eChapters

24.1 Introduction

In India, the commercial cultivation of oil palm started during 1970. Today about 16 countries of tropics cultivate oil palm. The palm oil has been safe and nutritious source of edible oil for human consumption for 1000 years. Palm oil and its liquid fraction palmolein is consumed worldwide as cooking oil. Malaysia, Indonesia, Nigeria and Columbia started to grow oil palm in a large scale during 1980. The commercial producing countries are Liberia, Peru, Angola, Senegal, Ghana, etc.

23.1 Introduction

Coffee is the second important popular beverage for its aroma and mild stimulant action. Its dried beans are roasted, ground and brewed to make a stimulating and refreshing beverage. Use of coffee has evolved from original chewing of leaves and beans of the plant to relieve pain, hunger and fatigue to the present more sophisticated uses, like espresso and decaffeinated coffees. Its use was first discovered in Arabia around mid-15th century.

22.1 Introduction

Tea is the most important non-alcoholic beverage gaining further popularity as an important “health drink”. It is consumed as a morning drink by 2/3rd of world population. Tea was initially used as a medicine and subsequently as beverage and now has proven to be a potential raw material for pharmaceutical industry. Tea is mainly consumed in the form of ‘fermented tea’ or ‘black tea’, ‘non-fermented tea’ or ‘green tea’ and ‘semi-fermented tea’ or ‘oolong tea’. Green leaves are also used as vegetables such as ‘leppet tea’ in Burma and ‘meing tea’ in Thailand. Tea leaves have more than 700 chemical constituents among which flavonoids, amino acids, vitamins (C, E, K), caffeine and polysaccharides are important to human health. The stimulating effect of tea is due to caffeine (1.25-4.5%). Per cup value of caffeine in tea is three and half times less than that of coffee and is less harmful than coffee. Importantly, the vitamin C content in leaves is comparable to that of lemon.

21.1 Introduction

Cashewnut or cashew is an export-oriented crop grown for its nuts, a true drupe. It is often referred to as ‘wonder nut’. It is highly delicious and nutritive. Its nutritional composition compares well with almond, hazelnut and walnut. The economic part is kernel which is a unique combination of fats (47%), protein (21%), carbohydrates, minerals and vitamins. Nutritionally it is equivalent to milk, egg and meat, it is good appetizer, an excellent nerve tonic, stimulant, body builder. India is the largest producer, processor and exporter; and second largest consumer in the world. Cashew is grown in Kerala, Karnataka, Goa and Maharashtra along the west coast and in Tamil Nadu, Andhra Pradesh, Odisha and West Bengal along the east coast.

20.1 Introduction

Arecanut (or betel palm) is chewed as narcotic either alone or more commonly along with betel leaf. Its nut has medicinal and pharmaceutical properties. About 1/10th of world population has a habit of betel chewing. Nut contains polyphenol (11-18 %) which includes flavonols like catechin, epicatechin, leucocyanidin, fat (9-15%), polysaccharides (17-25%), fibre and protein (6- 7.5%) and some minerals like calcium, phosphorous and iron.

19.1 Introduction

Coconut is valued most for its nut. The other components are coconut water, kernel, shell and husk. Coconut water is one of the most refreshing drinks. It is also known for its medicinal value. It increases blood circulation in kidneys, causes profuse diuresis and eliminates mineral poisoning. It is a good substitute for saline glucose under gastroenteritis conditions. The coconut has been regarded as ‘Kalpavriksha’ because all the parts of the palm is useful to mankind in one way or other. The dried kernels commonly called as copra is the richest source of vegetable fat (60-67 % oil). Coconut is major source of edible oil. India is the largest producer of coir and its product. The major coconut growing areas of the world are found in Asia, Oceania, West Indies, Central South America, East and West Africa. In India, coconut is commercially grown in Kerala, Karnataka, Tamil Nadu, Andhra Pradesh, Gujarat, Odisha and West Bengal.

