In the mid-1960s, India was frighteningly close to running out of food. Two monsoons had failed one after another, grain stores were emptying, and the country was surviving largely on wheat shipped in from abroad. Newspapers of the time carried a phrase that no nation wants to hear about itself: living from ship to mouth.
At the same time, in Mexico, an American biologist named Norman E. Borlaug had been working for years on an unusual idea. Tall wheat plants, he had noticed, grew heavy heads of grain and then simply fell over — the stem could not carry the weight. So he bred wheat that was short and stiff-stemmed. These semi-dwarf plants could hold a much larger head of grain upright, and they responded well to fertilizer and irrigation instead of collapsing.
Indian scientists, led by M. S. Swaminathan, brought this wheat to Indian fields, tested it, and adapted it to Indian conditions. Within a few seasons, wheat harvests in Punjab, Haryana and western Uttar Pradesh rose sharply. India stopped standing in line for food aid. The change was so dramatic that it earned a name of its own — the Green Revolution — and in 1970 Borlaug received the Nobel Peace Prize.
The lesson of that decade is the heart of this chapter: food production is not only about farming harder. It is about understanding the biology of the organisms we depend on — crops, livestock, and even microbes — and improving them with science.
Keep these questions in mind as you read. This chapter will answer every one of them.
11.2 Plant Breeding
What Plant Breeding Means
Wild plants were not designed for us. They were shaped by natural selection to survive and reproduce, not to fill a granary. Plant breeding is the deliberate human effort to change that — the improvement, or purposeful manipulation, of the heredity of crops so that new superior varieties of existing crop plants are produced.
Because it takes scientific principles and puts them to practical use in the field, plant breeding is an applied branch of botany. It is, in fact, both an art and a science: the art of choosing which plants look promising, and the science of changing and improving the heredity of plants in a predictable way.
(Plant breeding is the improvement or purposeful manipulation in the heredity of crops, leading to the production of new superior varieties of existing crop plants.)
Plant breeding draws on many disciplines at once. It uses the principles of genetics (how characters are inherited), taxonomy (which wild relatives are close enough to cross with), physiology (how the plant grows and responds), pathology (which pathogens attack it), agriculture (how it is actually cultivated) and, in modern times, rDNA technology.
Put in genetic language, plant breeding is a method of altering the genetic pattern of plants to increase their value and utility for human welfare. Everything the breeder does — every cross, every selection — is ultimately a way of deciding which alleles will be present in the next generation.
Why We Breed: The Objectives
Plant breeding is not carried out for one purpose alone. Broadly, breeding programmes aim to:
- increase crop yield, so that more is harvested from the same area of land;
- improve the quality of the produce, such as grain protein, oil content, cooking or milling quality;
- increase tolerance to environmental stresses such as drought, salinity, heat or cold;
- make plants resistant to pathogens, which cause plant diseases;
- increase tolerance to insect pests, which damage crops in the field and in storage.
The primary aim of plant breeding is to obtain a new crop variety that is superior to the existing type in all characters. This purposeful manipulation carries various objectives, and these differ from plant to plant depending on its type and its use. Some objectives are common to almost every crop — yield, quality, resistance and life-cycle span — while others depend entirely on how the plant is used. A breeder working on cotton is not chasing the same target as a breeder working on tomato.
The Green Revolution
None of these objectives were achieved by breeding alone. The Green Revolution was the result of a whole sequence of scientific breakthroughs and developmental activities that together fought hunger by increasing food production.
Several things had to arrive at once. Seeds of superior quality were combined with the use of chemicals — pesticides and fertilizers — and with a multiple cropping system, supported by modern farm machinery and a proper irrigation system. Together, these helped in developing high-yielding and disease-resistant varieties of wheat, rice, maize and other crops.
Notice why fertilizer and improved seed had to come together. A high-yielding variety only shows its potential when it is well fed and well watered; the same seed sown in a starved, unirrigated field gives an ordinary harvest. Equally, pouring fertilizer onto a traditional tall variety made it grow leaner and taller and simply fall over. Neither input worked without the other — which is exactly why the Green Revolution is described as a package, not a single invention.
Breeding itself, however, is far older than any revolution. Plant breeding dates back to about 10,000 years ago, when early farmers began saving seed from the best plants. The crops we grow today are the result of that long process of domestication (bringing wild species under cultivation) and acclimatization (adapting them to new climates and conditions).
The different methods of plant breeding include Introduction, Selection, Hybridization, Mutation breeding, Polyploidy breeding, Molecular plant breeding, Tissue culture, rDNA technology, SCP, etc. Of these, hybridization has historically been the workhorse of crop improvement, and it is the method examined in detail below.
Know the Scientist Dr. Norman E. Borlaug: An American biologist, called the “Father of the Green Revolution”, “Agriculture’s greatest spokesperson” and “The Man Who Saved a Billion Lives”. A Nobel Laureate of 1970, he was honoured for his work in the Green Revolution, which saved millions of lives from famine in India, Mexico and the Middle East. Dr. M. S. Swaminathan: Called the “Father of the Green Revolution in India” for his role in introducing and further developing high-yielding varieties of wheat in India. He advocated moving India towards sustainable development — especially environmentally sustainable agriculture, sustainable food security and the preservation of biodiversity. He is a pioneer of mutation breeding in India, and developed new wheat varieties such as Sonora, NP 165 and Sharbati.
Hybridization and Its Technique
Hybridization is the chief method of crop improvement, and it offers greater possibilities than any of the other methods. It is the only effective means of combining together the desirable characters of two or more varieties into a single plant.
Think about what that means genetically. One variety may carry the allele for high yield, another the allele for rust resistance. Neither plant alone is good enough. By crossing them, the breeder can create new genetic combinations of characters that already exist, and even generate new genetic variations. Hybridization also exploits and utilizes hybrid vigour — the tendency of a hybrid to outperform both its parents in growth, size or yield.
Find out more about hybrid vigour. Why should a cross between two different parents produce offspring stronger than either one?
Types of Crosses
Hybridization is classified by how closely related the two parents are.
Types of hybridization | Type of cross | Parents belong to | Example of relationship | |—|—|—| | Intravarietal | Plants of the same variety | Closest possible cross | | Intervarietal | Two varieties of the same species | Two wheat varieties crossed | | Interspecific | Two species of the same genus | Two Saccharum species crossed | | Intergeneric | Two genera of the same family | Two different genera of grasses |
When the parental plants are distantly related — as in interspecific and intergeneric crosses — such crosses are also called wide or distant crosses. Interspecific and intergeneric hybrids are seldom found to occur in nature, because barriers to fertilization normally keep species apart. This is why the breeder must bring them together artificially.
Interspecific vs Intergeneric cross (Interspecific = two species of the same genus. Intergeneric = two genera of the same family. Move one rank up the taxonomic hierarchy and the name of the cross changes.)
The Steps of a Plant Breeding Programme
The hybridization programme is not a single act of crossing. It is a long, ordered sequence, and every step exists for a reason. The complete sequence is summarised in FC 11.2.
Step 1 – Collection of Variability
A breeder can only combine characters that exist somewhere. So the first step is to collect and preserve wild species and relatives of the cultivated species that carry desired traits.
The entire collection having all the diverse alleles (that is, all the variations) for all the genes in a given crop is called germplasm collection. It is, in effect, the breeder’s raw material store. Variations are useful in the selection that follows — without variation, there is nothing to select.
(Germplasm collection is the entire collection of all the diverse alleles — that is, all the variations — for all the genes in a given crop.)
Germplasm conservation can be done in two ways, and the difference is simply where the organism is kept.
- In situ conservation: the germplasm is conserved in its own natural surroundings, with the help of forests and natural reserves.
- Ex situ conservation: the germplasm is conserved away from its natural home, through botanical gardens, seed banks, and similar facilities.
In situ vs Ex situ conservation (In situ = “in the original place” — the plant stays where it naturally grows. Ex situ = “out of the place” — the plant, or its seed, is stored elsewhere under human care.)
Step 2 – Evaluation and Selection of Parents
This is an important and essential step. The collected germplasm is evaluated — that is, screened — to identify plants with desirable characters.
The two plants chosen as parents must be healthy and vigorous, and should show desirable but complementary features. Complementary is the key word: there is no point crossing two plants that are good at the same thing. The whole purpose is that what one lacks, the other supplies.
The selected parents are then selfed for three to four generations to make them pure, or homozygous. Only pure lines are selected, multiplied and used in hybridization. The reason is straightforward — if a parent is heterozygous, it will not pass on its good characters reliably, and the hybrid generation will be a genetic muddle rather than a clean combination of the two parents.
Step 3 – Hybridization
Now the actual cross is made. The variety showing the maximum number of desirable features is selected as the female (recurrent) parent, and the other one is used as the male parent (donor), which lacks the good characters found in the recurrent parent.
Pollen grains from the anthers of the male parent are collected. They are then artificially dusted over the stigmas of emasculated flowers of the female parent — emasculation being the removal of the stamens from a bisexual flower before its anthers mature, so that the flower cannot pollinate itself. The emasculated flower is bagged and tagged, so no unwanted pollen from another plant can reach the stigma. The steps are shown in Fig. 11.3.
Pollination is followed, in due course, by seed and fruit formation. The seed thus obtained represents the hybrid generation. The hybrid F₁ progeny grown from it is selected and evaluated for the desired combinations of characters.
Step 4 – Selection and Testing of Superior Recombinants
Not every F₁ plant is worth keeping. Those F₁ hybrid plants that show superiority over both the parents, and that possess high hybrid vigour, are selected.
Such hybrids are then selfed for a few generations. Selfing steadily increases homozygosity, so the plants become homozygous for the desired characters until a state of uniformity is reached and the characters will not segregate further. Only then can a farmer sow the seed and be confident that the crop will look and yield as promised, season after season.
Step 5 – Testing, Release and Commercialization of New Cultivars
The newly selected lines are evaluated for productivity and other features such as disease resistance, pest resistance and quality.
Initially these plants are grown under controlled conditions of water, fertilizers and so on, and their performance is recorded. The selected lines are then grown for at least three generations in natural fields, in different agroclimatic zones — because a variety that thrives in one climate may fail completely in another. Finally, the variety is released as a new variety for use by farmers.
The cultivation of many high-yielding hybrid varieties of rice, wheat, sugarcane and millets developed through hybridization, together with fertilizers, pesticides and proper irrigation, has helped the farming community attain record agricultural production in India since 1961. This is called the green revolution.
The five steps of a plant breeding programme (C-E-H-S-T — Collect variability → Evaluate and select parents → Hybridize → Select superior recombinants → Test, release and commercialize.)
