–40 words)
Which type of solution will bring about deplasmolysis? (30–40 words)
Which type of plants have negative root pressure? (30–40 words)
In which conditions will the transpiration pull be affected? (30–40 words)
Mention the shape of guard cells in Cyperus. (30–40 words)
Why do diurnal changes occur in the osmotic potential of guard cells? (30–40 words)
What is symplast pathway? (30–40 words)
Describe the mechanism of absorption of water.
Discuss the theories of water translocation.
What is transpiration? Describe the mechanism of opening and closing of stomata.
What is transpiration? Explain the role of transpiration.
What is the significance of transpiration? Explain the root pressure theory and its limitations.
Explain the capillarity theory of water translocation.
Why is transpiration called ‘a necessary evil’?
Explain the movement of water in the root.
Define and/or explain the terms: osmosis, diffusion, plasmolysis, imbibition, guttation, transpiration, ascent of sap, active absorption, DPD, turgor pressure, water potential, wall pressure, root pressure.
Distinguish between: (a) osmotic pressure and turgor pressure; (b) diffusion and osmosis.
Describe the structure of root hair. (150–200 words)
Write on the journey of water from soil to xylem in roots. (150–200 words)
Explain the cohesion theory for the translocation of water. (150–200 words)
Write on the mechanism of opening and closing of stoma. (150–200 words)
Explain the active absorption of minerals. (150–200 words)