18.1 Introduction

Among nut crops, ‘Walnut’ or ‘Akhrot’ is the most important crop. It has immense commercial importance even in the areas situated in remotes without having good market and transport infrastructure. Because of long storability, growers have opportunity to hold the produce at room temperature for two to three months easily and sell out at intended time. Walnut is rich in protein, fats and vitamins, especially vitamin B6 . The walnut kernel contains about 90% PUFA of which 26% is the Omega-3 fatty acid, alpha-linolenic acid. But the problem with this type of kernel is that higher the percentage of PUFA in it, the more is the chances of getting the oil oxidized and thereby reducing the shelf life of the walnut kernel.

17.1 Introduction

Almond or badam is a high energy nut. Almond kernels contain lipids (52.2%), protein (20%), carbohydrate (20.4%) and water-soluble sugars (4.4%). Irrigation affects almond yield and quality. Almond quality is primarily genotype dependent. Maturity at harvest can also influence kernel quality.

16.1 Introduction

Peach is one of the important fruits grown in warm temperate zones of the world. Fruits are rich in protein, sugar, minerals and vitamins. Peaches are highly valued as table fruit for their attractive colour and palatability. Canned peaches, dried frozen preserves, jams, nectar are principal processed products of peach. It is grown mainly in Jammu and Kashmir, Himachal Pradesh, Punjab, Uttarakhand, Nilgiri hills, Jharkhand and North-Eastern States.

15.1 Introduction

Pear is one of the important fruit crops that belong to the family Rosaceae. In the world pear is cultivated in China, USA, Argentina, Italy, Turkey, Spain, India, South Africa, Japan, Belgium etc. It is grown under a wide variety of climatic regimes ranging from cold dry temperate hilly conditions to warm humid sub-tropical conditions on the plains of northern India. In India pear is widely cultivated in Punjab, Himachal Pradesh, Jammu and Kashmir, Uttar Pradesh, Uttarakhand etc.

14.1 Introduction

Apple is the most widely grown temperate fruit of the world. It is considered as King of temperate fruits. It is rich in carbohydrate, calcium, phosphorous, iron, potassium, vitamin B6 . Apple is rich in antioxidant, flavonoids and dietary fibre. It is recommended to reduce the incidence of dental caries. The phytonutrients and antioxidants may help reduce the risk of developing cancers, hypertensions, diabetes and heart disease. Apple is mainly used for table purpose. The various processed products are prepared from fruit, e.g., juice, jelly, preserve, canned slice, wine, concentrated juice, cider and powder. It is widely cultivated in Afghanistan, Armenia, Azerbaijan, Bhutan, China, India, Indonesia, Iraq, Iran, Israel, Japan, Jordan etc.

13.1 Introduction

Pomegranate (Punica granatum L.), an economically important fruit plant species, belongs to the family Punicaceae. It is an important fruit crop of arid and semiarid regions of the world and has been believed to be the symbol of fertility and abundance. Pomegranate is a highly remunerative crop for replacing subsistence farming and alleviating poverty. The plant is drought tolerant, winter hardy and can thrive well under desert condition. It is a good source of protein, carbohydrate, minerals, antioxidants, vitamins A, B and C, and has also been used in controlling diarrhoea, hyperacidity, tuberculosis, leprosy, abdominal pain and fever. Pomegranate juice contains antioxidants such as soluble polyphenols, tannins, anthocyanins and can be used in the treatment of cancer and chronic inflammation. It has enjoyed a reputation for its healthy dietetic and medicinal properties.

12.1 Introduction

Aonla (also called Amla, Indian gooseberry) is a tropical and sub-tropical fruit. It is mainly recognized due to its high nutritive, medicinal and therapeutic properties. It contains high amount of vitamin C (500-1500 mg/100 g); it is also rich in pectin, iron, calcium and phosphorus. Dried fruits of aonla are used in curing haemorrhage, diarrhoea, chronic dysentery, diabetes, jaundice, dyspepsia, cough etc. Aonla is the main ingredient of Chavanprash and Triphala powder. The fruit contains gallic acid and leucoanthocyanin which have antioxidant property. Its cultivation is more common and widely grown in Uttar Pradesh, Gujarat, Rajasthan, Madhya Pradesh and Tamil Nadu.