1. Wheat and Rice
In the 1960s, wheat and rice production in India increased tremendously. Norman E. Borlaug developed semi-dwarf varieties of wheat, and Sonalika and Kalyan Sona are two of the hybrid wheat varieties grown in India.
For rice, semi-dwarf varieties were taken from IR-8 (developed at the International Rice Research Institute) and Taichung Native-1 (from Taiwan) and introduced in India. Jaya, Padma and Ratna are the better-yielding semi-dwarf rice varieties that were developed later.
Q1. How can making a wheat plant shorter give a farmer more grain, not less?
A. Grain yield does not come from the height of the stem — it comes from the ear. A tall variety spends much of its energy building a long stem, and when fertilizer is added, that stem grows longer and weaker until the plant bends over and falls (lodging), so the grain is lost. A semi-dwarf plant is short and stiff. It diverts more of its resources into grain instead of stem, and its sturdy stem can hold a much heavier ear upright without lodging, even under heavy fertilizer and irrigation. Shorter plant, heavier harvest.
2. Sugarcane
Sugarcane offers a clear example of why breeders cross two species instead of simply picking the better one.
- Saccharum barberi is a native of North India, while Saccharum officinarum belongs to South India.
- S. officinarum has a thicker stem and high sugar content, but it does not grow well in North India.
- The two varieties were crossed to obtain the desirable qualities of both — high sugar content, thicker stem, and the ability to grow in North India. CO-419, CO-421 and CO-453, which are high-yielding and have high sugar content, were developed in India at Coimbatore (Tamil Nadu).
thick, sugar-rich sugarcane stem (Thicker stem → more storage parenchyma in the internodes → more sucrose stored per cane → higher sugar recovery at the mill. This is why “thicker stem” is not a cosmetic character but a yield character.)
3. Millets
- Hybrid maize (Ganga-3), jowar (CO-12) and bajra (Niphad) have been successfully developed in India.
- These varieties are high-yielding and resistant to water stress.
Water-stress resistance matters here for an ecological reason: millets are grown largely in dry, rain-fed regions where irrigation is uncertain. A variety that collapses in a dry spell is useless to the farmer who needs it most.
Plant Breeding for Disease Resistance
Plant diseases are caused by pathogens — disease-causing organisms. Some of the diseases caused in plants are listed below.
| Fungi |
Brown rust of wheat, red rot of sugarcane, late blight of potato |
| Bacteria |
Black rot of crucifers |
| Viruses |
Tobacco mosaic virus |
The basic objective of breeding for disease resistance is to develop an inherent quality in the plant that prevents the pathogen from causing the disease. Such varieties of plants are called disease resistant plants.
Notice what “inherent” means here. Spraying a fungicide protects a crop from outside, season after season, at continuing cost. A resistance gene bred into the plant travels inside every seed, free of charge, generation after generation. The basic technique used is the same as for the normal hybridization process described above — the resistance allele is simply one more desirable character to be transferred from a donor into a superior variety.
Some disease-resistant plants developed are:
| Wheat |
Himgiri |
Leaf and stripe rust, hill bunt |
| Brassica |
Pusa Swarnim |
White rust |
| Cauliflower |
Pusa Shubra |
Black rot and curl blight black rot |
| Chilli |
Pusa Sadabahar |
Chilli mosaic virus, tobacco mosaic virus, leaf curl |
Late blight of potato is not merely a textbook entry. The fungus-like pathogen that causes it destroyed the potato crop of Ireland in the 1840s, and the resulting famine killed and displaced enormous numbers of people. The Irish fields were planted with a single, uniform potato type — so once the pathogen found a way in, nothing in the field could resist it. It is the sharpest possible argument for both disease-resistance breeding and the conservation of genetic variability.
Plant Breeding for Developing Resistance to Insect Pests
Insects, being herbivores, incur heavy loss in both the quantity and the quality of crops — they eat the plant, and they damage what is left. Resistance in crops can be developed in the following ways.
Through morphological characters — features of the plant’s form that physically discourage the insect:
- Hairy leaves in cotton and in wheat develop resistance against the vectors jassids and cereal leaf beetle respectively.
- A solid stem in wheat leads to resistance to stem borers.
Through biochemical characters — the plant’s chemistry makes it a poor meal:
- High aspartic acid content, together with low nitrogen and low sugar content, in maize leads to resistance against stem borers.
- Nectar-less cotton having smooth leaves develops resistance against bollworms.
solid stem in wheat (Stem borer larvae need a hollow space inside the stem to tunnel and feed → a solid stem leaves them nowhere to bore → the larva cannot establish itself → the crop escapes damage. The defence here is purely architectural, not chemical.)
Some pest-resistant varieties are:
| Brassica |
Pusa Gaurav |
Aphids |
| Flat bean |
Pusa Sem 2, Pusa Sem 3 |
Jassids, aphids and fruit borer |
| Okra |
Pusa Sawani, Pusa A-4 |
Shoot and fruit borer |
Mutation Breeding
Hybridization can only shuffle alleles that already exist somewhere in the germplasm. But what if the character the breeder wants exists nowhere at all? Then new variation must be created — and that is the purpose of mutation breeding.
A mutation is a sudden heritable change in the genotype, caused naturally. It can also be induced by the application of chemicals or by physical agents. The agents that cause mutations are called mutagens.
(Mutation is a sudden heritable change in the genotype. It occurs naturally, and can also be induced artificially — for example, by the application of chemicals.)
Natural (physical) mutagens are: high temperature, high concentration of \(CO_2\), X-rays and UV rays.
Chemical mutagens are: nitrous acid, EMS (Ethyl Methyl Sulphonate), mustard gas, colchicine, and others.
In practice, seedlings or seeds are irradiated by \(Co^{60}\) (cobalt-60), or exposed to UV bulbs, X-ray machines, and similar sources. Mutagens cause gene mutations (changes within a gene) and chromosomal aberrations (changes in chromosome structure or number).
Mutations are random. Most of them are useless, and many are harmful — so the breeder cannot simply order the mutation he wants. What he can do is create a very large number of mutants and then look hard for the rare useful one. Accordingly, the treated seedlings are screened for resistance to diseases and pests, for high yield, and for other desirable characters. The sequence is summarised in FC 11.3.
Varieties produced through mutation breeding include the Jagannath variety of rice, the NP 836 variety of wheat (rust resistant), the Indore-2 variety of cotton (resistant to bollworm) and the Regina-II variety of cabbage (resistant to bacterial rot).
Hybridization vs Mutation breeding (Hybridization combines alleles that already exist in two different parents. Mutation breeding creates a new allele that did not exist before, by damaging and altering the DNA with a mutagen.)
11.3 Tissue Culture
Growing a Plant Outside a Plant
Breeding, as you have just seen, works with whole plants, flowers and seeds, and takes years. Tissue culture works with something far smaller — a fragment of living tissue — and does its work in a glass vessel on a laboratory shelf.
Tissue culture is not one technique but a collection of different techniques, and it has emerged as one of the central techniques of plant biotechnology. In all of them, isolated cells, tissues or organs are grown in vitro — literally “in glass”, meaning outside the living plant — on a solid or liquid nutrient medium, under aseptic (germ-free) and controlled conditions of light, humidity and temperature, in order to achieve different objectives.
The part of the plant that is taken and used to start the culture is called the explant. It may be a piece of stem, a leaf segment, a root tip, an anther or an embryo.
(Tissue culture is the technique of growing isolated plant cells, tissues or organs in vitro on a solid or liquid nutrient medium, under aseptic and controlled conditions of light, humidity and temperature.)
where tissue culture sits (Zoom in: the working unit is a single living cell with its intact nucleus and full set of genes. Zoom out: from that one cell, an entire multicellular plant — with root, shoot, leaves and eventually flowers — can be rebuilt in a test tube.)
The Principle: Totipotency
Why should a scrap of stem be able to produce a whole plant at all? The answer is the principle on which all of plant tissue culture rests — totipotency.
Every living cell of a plant, however specialised it looks, still carries the complete set of genes of that plant. Given the right nutrients and hormones, it can switch that full genetic programme back on: it can grow, divide, redivide, and give rise to a whole plant.
(Totipotency is the inherent ability of a living plant cell to grow, divide and redivide, and to give rise to a whole plant.)
The idea did not come from a laboratory success but from a laboratory failure. In 1902, the German botanist Gottlieb Haberlandt was the first to conceive the idea and to develop the concept of in vitro cell culture (plant morphogenesis). He isolated single plant cells and tried to grow them in a nutrient solution. The cells survived but did not divide — he did not have the plant growth hormones that later workers would use. His experiment failed, but the concept he proposed was correct, and the whole field of plant tissue culture grew out of it.
The Culture Medium
A cell in a test tube gets nothing from the parent plant, so everything it needs must be supplied in the medium. The plant tissue culture medium contains all the essential minerals, sources of carbohydrates, proteins and fats, water, growth hormones, vitamins, and agar (for callus culture, where a solid medium is needed).
The most commonly preferred medium for tissue culture is MS (Murashige and Skoog) medium. Because its composition is exactly known, results obtained in one laboratory can be repeated in another.
Certain physical conditions are also maintained: a temperature of \(18\,^\circ C\) to \(20\,^\circ C\), a pH of the nutrient medium of 5 to 5.8, and aeration — particularly for suspension culture, where the cells are submerged in liquid and would otherwise be starved of oxygen.
Maintenance of Aseptic Conditions
Aseptic conditions are essential, and the reason is unforgiving. The nutrient medium is rich in sugars and minerals — it is as good a food for bacteria and fungi as it is for plant cells, and those microbes grow far faster. A single contaminating spore can overrun a culture flask in a couple of days. Aseptic conditions must therefore be maintained to avoid contamination by other harmful micro-organisms.
This is accomplished by sterilization at every point of entry:
- Glassware: cleaned with detergents and sterilized in a hot air oven.
- Nutrient medium: sterilized in an autoclave under a constant pressure of 15 lb/sq inch for a continuous 20 minutes.
- Explant: surface-sterilized by treatment with 20% ethyl alcohol and 0.1% \(HgCl_2\) (mercuric chloride).
- Inoculation chamber (laminar air flow): exposed to a UV ray tube for 1 hour before the actual inoculation of the explant onto the sterilized nutrient medium is performed.
Notice that the explant is only surface-sterilized. The chemicals must kill the microbes clinging to the outside without killing the plant tissue inside — which is why the treatment is brief and is followed by rinsing.
Sterilization vs Disinfection of the explant (The medium and glassware are completely sterilized — every microbe on them is killed. The explant is only surface-sterilized, because a treatment strong enough to sterilize it through and through would kill the living tissue you are trying to culture.)
Types of Culture
Cultures are classified in two different ways, and it is worth keeping the two criteria apart.
Bases of classifying tissue cultures | Basis of classification | Types | |—|—| | Nature of the explant | Cell culture, organ culture, embryo culture | | Type of in vitro growth | Callus culture (solid medium), suspension culture (liquid medium) |
Callus Culture
In callus culture, a solid medium is used. In the explant placed on it, the development and organisation of the tissue is lost — the cells forget, in effect, that they were once leaf or stem.
The cells of the explant then divide and redivide to form a mass of undifferentiated cells called the callus. It is maintained on the solid medium, and no shaker (agitator) is needed.
(Callus is the mass of undifferentiated cells formed when the cells of an explant divide and redivide on a solid nutrient medium, having lost their original tissue organisation.)
The callus is not the goal, only the halfway house. It can be induced to form organs — roots by rhizogenesis and shoots by caulogenesis — and thus to give rise to a plantlet. The whole route from mother plant to a field-ready plant is shown in Fig. 11.2.
Suspension Culture
In suspension culture, small groups of cells, or even a single cell, are used as the explant in a liquid medium.
The liquid medium is constantly agitated using shakers (agitators), so that there is constant mixing of the medium and the explant. This keeps the cells evenly supplied with nutrients and with oxygen, and prevents them from settling into a starved heap at the bottom of the flask.
Callus culture vs Suspension culture | Feature | Callus culture | Suspension culture | |—|—|—| | Medium | Solid (agar added) | Liquid | | Explant | A tissue piece that forms a callus | Small groups of cells or a single cell | | Agitation | No shaker needed | Constantly agitated on a shaker | | Growth form | An undifferentiated cell mass sitting on the medium | Cells dispersed and mixed through the liquid |
Both the callus and the suspension cultures die in due course of time, as the nutrients run out and waste products accumulate. Therefore subculturing — transferring a portion of the culture to fresh medium — is necessary for the continuation of the technique.
The Complete Tissue Culture Procedure
Put together, the technique follows a fixed order, and each step protects or feeds the one after it. The complete sequence is given in FC 11.4.
Hardening deserves a second look. A plantlet raised in a flask has lived in perfectly humid, sterile, low-light comfort, with sugar supplied to it; its cuticle is thin and its stomata do not close efficiently. Moved straight into a field, it would simply dry out. Hardening gradually accustoms it to real air, real light and real soil.
Micropropagation (Clonal Propagation)
Organogenesis via shoots is considered one of the most widely used commercial methods of regenerating plants. When it is used to multiply a plant on a large scale, it is called micropropagation.
Micropropagation is also known as clonal propagation, because all the plants obtained from one parent by this method are genetically identical to one another and to the parent — they are a clone. It is the only process adopted by Indian plant biotechnologists in different industries, mainly for the commercial production of ornamental plants like orchids, Chrysanthemum and Eucalyptus, and fruit plants like banana, grapes and Citrus.
(Micropropagation, or clonal propagation, is the production of a large number of genetically identical plants (clones) from a small explant, by tissue culture.)
Q2. Can a small piece cut from one plant really be grown into thousands of identical plants?
A. Yes — and the reason is totipotency. A small explant is placed on a nutrient medium, where its cells divide to form a callus, or shoots are made to multiply directly. Each shoot can be separated, rooted, hardened and grown into a complete plant. Because all of them arose from the cells of one parent by mitosis, they are genetically identical — a clone. This is exactly how thousands of uniform orchid, banana and Eucalyptus plants are produced commercially, in a small space, within a short time.
Advantages of Micropropagation
- It helps in the rapid multiplication of plants.
- A large number of plantlets are obtained within a short period and in a small space — a shelf of culture flasks does the work of a nursery field.
- Plants are obtained throughout the year under controlled conditions, independent of seasons.
- Genetically identical plants (clones) are produced by this method. Therefore, the desirable characters (genotype) and the desired sex of a superior variety are kept constant for many generations.
- Rare plants and endangered species are multiplied by this method, and such plants are saved from extinction.
- With the help of somatic hybrids (cybrids), we are able to obtain a new variety in a short time span.
Point 4 is the one to think about hardest. A seed is the product of fertilization, so it carries a reshuffled genotype and may not resemble its parent. A clone carries no such surprise. When a grower has finally obtained one outstanding banana or orchid plant, cloning is the only way to reproduce that exact plant, not merely one of its descendants.
Nearly every commercial banana plantation you see is a clone. Banana varieties are propagated vegetatively because their fruit is seedless, and micropropagation supplies uniform, disease-free planting material in bulk. The same uniformity, however, is a weakness: because every plant in the field carries the same genotype, a pathogen that can defeat one plant can defeat them all. Cloning buys uniformity, and pays for it with a loss of genetic variability.
High-yielding varieties of banana such as ‘Shrimati’, ‘Basarai’ and G-9 are the ones mostly used in Maharashtra.
Applications of tissue culture. Tissue culture has various applications in forestry, agriculture, horticulture, genetic engineering, physiology and other fields. The different applications include the production of disease-free plants and haploid plantlets, micropropagation, the production of secondary metabolites, protoplast culture, the culture of rare plants, somaclonal variations, and the production of stress-resistant plants.
11.6 Animal Husbandry
What Animal Husbandry Covers
Everything up to this point has dealt with plants. But a large share of the protein, fat and other nutrients in our diet comes from animals, and improving that supply is a science of its own.
Animal husbandry is the agricultural practice of breeding and raising livestock. Like plant breeding, it is not merely a skill of farmers — it is as much a science as it is an art.
(Animal husbandry is the agricultural practice of breeding and raising livestock, and of caring for them so as to obtain useful products from them.)
Animal husbandry deals with the care and breeding of livestock such as buffaloes, cows, pigs, horses, cattle, sheep, camels and goats, which are useful to humans. It also includes poultry farming, fish farming, bee keeping, sericulture and lac culture.
Look at the range of organisms involved: honey bees, silk worms, prawns, crabs, birds, fish, pigs, cattle, sheep and camels have all been used by humans for products like milk, eggs, meat, wool, honey and silk. An insect that secretes a resin and a buffalo that gives milk sit in the same branch of applied biology, because the underlying task is the same — to rear a useful animal well and to obtain its product efficiently.
Why Better Management Is Needed
During the conventional practices of animal breeding, simply taking care of the animals is not enough to give the maximum yield. The figures make this uncomfortably clear.
India and China together have 70% of the world’s livestock population — yet this vast herd supplies only 25% of the world’s farm produce. In other words, we keep most of the world’s animals and get a small share of the output.
So a professional approach is needed to boost production. It requires management procedures and new technologies to be employed in various farm systems, in order to achieve improvement in quality and productivity. Industrial principles of production, processing and marketing are to be employed.
Management of Farms and Farm Animals
Farm management starts from the selection of high-yielding breeds, and continues through their food requirements, the supply of adequate nutritional sources, cleanliness of the environment, and maintenance of health.
Management of farm animals includes veterinary supervision, vaccination, the development of high-yielding cross breeds, the production and preservation of products, and their distribution and marketing.
Notice how the chain runs from the animal all the way to the market. A farm that breeds a superior animal but cannot keep its milk clean and cold, or cannot get it to a buyer, has still failed.
A. Animal Breeding
Breeding of animals is an important aspect of animal husbandry. Animal breeding aims at increasing the yield of animals and improving the desirable qualities of the products obtained from them.
Before we go further, one term must be defined precisely, because everything in animal breeding is built on it.
(A breed is a group of animals related by descent and similar in most characters — such as general appearance, features, size and configuration.)
Animal breeding is done to obtain improved breeds with desirable qualities of product, and also to increase the yield of animals. Desirable characters such as increased production of milk, quality of product, quality of meat, or maximum yield of eggs per year, are necessarily achieved through animal breeding.
Find out the names of the common breeds of cattle and poultry kept on the farms in your own area.
Breeding can be of two main types — inbreeding and outbreeding. The classification is shown in FC 11.7.
a. Inbreeding
Inbreeding involves the breeding of closely related individuals, for 4 to 6 generations.
Its genetic effect is straightforward: inbreeding increases homozygosity. When close relatives mate, the offspring is likely to receive the same allele from both parents, so more and more gene pairs become homozygous with each generation. By inbreeding, pure lines of animals can be obtained.
This has two useful consequences. Inbreeding is helpful in the elimination of harmful recessive genes, and in the accumulation of superior genes.
Inbreeding has one serious demerit: it usually reduces fertility and productivity. This decline is what breeders call inbreeding depression, and it sets a limit on how long inbreeding can be continued.
b. Outbreeding
Outbreeding involves the breeding of unrelated animals. The animals may be of the same breed, but must have no common ancestors for 4 to 6 generations.
Outcrossing involves breeding between the animals of different species. It is also known as interspecific hybridization. Outcrossing helps to remove the inbreeding depression, because bringing in unrelated genetic material restores heterozygosity.
Crossbreeding involves the breeding of a superior male of one breed with a superior female of another breed. By cross-breeding, new animal breeds with desirable characters are developed. For example, Hisardale is a new breed of sheep developed from crossing Bikaneri ewes with Marino rams in Punjab.
Interspecific hybridization involves the breeding of animals of two different but related species. It results in the production of animals with desirable characters from both parents. But such breeding is not always successful — the mule, a breed obtained from a horse and a donkey, is strong and hardy but sterile, and so cannot found a breed of its own.
Inbreeding vs Outbreeding | Feature | Inbreeding | Outbreeding | |—|—|—| | Animals mated | Closely related individuals | Unrelated animals | | Generations considered | Continued for 4 to 6 generations | No common ancestors for 4 to 6 generations | | Effect on genotype | Increases homozygosity | Restores heterozygosity | | Main use | Produces pure lines; eliminates harmful recessives; accumulates superior genes | Removes inbreeding depression; produces new breeds with combined characters | | Main drawback | Usually reduces fertility and productivity | Desired combination is not always achieved; hybrids may be sterile |
Crossbreeding vs Interspecific hybridization (Crossbreeding = superior male of one breed × superior female of another breed — both belong to the same species, e.g. Hisardale sheep. Interspecific hybridization = animals of two different but related species, e.g. the mule from a horse and a donkey.)
Artificial Insemination
The artificial insemination technique involves controlled breeding experiments. Semen from selected superior males is collected and either preserved in a frozen state, or injected into the genital tract of the female immediately.
It is useful because it overcomes the problem of normal mating and the inconvenience of transportation. A single outstanding bull, whose frozen semen can be shipped anywhere, can father calves in herds hundreds of kilometres apart — something impossible if the animals themselves had to be brought together.