11.1 Introduction

Ber (also called Indian Jujube, Indian Plum, Chinese Apple and Masau) is an important minor fruit of India. It is most widely cultivated in Punjab, Haryana, Rajasthan, Uttar Pradesh, Madhya Pradesh, Bihar, Gujarat, Maharashtra etc. Fruits are mostly eaten fresh but other forms, such as dried, candied, squash etc. can be prepared from ber. Stem, bark, root and leaves have some medicinal values. Leaves of ber are used as fodder in dry regions.

10.1 Introduction

Litchi is a delicious fruit and generally consumed as a table fruit. It is one of the most popular fruits in India. It is also recognized as “Queen of Fruits”. It is a subtropical evergreen fruit crop. Its climatic requirements are highly specific. Thus, its commercial cultivation is restricted to only a few subtropical countries of the world. The major litchi producing countries are China, India, Taiwan, Thailand and Vietnam. In India, the important litchi growing states are Bihar, Jharkhand, West Bengal, Tripura, Uttar Pradesh, Uttarakhand, Chhattisgarh, Punjab and Himachal Pradesh. Litchi makes an excellent canned fruit and highly flavoured squash is prepared from inferior fruits.

9.1 Introduction

Pineapple is one of the most important commercial fruits of the world. It is good source of vitamin A, B and fairly rich in vitamin C, minerals like calcium, phosphorous and iron. Fresh fruit is used as dessert. Fruit is also used for juice, squash, jam, mixed jam and in canned form. Fruit core is used for preparing candy. Some cultivars of pineapple are used for extraction of fibre. The important pineapple growing states are Assam, West Bengal, Tripura, Kerala, Andhra Pradesh, Bihar, Karnataka and Madhya Pradesh.

8.1 Introduction

Guava is the fourth important fruit crop next to mango, banana and citrus. It is known as the ‘Apple of tropics’. It is very rich and cheap source of Vitamin C and also contains a fair amount of calcium. Important guava growing states are Uttar Pradesh, Bihar, Madhya Pradesh and Maharashtra. Allahabad district of Uttar Pradesh has the reputation of growing the best quality of guava fruits in the world. The importance of guava is due to the fact that it is the hardy fruit which can be grown in alkaline and poorly drained soil. Guava fruits are used for making jam, jelly and various culinary purposes. The fruit can be canned in sugar syrup.

7.1 Introduction

Papaya is a fruit crop grown in kitchen garden, backyards of home and orchards. It is grown as a filler crop in orchard. India is the largest producer of papaya in the world. It is also cultivated in Brazil, Mexico, Australia, Hawaii, Malaysia, Taiwan, Peru, Florida, Gold Coast, South Africa and Bangladesh. In India, it is widely cultivated in Uttar Pradesh, Bihar, Karnataka, Jharkhand and Madhya Pradesh. The ripe fruits of papaya are consumed throughout the tropics and sub-tropics. Fruits are also used in the preparation of jam, soft drinks, ice cream flavour, crystallized fruits and syrups. Unripe fruits are commonly used as vegetables. Papain is prepared from the latex of immature fruits. It is a proteolytic enzyme used for tendering of meat, preparation of chewing gum and cosmetics. The ripe fresh fruit is rich source of vitamin A, carbohydrates and minerals. The fruits are prescribed for piles, dyspepsia, liver, spleen and digestive disorders. Roots are used as nerve tonic. The plants flower five months after planting and fruits are ready five months after flowering. So, it has attracted attention of fruit growers to cultivate on a large scale in the country.

6.1 Introduction

Grapes are being grown commercially in the tropics. The region between the tropics of Cancer and Capricorn which was once considered to be unsuitable for growing quality grapes is producing good quality grapes. Tropical countries where grapes are grown are India, Indonesia, the Philippines, Thailand, Taiwan, South China, Australia, Brazil, Columbia, Ecuador, Mexico, Venezuela, West Indies, Kenya, Nigeria and Zimbabwe. In India, about 90% of grapes are being produced in the tropical states including Maharashtra, Andhra Pradesh, Karnataka and Tamil Nadu. There are three main commercial uses of grape, viz., table, raisin and wine.