Multiple Ovulation Embryo Transfer (MOET)
Artificial insemination solves the problem of the male, but not of the female: a superior cow still produces only one calf at a time. MOET is the technology designed to get around exactly that limit, by providing the chance of successful production of hybrids in larger numbers.
In this method, the cow is administered with an FSH-like hormone, to induce follicular maturation, and then super-ovulation is brought about. In each cycle, 6 to 8 eggs mature simultaneously, instead of the usual one.
The cow is either mated with an elite bull or artificially inseminated. The blastocysts, at the 8 to 32 cell stage, are recovered non-surgically and transferred to surrogate mothers, which carry the pregnancies to term. The steps are summarised in FC 11.8.
This technology is successfully used in cattle, sheep, rabbits, buffaloes and other animals. High milk-yielding breeds of females, and high quality meat-yielding bulls, have been found successful in increasing herd size in a short period.
B. Dairy Farm Management
The dairy industry involves the production, processing and distribution of milk and milk products. Milk is a valuable foodstuff, universally consumed by human beings.
Milk yield mainly depends on the quality of the breeds kept on the farm. The basic requirement is therefore the selection of good breeds — ones that have a high yielding potential under the climatic conditions of the area they will live in, and that carry disease resistance. A breed that gives magnificent yields in a cold country may struggle and fall sick in an Indian summer, and its record yield on paper is then worth nothing.
Breeds Used in India
In India, cows and buffaloes are mainly used for dairy farms.
- Indian breeds of cattle: Sahiwal, Sindhi, Gir.
- Exotic breeds of cattle: Jersey, Brown Swiss, Holstein.
Buffaloes are restricted to some parts of Asia only. In India, six breeds of buffalo occur — Jaffarabadi, Mehsana, Murrah, Nagpuri, Nili and Surati — which are all good milk producers.
The six Indian buffalo breeds (“JM-MN-NS” — Jaffarabadi, Mehsana, Murrah, Nagpuri, Nili, Surati.)
Feeding and Hygiene
Cattle have to be well looked after. The quality and quantity of fodder must be given in the proper ratio.
Silage made from legumes and grasses, maize and jowar makes good feed. Silage is supplemented with oil cakes, minerals, vitamins and salts — the supplements supply what green fodder alone cannot, particularly for an animal being asked to produce many litres of milk a day.
Cleanliness and hygiene of the cattle and of the handlers is of great importance during milking, and during the storage and transport of milk and milk products. Milk is an excellent growth medium for bacteria, which is precisely why it spoils so quickly and why hygiene is not optional here. In recent years much of this process has been mechanised, which reduces the chance of direct contact with the product.
The shed must be cleaned daily. It should be spacious, with adequate facilities for feeding, watering and lighting.
A regular visit by a veterinary doctor to a dairy farm is mandatory. Why?
Products and By-products
Milk processing, marketing and distribution play an important role in the dairy industry. A variety of milk products — curd, cream, butter, ghee, condensed milk, khoa and cheese — must be prepared from the extra milk, and on demand.
Additional income can be obtained on cattle farms from cow dung, manure, fuel cakes, and gobar gas for cooking and lighting. Nothing on a well-run dairy farm is treated as waste.
C. Poultry Farm Management
Poultry includes a number of bird species such as chicken, ducks, turkey and fowls, which are domesticated for their eggs and meat.
- What is a layer? 2. What is a broiler?
Allied professions to poultry include the processing of eggs and meat, the marketing of poultry products, the compounding and sale of poultry feed, poultry equipment, pharmaceuticals and feed additives.
The important requirements for poultry farm management are: selection of a proper and disease-free breed, suitable and safe farm conditions, proper feed and water, and hygiene and health care.
Breeds of Poultry
On the basis of their origin, the different types of poultry breeds are:
| American breeds |
Plymouth Rock, New Hampshire, Rhode Island Red |
| Asiatic breeds |
Brahma, Cochin, Langshan |
| Mediterranean breeds |
Leghorn, Minorca |
| English breeds |
Australorp |
| Indian breeds |
Chittagong, Aseel, Brahma, Kadaknath |
Leghorn is the best layer (for eggs), while Plymouth Rock, Rhode Island Red, Aseel, Brahma and Kadaknath are preferred as broilers (for meat).
Layer vs Broiler (A layer is a bird kept for the eggs it lays, so it is selected for a high rate of egg production over a long period. A broiler is a bird kept for meat, so it is selected for fast body growth and good meat quality. The same species, bred towards two entirely different goals.)
Management of Layers and Broilers
Management of layers requires the purchase of high-yielding chicken, well ventilated farms, proper feed, debeaking, lighting, waterers, sanitation, culling and vaccination.
Management of broilers requires selection of the breed, housing, temperature, ventilation, lighting, adequate floor space and broiler feed.
Lighting appears in both lists, and for a reason worth knowing: the laying of eggs is influenced by the length of the lighted period, so controlled lighting is used to keep layers productive through the year rather than only in the natural laying season.
Poultry Diseases
Birds kept crowded together are birds at risk — an infection that reaches one can sweep through the flock. The different types of poultry diseases are:
i. Viral diseases include Ranikhet, Bronchitis and Avian influenza (bird flu). A few years ago, bird flu seriously influenced poultry farming and caused human infection too.
ii. Bacterial diseases mainly include Pullorum, Cholera, Typhoid, TB, CRD (chronic respiratory disease) and Enteritis.
iii. Fungal diseases are Aspergillosis, Favus and Thrush.
iv. Parasitic diseases include infections by lice, round worm and caecal worm.
v. Protozoan diseases — for example, Coccidiosis is a protozoan disease.
Avian influenza is the reason poultry farms are placed under surveillance, and whole flocks are culled, whenever an outbreak is detected. The virus can cross from birds to humans, which turns a poultry disease into a public health problem. This is also why vaccination and sanitation appear in the list of management practices rather than as optional extras.
D. Apiculture or Bee Keeping
Apiculture, or bee keeping, deals with the artificial rearing of honey bees in order to obtain bee products — honey, wax, pollen, bee venom, propolis (bee glue) and royal jelly — and also to use the bees as pollinating agents for crop plants.
(Apiculture is the artificial rearing of honey bees for the production of honey, wax and other bee products, and for their use as pollinators of crops.)
Bee keeping is an ancient cottage industry. Honey is a food of high nutritive value, and also finds uses in the indigenous systems of medicine. A colony of bees living in an artificial hive is shown in Fig. 11.4.
Why are honey bees called the best pollinators?
Species of Honey Bees
The four species of honey bees commonly found in India are:
- Apis dorsata — rock bee, or wild bee
- Apis florea — little bee
- Apis mellifera — European bee
- Apis indica — Indian bee
For bee keeping, Apis mellifera and Apis indica are the suitable species — hence they are known as the domesticated species.
Polymorphism in Honey Bees
A honey bee colony is not a crowd of identical insects. It shows polymorphism: three distinct forms — the queen, the workers and the drones — live in one colony, each with a different body form and a different job. The three castes are shown in Fig. 11.5.
the worker honey bee (Hairy body → pollen grains cling to it as the bee moves from flower to flower → pollen is carried between flowers, so the bee pollinates the crop even while it is only collecting food for itself. The very structure that feeds the colony is what makes the bee valuable to the farmer.)
Bee keeping is practised in areas where sufficient wild shrubs, fruit orchards and cultivated crops are present — that is, where the bees can find enough flowers to forage on. Bee keeping requires equipment such as bee hive boxes with comb foundation sheets, a bee veil, a smoker, a bee brush, gloves, gumshoes, an uncapping knife, a swarm net, a queen excluder, and an overall hive tool.
The Artificial Bee Hive
The artificial hive is a stack of removable parts, so that the beekeeper can inspect the colony and remove honey without destroying the comb. Its parts are shown in Fig. 11.6.
The queen excluder is a good example of design following biology. Its gaps are wide enough for a worker to pass through but too narrow for the larger queen — so the queen is confined to the lower hive bodies and cannot lay eggs in the honey super above, and the honey collected there stays free of brood.
For successful bee keeping, one must be familiar with the habits of bees, the selection of a suitable location, the catching and hiving of swarms, the management of hives during different seasons, and the handling and collection of honey, bee wax and other products. Periodic inspection is very necessary — for cleanliness of the hive boxes, activity of the bees and the queen, condition of the brood, and provision of water.
Bees and the Crop
Many Indian crop fields need the services of honey bees as pollinators. Bee keeping in the crop fields of sunflower, mustard, safflower, chilly, cabbage, cucumber and legumes, and in fruit orchards of apple, mango and citrus, helps to increase the productivity of honey as well as of the crops tremendously.
This is worth pausing on, because it is a rare case where two industries help each other for free. The farmer’s flowers feed the bees; the bees, in moving from flower to flower, pollinate the crop and raise its yield. Placing hives in an orchard is therefore not charity to the beekeeper — it is a way of increasing the fruit set.
E. Fishery
Fishery is a branch of applied biology which deals with the catching, processing, farming and marketing of fish, and of other useful aquatic animals such as prawns, lobsters, oysters, mussels and crabs.
The three divisions of fishery are inland fishery, marine fishery and estuarine fishery — and the division is based simply on the kind of water body involved.
Inland Fishery
Inland fishery includes the culturing and capturing of fish from fresh water bodies like ponds, lakes, dams and rivers. The inland aquatic area of our country covers about 40 to 50 lakh acres.
The common fresh water fish are Labeo rohita (rohu), Catla (catla), Cirrhina mrigala (mrigala) and other carps. Some of the common freshwater forms are shown in Fig. 11.7.
Fish farming — that is, the culturing of edible and commercially important fish — is only possible in fresh water bodies.
Marine Fishery
Marine fishery includes the capture of fish from sea water. The Indian coastline is about 7500 km long, which is why marine fishery supports so many coastal communities.
The common marine fish are Harpadon (Bombay duck), Sardinella (sardine), Rastrelliger (mackerel) and Stromateus (pomphret).
Estuarine Fishery
Estuarine fishery includes the capture of fish from an estuary.
(An estuary is a place where a river meets the sea — for example, the Sunderban area in West Bengal.)
An estuary is a demanding place to live, because the salt concentration of the water rises and falls with the tide and with the river’s flow. The fish found there must tolerate this changing salinity, which is why estuarine catches differ from both freshwater and purely marine catches.
- Give the names of estuaries found in Maharashtra, and mention their locations. 2. Enlist the names of different fish found at an estuary.
Fish Farm Management
The common factors for the maintenance of a fish farm include the selection of a suitable site, excavation of ponds, the requirement of a hatchery tank, nursery tank, rearing tank and stocking tank or ponds, a water source, manures and supplementary feed.
Notice that the tanks are not interchangeable — eggs hatch in the hatchery tank, tiny fry grow in the nursery tank, young fish grow on in the rearing tank, and only then are they moved to the stocking pond. Each stage of the fish’s life needs a different depth, feed and density, and mixing the stages together would let the larger fish eat the smaller.
Culture fishery may be of two kinds:
Monoculture vs Polyculture | Feature | Monoculture | Polyculture | |—|—|—| | Species stocked | Only one species | Many species together | | Use of the pond | One feeding zone of the pond is exploited | Species feeding at different levels are combined, so the whole pond is used |
Preservation and By-products
After the catching of fish, spoilage is prevented by different preservation methods — chilling, freezing, freeze drying, sun drying, smoke drying, salting and canning. Fish flesh is soft, moist and rich in protein, so it decays quickly; every one of these methods works by either lowering the temperature or removing the water that microbes need.
In addition to being a source of nutritious food, fish yield a number of by-products of commercial value: fish oil, fish meal, fertilizers, fish guano, fish glue and isinglass. These by-products are widely used in paints, soaps, oils and medicine. Prawns and lobsters have market value all over the world. Fishery provides good job opportunities and self-employment to many people.
F. Sericulture
Sericulture is the branch of applied zoology which deals with the rearing of silkworms and the production of silk.
Like other kinds of farming, sericulture involves skill and scientific knowledge for rearing and development. It requires less investment and can be started in a small space. It is the oldest of businesses, and a large number of families are associated with the production of silk in India. Disabled and older people can successfully do this job, since it demands care and patience rather than physical strength.
The Silkworm and Its Life Cycle
The best quality silk, called mulberry silk, is produced by the silkworm Bombyx mori, while Tussar silk and Eri silk are of inferior quality.
The quality and quantity of silk depend on the quality of the mulberry leaves on which the larvae feed. This is a direct link worth remembering: the silk thread is made from protein, and that protein is built out of what the caterpillar eats. Poor leaves make poor silk.
The life cycle of the silk moth is shown in Fig. 11.8, and its sequence in FC 11.9.
- What are the different stages found in the life cycle of the silkworm? 2. Describe the process of cocoon formation. 3. Which process is involved in the production of silk from the cocoon?
Rearing the Silkworm
Rearing, development and looking after the silkworms involve skill and labour, and demand constant watch. A little negligence can spoil the complete industry — an entire rearing house of larvae can be lost in a few days.
Silkworm larvae may be infected by protozoans, viruses and fungi. Besides these, ants, crows, birds and other predators are always ready to attack these insects. Hence the cages of these larvae must be managed carefully to prevent attack by predators.
G. Lac Culture
Lac is produced by an insect, Trachardia lacca, which is quite small in size and colonial in habit — that is, enormous numbers of them settle together on the same twig.
A resin-like substance is produced by the dermal glands of the female lac insect. The insect feeds on the succulent twigs of certain plants like ber, peeple, palas, kusum and babool, and secretes a pink coloured resin, which hardens on coming into contact with air, forming lac. It is produced on a large scale all over India. The life cycle of the insect is shown in Fig. 11.9.
Lac is a complex substance, having a large amount of resin together with sugar, water, minerals and alkaline substances.
Natural lac is always contaminated. Shellac is the pure form of lac, obtained by washing and filtering.
The lac insect is a native of India, and our share is 85% of the total lac produced in the world. The products of lac play a vital role in the economy of farmers. Lac is used in bangles, toys, woodwork, inks and mirrors.
The production of lac requires an artificial inoculation of the host plants — that is, twigs carrying the insects are deliberately tied onto fresh host trees so that the crawlers can settle on them. Left to themselves, the insects would spread unevenly; artificial inoculation gives a better and more regular supply of good quality and quantity of lac.
from a gland to an industry (Zoom in: the whole of lac culture rests on the secretion of the dermal glands of one small female insect. Zoom out: those secretions, gathered from millions of insects on thousands of trees, support a national industry that supplies 85% of the world’s lac.)
11.7 Microbes in Human Welfare
Biotechnology: Putting Living Agents to Work
We have improved plants, and we have improved animals. The third partner in food production is the one we cannot see — the microbe. Putting such living agents to work for us in a planned, engineered way is what we call biotechnology.
(Biotechnology is the application of scientific and engineering principles to the processing of materials by biological agents, in order to provide goods and services to humans, that is, for human welfare.)
Read that definition carefully, because two ideas sit inside it. “Biological agents” means the actual work — the chemical conversion — is done by a living organism or its enzymes. “Engineering principles” means we control the conditions under which it works: the temperature, the vessel, the substrate, the time. A microbe fermenting a spoiled fruit in a forest is doing biology; the same microbe fermenting a controlled substrate in a vessel we designed is doing biotechnology.
There is a wide variety of microorganisms available to us — algae, fungi, bacteria, viruses, protozoans, nematodes and others. Their products, having beneficial activities, are used for the welfare of humans in regard to food, health, industry, agriculture, medicine and biocontrol. These organisms are used variously in food and feed technology, in industry, in waste utilization and in energy generation.
the reach of one microbe (Zoom in: a single bacterial cell secreting an enzyme or an acid into its surroundings. Zoom out: that same activity, multiplied across a fermenter, a farm, a sewage plant or a biogas digester, is what the remaining sections of this chapter are about.)
Microbes in Food Preparation
The development of biotechnology occurred in two phases: traditional biotechnology (up to 1970) and modern biotechnology (after 1970).
Traditional biotechnology is based on the fermentation principle, using fermenting bacteria. These were used in the preparation of a variety of indigenous fermented food products — long before anyone knew that microbes existed at all. Cooks in every part of the world had learned, purely by observation, that certain foods left to stand under certain conditions became tastier, softer, safer and longer-lasting.
Traditional vs Modern biotechnology (Traditional biotechnology, up to 1970, uses whole microbes and the natural process of fermentation — curd, idli, wine. Modern biotechnology, after 1970, works at the level of the gene itself. The dividing line is not the organism used, but how deeply we intervene in it.)
1. Dosa, Dhokla and Idli
Dosa, idli and dhokla are fermented products, produced due to the activity of bacteria. They are fermented preparations of rice and black gram, made with the help of air-borne Leuconostoc and Streptococcus species of bacteria.
The batter is left to stand, and the bacteria multiply in it. \(CO_2\) produced during fermentation causes the puffing up of the dough. Trapped as bubbles within the thick batter, the gas cannot escape, so the batter rises — and this is exactly why a well-fermented idli is soft and spongy while an unfermented one is dense and hard.
Name the microbes used in the fermentation of dhokla.
2. Microbes as a Source of Food
Some microbes, or their fruiting bodies, are used directly as a source of food, because they are rich in vitamins and proteins.
You have already met one form of this in Section 11.4: the term SCP, or single cell protein, denotes the dead and dried cells of microbes such as bacteria, algae, molds and yeasts.
But microbes also reach our plate in a much more familiar form. Some mushrooms and truffles are used directly as food. They belong to the higher fungi, and they produce large, fleshy fruiting bodies which are edible.
The nutritional profile of these fruiting bodies is unusual and worth noting. They are sugar free and fat free, but rich in proteins, vitamins, minerals and amino acids. The food in the fruiting body is therefore low in calories — which is precisely why mushrooms suit a diet that needs protein without an energy load.
The same feature that makes mushrooms valuable makes them dangerous. Edible and poisonous mushrooms can look remarkably alike, and some of the most toxic species are the ones most easily mistaken for harmless ones. Every year, people are poisoned by wild mushrooms picked in good faith. This is why commercial mushroom cultivation, using known and identified species, matters as much as the nutrition does.
Find out the names of some edible mushrooms and some poisonous mushrooms.
3. Dairy Products
Lactic acid bacteria (LAB), such as Lactobacillus, are added to milk. The bacterium ferments the lactose sugar of milk into lactic acid.
That single chemical change explains everything that follows. Lactic acid causes coagulation and partial digestion of the milk protein casein — the acid makes the casein molecules clump together, so the liquid milk sets into a semi-solid. In this way milk is changed into curd, yoghurt and cheese.
The starter, or inoculum, used in the preparation of milk products actually contains millions of lactic acid bacteria. The sequence is summarised in FC 11.10.
why lactic acid sets milk (Casein in milk exists as tiny suspended particles that repel one another and stay dispersed → lactic acid makes the medium acidic → the particles lose that mutual repulsion and clump together → the liquid sets into a solid curd. The souring and the setting are not two events; they are the same event.)
Curd
Indian curd is prepared by inoculating milk with Lactobacillus acidophilus. It also checks the growth of disease-causing microbes.
That last point is more important than it sounds. The lactic acid makes the medium too acidic for many spoilage and pathogenic organisms to survive in. Curd is thus a food that partly preserves itself — one reason fermented milk was a safe food long before refrigeration existed.
Yoghurt
Yoghurt is produced by curdling milk with the help of Streptococcus thermophilus and Lactobacillus bulgaricus.
Curd vs Yoghurt (Both are set by lactic acid bacteria, but by different ones. Indian curd uses Lactobacillus acidophilus; yoghurt is curdled by Streptococcus thermophilus together with Lactobacillus bulgaricus. The organisms differ, and so do the texture and flavour they produce.)
Butter Milk
The acidulated liquid left after the churning of butter from curd is called butter milk.
Cheese
Cheese is the partially degraded concentrate of milk fat and protein — that is, the fat and protein of milk, concentrated and then deliberately allowed to break down a little further, which is what develops its flavour.
The milk is coagulated with lactic acid bacteria, and the curd formed is filtered to separate the whey (the watery liquid). The solid mass that remains is then ripened with the growth of a mould, which develops flavour in it.
Different varieties of cheese are known by their characteristic texture, flavour and taste, which are developed by different specific microbes. This is a striking point: the milk may be the same, but the microbe decides what the cheese becomes.
- The Roquefort and Camembert cheeses are ripened by the blue-green molds Penicillium roquefortii and P. camembertii respectively.
- The large holes in Swiss cheese are developed due to the production of a large amount of \(CO_2\) by a bacterium known as Propionibacterium shermanii.
11.8 Role of Microbes in Industrial Production
Fermentation on an Industrial Scale
In the kitchen, fermentation gives us a bowl of curd. In a factory, the same biological process is scaled up until it supplies medicines, acids, vitamins and fuels to the world.
During fermentation, a variety of products are produced — alcoholic beverages, organic acids, vitamins, growth hormones, enzymes, antibiotics, and many others.