5.1 Introduction

Citrus is one of the most important fruit crops in the world. It is grown in the tropical and sub-tropical regions of the world. The top ten citrus producing countries are China, India, Nigeria, Brazil, Mexico, USA, Spain, Egypt, Italy and Argentina. The citrus industry in India is the third fruit industry after mango and banana. The important groups of crops in citrus are sweet orange, mandarins, limes, lemons, grape fruits and pummelo. The most important commercial Citrus species grown in India are mandarin (Citrus reticulata Blanco.), followed by sweet orange (Citrus sinensis (L.) Osbeck) and acid lime (Citrus aurantifolia Swingle). The major citrus producing states are Maharashtra, Karnataka, Andhra Pradesh and Punjab.

4.1 Introduction

Banana (Musa spp.) is one of the oldest fruits and second largest growing fruit crop in the world. It is also known as Adam’s Fig and Apple of Paradise. Important banana growing states are Maharashtra, Karnataka, Kerala, Tamil Nadu, Andhra Pradesh, Odisha, Bihar, West Bengal and Assam. In South India, banana is extensively used in all auspicious occasions such as wedding, festival and worship. Banana is a good table fruit. The cultivar Nendran is used for cooking; it is also used for preparation of halwa, sweet, chips etc. Main bud or heart of banana bunches is also used as vegetable in India, Thailand, Philippines, Malaysia and Indonesia. Musa textiles is known for strong fibre quality. Ripe banana contains about 20% sugar (but no starch) and about 4.7% protein. Further, it contains vitamins A, B, C, D and E and minerals such as K, P, Ca and Fe.

3.1 Introduction

The Mango (Mangifera indica L.) belonging to the family Anacardiaceae in order Sapindales, is amongst the most important tropical fruits of the world. In India, it is also known as the ‘king of fruits’ owing to the delicious quality of the fruit rich in vitamins and minerals. Its long period of domestication is evident from its mention in the ancient Indian scriptures. In India, mango has been valued not only for religious consideration but also for its importance in the economic and cultural life of the society. Mango is rich in Vitamin A and C, flavonoids, carotenes, glucosides, sterols, terpenes, aromatic acids, essential oils, fatty acids and phenolics. It is a powerful nutritive fruit, containing most of the essential substances needed by the human body. The most important single factor limiting the production of mango fruit at the present time is the erratic fruiting habits of this species. Some cultivars produce a good crop in most years and a poor crop in other years, while still other cultivars produce only a few fruits each year. For many years new cultivars have been selected and grown on the basis of size, appearance, and dessert quality with very little attention given to their production capabilities. The improvement of mango needs to explore new recombinants primarily by means of exploiting the breeding methodologies. Diversity or heterogeneity is the main character desired for breeding either natural or manmade. It is required to have vast genetic pool to get new combinations of desired nature and developing new hybrids. It has been mentioned earlier that the development of mango in India is the result of selection from the amateur gardeners. Now, the scientists have developed certain hybrids of mango.

2.1 Introduction

The different breeding strategies (methods) that can be employed for improvement of fruit crops include introduction, open pollinated selection, clonal selection, hybrid breeding, polyploidy breeding, mutation breeding, molecular breeding, in vitro breeding and transgenic breeding. Combining traditional approaches with biotechnological approaches will speed up the rate of genetic improvement. Thus, what is required is to adopt a coordinated approach involving breeders, biotechnologists, horticulturists, pathologists, entomologists and statistician for making the cultivation of fruit crops more scientific and more profitable and thereby contributing more to improving the health of the people in a sustainable manner.

1.1 Introduction

Fruit breeding is an ancient technology with dynamic current technology and an exciting future (Janick and Moore 1975, 1996). In its broadest sense, fruit breeding refers to the purposeful genetic improvement of fruit crops through various techniques including selection, hybridization, mutation induction and molecular techniques. Its origins trace to the domestication process in prehistory and antiquity, where useful species were chosen and cultivated, and improved by continuous selection (Janick 2005, 2011). Through the millennia genetic improvement of these species have been achieved by grower selection, first from natural seedling populations and then from grower field that continued unique genotypes fixed by vegetative propagation. Spontaneous hybridization between wild plants and cultivated clones was critical to the early domestication of fruits.