Most of these are actually secondary metabolites, produced during the idio phase of microbial growth. They are not required by the micro-organisms for their own growth. The type of substrate and the type of micro-organism together decide which particular secondary metabolites are produced.
That fact is worth pausing on. A primary metabolite is something the microbe cannot live without. A secondary metabolite is something it makes only after the main phase of growth is over — and almost every industrially valuable microbial product falls into this second class. In other words, industry lives off what the microbe produces once it has finished growing.
The Fermenter
Production on an industrial scale requires growing microbes in very large vessels, called fermenters.
The main function of a fermenter is to provide a controlled environment for the growth of a microorganism, or of a defined mixture of microorganisms, in order to obtain the desired product.
(A fermenter is a large vessel that provides a controlled environment — of temperature, pH, aeration and mixing — for the growth of a microorganism, or a defined mixture of microorganisms, so as to obtain a desired product.)
Every word in “controlled environment” is earning its place. The microbe will make the product only within a narrow range of temperature, pH and oxygen supply. Outside that range, it either stops producing or dies, and the entire batch is lost. The general route from raw material to finished product is shown in FC 11.11.
a. Production of Alcoholic Beverages
A beverage is an alcoholic or non-alcoholic liquid produced or prepared and used for drinking — for example tea, coffee, beer or wine — which acts as a stimulant. Alcoholic beverages are the products of the alcoholic fermentation of specific substrates.
The use of microbes in making alcoholic beverages is not new: it is known in India since the vedic period, 5000 to 7000 BC.
Microbes, especially yeast, have been used from time immemorial for the production of beverages like wine, beer, whiskey, brandy and rum. For this purpose, the yeast Saccharomyces cerevisiae var. ellipsoidis — commonly called Brewer’s Yeast — is used for fermenting malted cereals and fruit juices to produce ethanol.
Undistilled vs Distilled beverages | Feature | Wine and Beer | Whiskey, Brandy and Rum | |—|—|—| | Produced by | Fermentation, without distillation | Fermentation followed by distillation | | Consequence | Alcohol content stays as fermentation left it | Alcohol is concentrated by distillation |
A tubular tower fermenter is used for the large-scale production of alcohols, as shown in Fig. 11.10.
Several traditional drinks and foods are also made through fermentation by yeast. Today, a traditional drink of the coastal region of South India is made by fermenting the sugar sap extracted from palm plants and coconut palm. The famous wine of Goa, traditionally known as ‘fenny’, is made by fermenting the fleshy pedicels of cashew fruits.
b. Production of Organic Acids
Microbes are also used for the commercial and industrial production of certain organic acids. These compounds can be produced directly from glucose (for example, gluconic acid), or formed as end products from pyruvate or ethanol.
| Citric acid |
Aspergillus niger |
| Gluconic acid |
Aspergillus niger |
| Fumaric acid |
Rhizopus arrhizus |
| Acetic acid (vinegar) |
Acetobacter aceti |
The organic acids are further used in various ways: citric acid is used in confectionery, fumaric acid in resins as a wetting agent, and gluconic acid in medicine for the solubility of \(Ca^{++}\).
c. Production of Vitamins
Vitamins are complex organic nitrogenous compounds, required in small amounts, and capable of performing many life-sustaining functions inside our body.
These compounds cannot be synthesized by humans (except vitamin D), and therefore they have to be supplied in small amounts in the diet.
Microbes, however, are capable of synthesizing vitamins, and hence they can be successfully used for the commercial production of many of them — for example thiamine, riboflavin, pyridoxine, folic acid, pantothenic acid, biotin, vitamin \(B_{12}\), ascorbic acid, beta-carotene (pro-vitamin A) and ergosterol (provitamin D).
Vitamins are manufactured by fermentation, using different microbial sources as listed below:
| Vitamin \(B_2\) |
Neurospora gossypii; Eremothecium ashbyi |
| Vitamin \(B_{12}\) |
Pseudomonas denitrificans |
| Vitamin C |
Aspergillus niger |
Vitamin \(B_{12}\) occurs naturally almost only in foods of animal origin, because plants do not make it — it is made by microorganisms. This is exactly why people on a strictly plant-based diet are at risk of \(B_{12}\) deficiency, and why the supplements they take are themselves manufactured by microbial fermentation. The vitamin comes from a microbe either way; industry simply cuts out the animal in the middle.
- What are antibiotics? 2. Who invented the first antibiotic?
d. Production of Antibiotics
Antibiotics are probably the most important group of compounds synthesized by industrial microorganisms. Most antibiotics are secondary metabolites.
They have therapeutic importance and are used in medical treatment. They are produced in small amounts by certain microbes — bacteria, fungi and a few algae — which thereby inhibit the growth of other microbial pathogens. This is the crucial point: an antibiotic is a weapon one microbe uses against another, and medicine simply borrows it. Antibiotics may be antibacterial, antifungal, and so on.
(Antibiotics are chemical substances produced in small amounts by certain microbes — bacteria, fungi and a few algae — which inhibit the growth of other microbial pathogens, and are therefore used in medicine.)
Antibiotics have greatly improved our capacity to treat deadly diseases such as plague, whooping cough, diphtheria and leprosy — diseases which, within living memory, killed most of the people who caught them.
The discovery was an accident that a careless observer would have thrown away. In the late 1920s, the Scottish bacteriologist Alexander Fleming returned to his laboratory to find that one of his culture plates of Staphylococcus bacteria had been contaminated by a mould. Around the mould, the bacterial growth had been dissolved away — a clear ring in a lawn of bacteria. Fleming realised that the mould, a Penicillium, was releasing something that killed bacteria, and he named the substance penicillin. It took another decade, and the work of Florey and Chain, to purify it and turn it into the drug that saved countless lives during the Second World War.
Can antibiotics kill viruses? Think about what an antibiotic actually attacks before you answer.
Some common antibiotics and their microbial sources are listed below.
Table 11.11: Antibiotic producing microbes
| Chloromycetin |
Streptomyces venezuelae |
| Erythromycin |
Streptomyces erythreus |
| Penicillin |
Penicillium chrysogenum |
| Streptomycin |
Streptomyces griseus |
| Griseofulvin |
Penicillium griseofulvum |
| Bacitracin |
Bacillus licheniformis |
| Oxytetracycline / Terramycin |
Streptomyces aurifaciens |
Which genus makes which antibiotic (Most of the “-mycin” and “-mycetin” antibiotics in the table come from Streptomyces — chloromycetin, erythromycin, streptomycin, oxytetracycline. Penicillin and griseofulvin come from Penicillium, and bacitracin from Bacillus.)
e. Production of Enzymes
What are enzymes? How are they classified?
In living beings, enzymes play a key role in metabolic reactions, and are therefore essential for the survival of such beings.
Enzymes are proteins known as biocatalysts, because of their ability to promote reactions more quickly, at body temperature, and more efficiently. A reaction that would need high heat and pressure in a factory can be carried out by an enzyme in a cell at \(37\,^\circ C\).
Many microbes synthesize and excrete large quantities of enzymes into the surrounding medium. This is the feature industry exploits — the enzyme does not have to be dug out of the cell; the microbe hands it over. Using this feature of these tiny organisms, many enzymes are produced commercially. These enzymes are amylase, cellulase, protease, lipase, pectinase, streptokinase and many others.
Several industrial sectors use enzymes from microorganisms for specific applications:
- In the textile industry, enzymes improve the quality of fabrics.
- In the pulp and paper industry, they are involved in biomechanical pulping and bleaching.
- In the food industry, they are used in the fermentation processes for the production of bread and of drinks such as wine and beer, and they also participate in the extraction of substances such as carotenoids and olive oil.
- Lipases are used in the detergent industry, because they have superior cleaning properties, increasing the brightness of the wash and removing oil stains. Lipases digest fat — which is precisely what an oil stain is.
- Enzymes are also used in cosmetics, animal feed and agricultural industries, among others.
A few examples of enzymes used in industrial food processing:
| Invertase |
Saccharomyces cerevisiae |
| Pectinase |
Sclerotinia libertine; Aspergillus niger |
| Lipase |
Candida lipolytica |
| Cellulase |
Trichoderma konigii |
Two microbial products that act inside the human body. Streptokinase, an enzyme (TPA) produced by the bacterium Streptococcus spp., has a fibrinolytic effect — it dissolves fibrin. It is therefore used as a ‘clot buster’ for clearing blood clots from the blood vessels of patients, clots which may otherwise cause a heart attack. Statins, produced by the yeast Monascus purpureus, have been produced on a commercial scale. A statin is a blood-cholesterol lowering agent. It acts as a competitive inhibitor of the enzyme responsible for the synthesis of cholesterol — that is, it resembles the enzyme’s normal substrate closely enough to occupy the active site, so the enzyme cannot make cholesterol while the statin is sitting there.
f. Gibberellin Production
Gibberellin is a group of growth hormones, mainly produced by higher plants and fungi, which promote growth by stem elongation.
Gibberellin was found because of a plant disease, not a plant experiment. Japanese rice farmers knew of a condition in which seedlings grew abnormally tall and spindly, then toppled over and yielded no grain. The cause turned out to be infection by the fungus Gibberella fujikouri, and in 1938 two Japanese scientists, Yabuta and Sumiki, isolated from those infected rice seedlings the first gibberellin. A substance that made a fungus’s victim grow itself to death became one of agriculture’s most useful growth regulators.
About 15 different types of gibberellins have been isolated. Gibberellins have many practical applications:
- They are used to induce parthenocarpy — the formation of fruit without fertilization — in apple, pear and other fruits.
- They are used in breaking the dormancy of seeds, so that seeds germinate when the grower wants them to.
- They are used in inducing flowering in Long Day Plants (LDP).
- They are also used to enlarge the size of grape fruits.
gibberellin and the stem (Gibberellin promotes elongation of the cells of the internode → the stem becomes longer without any increase in the number of cells → in a grape, the same effect stretches the fruit stalks and the fruit, so bunches become larger and looser. One hormone, one cellular action, several commercial uses.)
11.9 Microbes in Sewage Treatment
What Sewage Is
So far, microbes have been making things for us. In this section they do something equally valuable: they clean up after us.
Sewage is the matter carried off in drainage. It is a municipal waste containing human excreta, household waste, dissolved organic matter, and even pathogenic microbes — bacteria, viruses, protozoans, nematodes and microfungi.
It also includes discharged water from hospital waste, slaughter house waste and animal dung. Discharge from industrial waste — which contains toxic dissolved organic and inorganic chemicals — along with tannery and pharmaceutical waste, also adds to sewage.
(Sewage is the waste matter carried off in drainage — municipal waste containing human excreta, household waste, dissolved organic matter and pathogenic microbes.)
Sewage is a potential source of pathogenic bacteria, viruses and protozoa. The causative agents of dysentery, cholera, typhoid, polio and infectious hepatitis may occur in sewage, and bacteria from the soil are also present in it. During the course of sewage decomposition, aerobic and facultative anaerobic organisms predominate initially. These are then followed by strict anaerobes — especially methanogenic bacteria, which produce methane (\(CH_4\)) and \(CO_2\).
Composition of Sewage
The first surprise about sewage is how little of it is actually waste. Sewage consists of approximately 99.5% to 99.9% water, and only 0.1 to 0.5% inorganic and organic matter, present in suspended and in soluble form.
The composition of sewage varies depending upon the type of waste discharged into the water from different industries — textile, chemicals, pharmaceuticals, dairy, canning, brewing, meat packing, tannery, oil refineries and meat industries, among others.
Microorganisms in Sewage
Various types of micro-organisms are also present in sewage. Bacteria, viruses, fungi, protozoa, nematodes and algae are all found in it. However, their number and type fluctuate depending on the composition of the sewage and its source.
Raw sewage may contain millions of bacteria per ml. These include coliforms, fecal Streptococci, anaerobic spore-forming bacilli, and other types originating in the intestinal tract of humans.
Before waste water is made available for human use, it has to be treated properly, so as to remove organic matter, inorganic salts and pathogens as well.
Here is the central idea of the whole process, and it is a neat one. The sewage is full of organic matter that must be destroyed, and it is also full of microbes that eat organic matter. Sewage treatment does not fight the microbes — it employs them. The engineering merely provides the conditions in which they work fastest.
The sewage treatment process includes four basic steps, shown in Fig. 11.12 and summarised in FC 11.12.
1. Preliminary Treatment
The preliminary treatment includes screening and the grit chamber. Both are purely physical, and both exist to protect the biological treatment that comes later — a pump or an aeration tank clogged with rags, sticks and grit cannot do its job.
i. Screening: Sewage and waste water contain plenty of suspended and floating materials, coarse and solid particles, along with dissolved substances. The suspended objects are filtered and removed. This is done in screening chambers, where the sewage is passed through screens or nets. Larger suspended or floating objects are held back in the screening chambers. These have to be removed before the biological treatment.
ii. Grit Chamber: After screening, the filtered sewage is passed into a series of grit chambers. These chambers contain large stones (pebbles) and brick-ballast, which slow the flow of the water. Coarse particles — sand, grit, small stones — then settle down by gravity, because slower water can no longer carry them. Thus the passage of the filtered sewage through the grit chambers removes much of the coarse particulate matter.
2. Primary Treatment (Physical Treatment)
After the preliminary treatment, the sewage water is pumped into the primary sedimentation tank. Here the sedimentation of suspended solid and organic matter occurs, simply by allowing the water to stand quietly so that gravity can do the work.
About 50–70% of the solids settle down, and there is a reduction of about 30–40% (in number) of the coliform organisms — the bacteria settle along with the solids they cling to.
The organic matter which settles down is called primary sludge, and it is removed by mechanically operated devices.
The supernatant — the liquid left above, called the effluent — still contains a large amount of dissolved organic matter and micro-organisms. Sedimentation cannot remove these, because dissolved matter does not settle. They can only be removed by the secondary treatment.
Primary sludge vs Activated sludge (Primary sludge is the solid matter that simply settles out of the sewage in the primary sedimentation tank, before any biological treatment. Activated sludge is the sediment of bacterial flocs, formed later, after the aerobic bacteria have grown in the aeration tank.)
3. Secondary Treatment (Biological Treatment)
This is where the microbes take over.
The primary effluent is passed into large aeration tanks. Here it is constantly agitated mechanically, and air is pumped into it. Both the stirring and the air have the same purpose — to keep the effluent well supplied with dissolved oxygen, which is exactly what aerobic bacteria need.
Under these conditions, aerobic bacteria grow vigorously and form flocs.
(Flocs are masses of bacteria held together by slime and fungal hyphae, forming mesh-like masses.)
These aerobic microbes consume the major part of the organic matter present in the effluent, as they grow. In other words, the pollutant is their food, and they turn it into more bacteria.
Because the organic matter is consumed, the BOD (Biochemical Oxygen Demand) of the effluent is significantly reduced.
(Biochemical Oxygen Demand (BOD) is the amount of oxygen that would be consumed by bacteria in decomposing the organic matter present in a sample of water. The greater the amount of organic matter, the higher the BOD.)
BOD is therefore a measure of pollution by an indirect route: it does not weigh the organic matter, it measures how much oxygen the microbes would need to destroy it. A high BOD in water released into a river means the river’s own oxygen will be stripped away — and that is what kills fish. Reducing BOD before discharge is the whole purpose of the secondary treatment.
the bacterial floc (Bacteria embedded in slime and bound together by fungal hyphae → they form a large mesh-like mass instead of remaining as free cells → the mass has a huge surface area for absorbing organic matter, and yet it is heavy enough to sediment out of the water afterwards. The floc is thus both an efficient eater and an easily removable one — which is exactly what a treatment plant needs.)
4. Tertiary Treatment
Once the BOD of the waste water is reduced, the water is passed into a settling tank. Here the bacterial flocs are allowed to sediment.
The sediment is now called activated sludge. It is not thrown away as it stands. A small part of it is passed back into the aeration tank — because it is a ready-made, actively growing population of exactly the bacteria needed there, and returning it saves the plant from having to grow them afresh. (That is what “activated” means: alive and working.)
The major part of the activated sludge is pumped into large tanks called anaerobic sludge digesters. In these tanks, anaerobic bacteria grow and digest the bacteria and fungi present in the sludge. The bacteria that cleaned the water are, in their turn, digested.
During this anaerobic digestion, gases such as methane, hydrogen sulphide and \(CO_2\) are produced. This mixture is not a nuisance — it is a fuel, as you will see in the next section.
The effluents from these digester plants, after chlorination, are released into natural water bodies like rivers and streams. Chlorination kills pathogenic bacteria — the last safeguard, because reducing organic matter is not the same as making the water safe. The digested sludge is then disposed of.
Q5. Where does the dirty water that leaves our homes go, and who actually cleans it?
A. It goes to a sewage treatment plant, and the cleaning is done, above all, by microbes. First the plant physically removes what floats and what is coarse (screening and grit chambers), then lets the solids settle out (primary treatment). The real cleaning happens next: in the aeration tank, aerobic bacteria are supplied with air, they multiply into flocs, and they eat the dissolved organic matter — which lowers the BOD. The flocs are then settled out as activated sludge and handed to anaerobic bacteria in a digester, which break them down further and produce methane in the process. Finally the water is chlorinated to kill pathogens and released into a river. The engineering builds the tanks; the bacteria do the work.
Untreated sewage entering a river produces a predictable sequence: the organic matter feeds bacteria, the bacteria consume the dissolved oxygen, the oxygen level collapses, and fish and other aerobic aquatic life die. Stretches of Indian rivers downstream of large cities have suffered exactly this. It is why a sewage treatment plant is not a civic luxury but a condition for the river’s survival.
Oil-eating bacteria can clean up crude oil spills — the same principle as sewage treatment, applied to a pollutant floating on the sea. Collect more information about these bacteria.
11.12 Role of Microbes as Biofertilizers
Fertilizers: Two Very Different Kinds
Fertilizers are nutrients which are necessary for the growth of plants, and therefore for the productivity of cultivated plants. The use of fertilizers to increase productivity was, as you have seen, one of the pillars of the green revolution.
Fertilizers are classified as inorganic (chemical) and organic (biological).
Inorganic fertilizers are synthetic. Mineral salts of NPK — nitrogen, phosphorus and potassium — are mixed in definite proportions and dusted in the field. They act quickly, and they raised yields dramatically. But their non-judicious or excessive use leads to pollution of soil, air and ground water, and the soil becomes acidic. The nutrients that are not taken up by the crop are washed away into ponds, lakes and groundwater, where they cause problems of their own.
Organic fertilizers are biological in origin, and include Farm Yard Manure (FYM), compost and green manure. The use of these fertilizers increases the fertility of the soil.
Chemical fertilizer vs Biofertilizer (A chemical fertilizer supplies the nutrient directly — you spread the nitrogen salt and the plant absorbs it. A biofertilizer supplies living microorganisms, which then make the nutrient available — by fixing nitrogen from the air, or by dissolving phosphate that was already in the soil but locked up. One is a dose of nutrient; the other is a living workforce.)
What a Biofertilizer Is
Nowadays, for better and sustainable agricultural production, farmers use biofertilizers and practise organic farming.
Biofertilizers are mostly \(N_2\)-fixing living microorganisms, which enrich the nutrient quality of the soil. They include bacteria, cyanobacteria and fungi.
(Biofertilizers are commercial preparations of ready-to-use live bacterial or fungal formulations which, when applied to the plant, the soil or a composting pit, enrich the fertility of the soil through their biological activity.)
Because they are alive, biofertilizers work in a way a chemical never can: once established in the soil, they multiply, and they keep working season after season. Their use is cost-effective and eco-friendly, and they play a vital role in maintaining long-term soil fertility and sustainability.
Q4. Why would a farmer deliberately add living bacteria to a field instead of only adding chemical fertilizer?
A. Because the bacteria do work that the chemical cannot. Nitrogen-fixing microbes such as Rhizobium and Azotobacter take nitrogen straight from the air — a free, unlimited supply — and convert it into compounds the plant can absorb. Phosphate-solubilizing bacteria unlock phosphate that is already present in the soil but insoluble, so no new phosphate need be bought. Mycorrhizal fungi extend the root’s reach for water and minerals. And unlike a chemical fertilizer, which is used up, leaches away and acidifies the soil, these organisms are living, multiplying, non-polluting and cheap enough for a marginal farmer. The chemical is a one-time dose; the microbe is a permanent partner.
Types of Biofertilizers
On the basis of their nature and function, biofertilizers are divided into the following groups.
Classification of biofertilizers. On the basis of nature, or the group of organisms involved, biofertilizers are classified as bacterial fertilizers and fungal fertilizers. Bacterial fertilizers include eubacteria and cyanobacteria. On the basis of function, bacterial fertilizers are further grouped as nitrogen-fixing, phosphate-solubilizing and compost-making biofertilizers. Cyanobacterial biofertilizers, on the basis of function, are of the nitrogen-fixing type. Fungal biofertilizers include the mycorrhizal fungi, which, on the basis of function, are classified as ectomycorrhizae and endomycorrhizae.
The complete classification is shown in FC 11.14.