29.1 Introduction

An environmental factor that interferes with the complete expression of genotypic potential of a crop is called stress. The stresses may be biotic (pathogens, insect-pests, etc.) or abiotic. The main abiotic stresses are drought, mineral deficiency/toxicity and heat or cold. The losses due to abiotic stresses can be minimized by:

Crop management: The degree of stress can be reduced by suitable agronomic/soil management practices, e.g., irrigation to relieve drought stress, soil amendments to reduce salinity.

Development of resistant cultivars: This offers the cheapest, easiest to apply and most eco-friendly approach to reduce losses due to any stress.

Indirect breeding: Evaluation of materials under stress often helps in identification of a resistant line that may become an important commercial cultivar.

???????Direct breeding: It is deliberate breeding of materials for stress resistance. When a genetically variable material is grown under the stress environment, selection can be based on survival, yield and other traits contributing to stress resistance.

28.1 Introduction

The stress that is caused by biological agents or factors such as diseases, insect-pests and parasitic weeds is called biotic stress. The term disease can be broken into two words: “dis” (means negative, reverse or opposite) and “ease” (means comfort, or freedom from pain or discomfort). Hence, plant disease can be defined as any harmful alteration from the normal functioning of physiological processes. Diseases may be bacterial, fungal or viral. A wide range of insects infests various crop plants. The extent of damage caused to crop plants by various organisms is usually in the order fungi > bacteria > viruses > nematodes = insects. But the relative importance of these organisms varies to a great deal in different crops. Breeding of cultivars resistant to diseases and/or insect-pests is the cheapest, safest and most dependable means of limiting the losses due to them. Resistant cultivars form an important component of integrated management of diseases and insect-pests.

28.2 Breeding for Resistance to Biotic Stresses-Diseases

Agrios (1970) has defined disease as: “Disease is a series of invisible and visible responses of plant cells and tissues to a pathogenic microorganism or an environmental factor that result in adverse changes in form, function or integrity of plant and may lead to partial impairment or death of the plant or its parts.” The organism causing disease is called pathogen, while the plant affected by the disease is called host.

The principles underlying breeding for resistance to diseases are much the same as for other characters, except for one important difference, i.e., the diseases caused by pathogens are the product of interaction between two genetic systems, that of the host and that of the parasite. Both systems are capable of variation and evolution and affect the interaction between host and pathogen. Here the breeder is required to deal with the heritable traits of both the host plant and the pathogens invading it.

27.1 Introduction

Crop improvement depends on genetic variation present in nature or created by hybridization followed by segregation and recombination. Again, gene transfers are possible only from the lines and species that are sexually compatible with the concerned species. Interspecific gene transfers pose many problems, including the loss of F₁ hybrids during embryo development due to abortion of the endosperm in the developing hybrid seeds. Embryo rescue technique was developed to rescue such interspecific hybrid embryos, and it has been widely used for this purpose. It is to be mentioned that embryo rescue can be used only in such cases where sexual hybridization is successful in producing zygotes that begin to develop into embryos.

Besides, the technique of somatic hybridization by protoplast fusion was developed to enable gene transfer from the species that do not produce zygotes on hybridization. This method has been extensively used and some useful lines have been derived. But this method is limited to utilization of genes present in plant species. Again, the results of somatic hybridization are unpredictable since many somatic hybrids fail to regenerate complete plants and the transfer of specific genes or groups of genes depends entirely on chance.

The recombinant DNA technology (also called genetic engineering) has permitted the transfer of genes from any organism into any organism. Besides, chemically synthesized genes can be transferred and integrated into genomes of various organisms to enable the expression of these genes. A gene transferred into an organism by genetic engineering is called a transgene. 