1. Nitrogen-fixing Biofertilizers
Nitrogen is the nutrient plants need in the largest quantity, and there is an enormous supply of it — the air is about four-fifths nitrogen. But plants cannot use \(N_2\) gas at all, because the triple bond holding the two nitrogen atoms together is extremely stable. The gas is, for a plant, useless.
Nitrogen-fixing microorganisms can do what the plant cannot. They convert atmospheric nitrogen into nitrogenous compounds like nitrites and nitrates, via ammonia. The route is shown in FC 11.15.
Nitrogen-fixing microorganisms are also called diazotrophs, and they are of two types:
i. Symbiotic \(N_2\)-fixing microorganisms: for example Rhizobium, Anabaena and Frankia. These are always associated, generally, with the underground parts — that is, the roots — of higher plants.
ii. Free-living, or non-symbiotic, \(N_2\)-fixing microorganisms: for example Azotobacter, Nostoc, Clostridium, Beijerinkia and Klebsiella. These fix nitrogen while living independently in the soil.
Symbiotic vs Free-living nitrogen fixers (Symbiotic fixers live inside or on a host plant and pass fixed nitrogen directly to it — Rhizobium in the root nodule of a legume. Free-living fixers live independently in the soil and enrich it; the plant benefits indirectly, from the soil around it.)
2. Phosphate-solubilizing Biofertilizers
These are the bacterial species which solubilize insoluble inorganic phosphate compounds, such as rock phosphate.
The problem they solve is a subtle one. Many soils are not short of phosphorus at all — the phosphorus is simply locked up in insoluble compounds that roots cannot absorb. These microbes secrete organic acids that dissolve those compounds, releasing phosphate into the soil solution where the root can take it up. The nutrient was always there; the microbe merely unlocks it.
Examples include Pseudomonas striata, Bacillus polymyxa, Agrobacterium, Microccocus and Aspergillus spp.
3. Compost-making Biofertilizers
Composting is a natural process that turns organic material into a dark, rich substance called compost or humus.
The composting process is dependent on microorganisms to break down the organic matter into compost. There are many types of microorganisms found in active compost — bacteria, fungi, actinobacteria, protozoa and rotifers.
Notice the difference from the previous two groups. These organisms do not add a new nutrient to the soil; they release the nutrients already present in dead plant and animal matter, and convert them into a form plants can use, while improving the soil’s structure at the same time.
4. Cyanobacteria as Biofertilizers
Many cyanobacteria are aquatic and terrestrial, free-living or symbiotic, aerobic, photosynthetic and \(N_2\)-fixing, and may be heterocystous or non-heterocystous forms. Examples are Anabaena, Nostoc, Plectonema and Oscillatoria.
Anabaena, Nostoc and Tolypothrix are associated with lichens, while Anabaena is associated with plants like Azolla and Cycas.
Cyanobacteria are unusually self-sufficient partners. Being photosynthetic, they make their own food; being nitrogen-fixing, they make their own nitrogen. A field of flooded rice, lit by the sun, is close to an ideal home for them — which is why they matter so much to rice cultivation.
Fungal Biofertilizers: Mycorrhiza
Mycorrhiza is a fungus which forms a symbiotic association with the underground parts — such as rhizomes and roots — of higher plants occurring in thick, humid forests.
The association was described by the German scientist A. B. Frank in 1885, while he was investigating the roots of forest trees. He found fungal threads wrapped so intimately around and within the roots that he could not regard the fungus as a mere infection, and proposed instead that fungus and root were living together to their mutual benefit. It was a radical suggestion at a time when a fungus on a root was assumed to be a parasite — and it turned out to describe one of the most widespread partnerships on the planet.
There are two types of mycorrhiza: Ectomycorrhizae and Endomycorrhizae. The distinction is exactly what the names suggest — one stays outside the cells, the other goes inside them.
I. Ectomycorrhizae: These have a well developed mycelium that forms a mantle on the outside of the roots. This increases the absorptive surface area of the roots and accelerates the uptake of water and nutrients (N, P, Ca and K). Because of this, the plant’s vigour, growth and yield all increase. Some hyphae of the mycorrhizal fungus penetrate into the root and form the hartig-net in the intercellular spaces of the root cortex — that is, they run between the cells, but never enter them.
II. Endomycorrhizae: These grow in between and within the cortical cells of the roots. The fungal hyphae penetrate the cells and form finely branched arbuscules intracellularly, and form vesicles mostly in the intercellular spaces of the cortical cells. Hence they are called Vesiculo Arbuscular Mycorrhizae, or VAM. Nowadays they are described as AM fungi.
Plants with VAM grow luxuriantly in less irrigated lands. The association of VAM with crop plants helps in converting a less productive field into a more productive one.
Ectomycorrhizae vs Endomycorrhizae | Feature | Ectomycorrhizae | Endomycorrhizae | |—|—|—| | Position of the fungus | Forms a mantle outside the root | Grows between and within the cortical cells | | Penetration | Hyphae enter only the intercellular spaces, forming the hartig-net | Hyphae penetrate the cortical cells themselves | | Special structures | Mantle and hartig-net | Arbuscules (intracellular) and vesicles (mostly intercellular) | | Other name | — | Vesiculo Arbuscular Mycorrhizae (VAM), or AM fungi |
the fungal mantle and the arbuscule (The fungal hyphae are far finer than a root hair → they penetrate soil pores the root cannot enter, and they extend the absorbing surface enormously → water and mineral ions, especially phosphorus, are gathered from a much larger volume of soil → the arbuscule, finely branched inside the cortical cell, has a huge surface area of contact with the plant cell → nutrients are handed over efficiently, and the plant returns sugars to the fungus. Both partners gain, which is why the association is a symbiosis and not a disease.)
Benefits of Mycorrhiza: 1. Selective absorption of P, Zn, Cu, Ca, N, Mn, Br and Fe. 2. Enhanced water uptake. 3. Induction of growth, by secreting hormones. 4. Protection of the host plant from other microbes, by secreting antibiotics.
Nowadays, mycorrhizae are classified into 8 different types: Ectomycorrhizae, Endomycorrhizae, Ectendomycorrhizae, Orchidaceous mycorrhizae, Ericoid mycorrhizae, Arbutoid mycorrhizae, Monotropoid mycorrhizae and Ophioglossoid mycorrhizae.
Biofertilizer Microorganisms
1. Rhizobium
Rhizobia are rod-shaped, motile, aerobic, gram-negative, non-spore-forming, nitrogen-fixing bacteria, containing Nod genes and Nif genes. (The names tell you the jobs: the Nod genes are needed to bring about nodule formation, and the Nif genes for nitrogen fixation itself.)
They form a symbiotic association with the roots of leguminous plants. They bring about the formation of nodules on the roots, and multiply inside the nodule. Within it, they fix atmospheric nitrogen into organic forms, which can be used by the plant as nutrients. The root system of a leguminous plant is shown in Fig. 11.15, and the internal structure of a nodule in Fig. 11.16.
For example, R. leguminosarum is specific to pea, and R. phaseoli to beans. That specificity is important — a Rhizobium strain that nodulates one legume may fail entirely on another, which is why the right inoculum must be matched to the right crop.
Why are healthy root nodules pink in colour?
The old practice of rotating a cereal crop with a legume, or of ploughing a legume back into the field as green manure, works because of Rhizobium. The legume’s nodules leave the soil richer in nitrogen than they found it, so the cereal that follows needs less fertilizer. Farmers were exploiting biological nitrogen fixation for centuries before anyone knew that a bacterium was responsible.
2. Azotobacter
Azotobacter is an important and well known free-living, nitrogen-fixing, aerobic, non-photosynthetic, non-nodule-forming bacterium, which is intimately associated with the roots of grasses and certain plants.
Because it needs no nodule and no host, it is used as a biofertilizer for all non-leguminous plants — especially rice, cotton and vegetables. This is precisely why it matters: Rhizobium can serve only the legumes, and most of our food crops are not legumes.
3. Azospirillum
Azospirillum is a free-living, aerobic, nitrogen-fixing bacterium associated with the roots of corn, wheat and jowar.
It fixes a considerable quantity of nitrogen — 20–40 kg N/ha — in non-leguminous plants such as cereals, millets, cotton and oilseeds. That is a substantial saving of chemical nitrogen fertilizer, obtained from a living organism that costs almost nothing to apply.
4. Anabaena
Anabaena is a genus of multicellular, filamentous cyanobacteria that exists as plankton. It has the ability to fix nitrogen, and also forms symbiotic relationships with certain plants — such as the coralloid roots of Cycas, and the Anthoceros thallus.
It has some specialized, colourless cells called heterocysts, which are the sites of nitrogen fixation.
the heterocyst (The enzyme that fixes nitrogen is destroyed by oxygen → but Anabaena is photosynthetic and releases oxygen in its ordinary cells → so nitrogen fixation is confined to the heterocyst, a thick-walled, colourless cell that does not carry out oxygen-releasing photosynthesis → the enzyme is thus protected from the oxygen the rest of the filament produces. Two incompatible processes are separated by putting them in different cells.)
5. Azolla
Azolla is a free-floating water fern. The Azolla plant consists of a floating rhizome (stem) with small, overlapping, bi-lobed leaves and roots. The leaf shows a dorsal and a ventral lobe, as seen in Fig. 11.17 and Fig. 11.18.
In the dorsal lobe, filaments of Anabaena are present in the aerenchyma (the air-filled tissue), and it is these filaments that fix nitrogen.
Azolla can be used as a biofertilizer in rice fields. The arrangement suits the rice field perfectly: the fern floats on the standing water, its Anabaena fixes nitrogen from the air above, and when the fern dies and decays it releases that nitrogen into the water and mud where the rice roots are waiting.
Azolla in a paddy field (Zoom in: a filament of Anabaena fixing \(N_2\) inside a cavity in a fern leaf. Zoom out: a green carpet of Azolla on a flooded paddy field, quietly fertilising the rice crop growing through it.)
Benefits of Biofertilizers
- They are low in cost, and can be used even by marginal farmers.
- They are free from pollution hazards.
- They increase soil fertility.
- BGA (blue-green algae) used as biofertilizers secrete growth-promoting substances, organic acids, proteins and vitamins.
- Azotobacter supplies both nitrogen and antibiotics to the soil.
- Biofertilizers improve the physico-chemical properties of the soil — its texture, structure, pH and water-holding capacity — by providing nutrients and organic matter.
Point 6 is the one that separates a biofertilizer from a chemical most sharply. A chemical fertilizer feeds the crop and, over the years, degrades the soil; a biofertilizer feeds the crop by improving the soil.
Many biofertilizers are now available in the market in our country, so as to reduce the use of chemical fertilizers, and thereby the pollution they cause.