26.1 Introduction

We have already discussed the basics of tissue culture techniques in Chapter 2. Plant tissue culture techniques in combination with recombinant-DNA technology are essential requirements for development of transgenic plants (will be discussed in next chapter). However, tissue culture techniques like anther, pollen, ovule and meristem culture can themselves be used for improvement of different traits or as an aid to conventional breeding methods. In recent years, isolated microspore culture has been preferred as a breeding tool and an experiment system for various genetic manipulations. Besides, using protoplast fusion and hybrid embryo rescue, distant hybridization has been successfully achieved in many crops. Protoplast fusion technique can be used for the transfer of cytoplasmic male sterility from one species to another in very short duration. Somaclonal variation generated during tissue culture can also be utilized for recovery of variants showing disease resistance or other desirable characters.

25.1 Introduction

The method of selection in which a gene/QTL (Quantitative Trait Locus) is selected on the basis of the genotype of the molecular marker (also called DNA marker) linked to this gene/QTL is known as marker-assisted selection (also called marker-aided selection). The idea of marker-assisted selection (MAS) was first advocated by Tanksley (1983). MAS can be used in combination with the conventional breeding methods or novel breeding schemes could be devised to take full advantage of the marker data. MAS has been widely used for introgression of the desired genes and QTLs using backcross programmes.

25.2 Marker-Assisted Backcrossing (MABC)

The backcross programme based on molecular marker is usually called marker-assisted backcrossing (MABC). MABC has three activities: (a) foreground selection, (b) background selection and (c) recombinant selection.

25.2.1 Foreground Selection

The use of molecular marker to select the gene/QTL being introgressed is known as foreground selection. The objective of foreground selection is to select the target gene, i.e., the gene being introgressed, without phenotypic evaluation for the trait governed by the target gene. Foreground selection for a gene/QTL will be highly desirable in a variety of situation, some of which are listed below:

24.1 Introduction

When crosses are made between two different species of a genus or between two different genera, they are known as distant hybridization (also called wide hybridization). It includes both interspecific and intergeneric hybridization. Thomas Fairchild (1717) was the first man to do distant hybridization. He produced a hybrid between two species of Dianthus, viz., D. caryophyllus and D. barbatus (Carnation and Sweet William). Intergeneric hybrid produced by Karpechenko, a Russian scientist, in 1928 is Raphanobarssica. It is the amphidiploid of cross between radish (Raphanus sativus) and cabbage (Brassica oleracea var. capitata). Unfortunately, it had roots like cabbage and leaves like radish. Triticale was produced by Rimpau in 1890 itself. It is an amphidiploid obtained from a cross between wheat and rye (Triticum and Secale). Another example is Saccharum nobilisation involving Saccharum officinarum, S. barberi, S. sinensis, S. spontaneum. Wide hybridization has been used in the genetic improvement of some crops. It is an effective means of transferring desirable genes into cultivated plants from related species and genera.

23.1 Introduction

An organism or individual having more than two basic sets of chromosomes is called polyploid and the condition is known as polyploidy. Polyploidy is an important tool to increase genetic diversity by altering the chromosome number (2x) and consequently the number of genes within a single cell. Polyploidy has been one of the important mechanisms in the evolution of many cultivated crop plants.

23.2 Types of Polyploidy

23.2.1 Autopolyploidy

The condition that arises due to multiplication of the same chromosomes of a single species is known as autopolyploidy (also called ‘simple polyploidy’ or ‘single species polyploidy’). Autopolyploids have generally larger leaves, stem and other vegetative parts and flower, fruit and seeds than their normal diploids.

22.1 Introduction

Mutations are sudden heritable changes in the genotype of an organism. De Vries (1900) for the first time used the term mutation (mutare in Latin means ‘to change’) for the appearance of new types in evening primrose (Oenothera) plant. In 1901 he put forth the idea of producing mutations artificially for use in crop improvement. Alberto Pirovano (1922) was first to use X-rays and ultraviolet rays for inducing mutations in plants, but his work remained unknown. So, Muller (1927) became the first known to induce mutation in Drosophila fly using X-rays. Though Stadler (1928) had started working simultaneously on barley and maize and induced mutations, he was late in reporting mutation due to longer life cycle of these plants compared to Drosophila. Mutation may be gene mutation (the result of change in the nuclear gene or plasma gene) or chromosomal mutation (change in chromosome number, structure or chromosomal aberrations of various types).

21.1 Introduction

There are a number of field, vegetable, fruit and ornamental crops that are cultivated using asexual or vegetative parts such as stems, roots, modified flower etc. This is a heterogeneous group of plants and can be further categorized into four groups for ease in understanding the breeding methods applied to them. They are:

Group 1: Flowerless or Sterile

This group either does not bear flowers or if bears, they are sterile and do not set seed, e.g., ginger, mango ginger, turmeric, garlic, betel, many yams, etc.

Group 2: Apomicts

Apomixis has been reported in over 300 genera. But stable apomixis has been reported only in a few cultivated plants like meadow grass, citrus, mango, raspberry, etc.

Group 3: Flowers but Seed-Set Rare

In this group flowering takes place under specific situations but seed-set is limited, e.g., potato, sweet potato, sugarcane, etc.

20.1 Introduction

A hybrid cultivar (also called hybrid) is the F₁ progeny of a planned cross between inbreds, cultivars, clones or populations. Depending upon the breeding approach, the hybrid cultivar may involve two or more parents. A critical requirement of hybrid production is that the parents are divergent. This divergence is the cause of superior performance of hybrids. The outstanding performance of crops is due to the exploitation of the phenomenon of heterosis (hybrid vigour) which is high when parents are divergent. Much of what we know about heterosis breeding (also called hybrid breeding) came from the work on maize hybrid cultivar development. However, commercial hybrids are now available in many crops including self-pollinated crops.

19.1 Introduction

In self-pollinated crops, the methods of selection aim at improving individual plants. On the other hand, in cross-pollinated crops they aim at improving a population of plants. A population is a large group of interbreeding individuals. Hence, selection is called population improvement in cross-pollinated crops. Selection in self-pollinated crops results in isolation of pure breeding lines which is the end product. But in cross-pollinated crops, it results in an improved population which is used as the commercial cultivar. Improved inbred lines (also called inbreds) for producing hybrids can be obtained from such populations. Again, like many rounds of selection being practised in different inbreeding generations in self-pollinated crops we are also practising many rounds of selection in cross-pollinated crops. But, unlike self-pollinated crops selection in cross-pollinated crops is followed by intermating to develop a new population before starting another round of selection. One cycle of selection consists of selection of individual/family and intermating of the selected units. Such cycle is repeated till we get the improved population. This method of practising the cycles of selection and recombination in a population is called recurrent selection or population improvement. Frey (1983) prefers the use of recurrent selection for closed populations where no new germplasm is added to the population during selection. Population improvement was used for a group of breeding techniques where all aspects are similar to recurrent selection but with the provision of adding new sources of germplasm whenever feasible. It is only a matter of coincidence that past history of application of recurrent selection has been limited to closed populations, but its basic aim has always been the accumulation of favourable genes, without any restriction on the migration of new genes during the course of selection. Apart from a slight nuance, population improvement and recurrent selection are two titles of the same principle and procedure of manipulating the genetic potential of populations. Recurrent selection can also be applied in self-pollinated crops.

18.1 Introduction

The genetic variability present in populations of self-pollinated vegetable, spice and ornamental crops is rapidly exhausted by pureline selection. For further improvement, genetic variability must be created. This is generally achieved by hybridization. After the cross has been made, F₁ is grown and seeds set on it are harvested. This makes the seed source for raising F₂ population, and so on. The segregating populations from crosses are handled according to a suitable method. The three basic methods available for this purpose are: (1) pedigree method, (2) bulk method, and (3) backcross method. There are also certain modifications of these methods.

18.2 Pedigree Method

Pedigree method is developed from the pureline system and widely used method of plant breeding for self-pollinated crops. A main difference between pedigree selection and mass selection or pureline selection is that hybridization is used to create variability for the base population in case of the former, whereas in mass and pureline selections variability is naturally present in the base populations. The pedigree method was first described by H.H. Lowe (1927). This method predominated the crop improvement programme throughout the world until 1980s. Pedigree method is common in horticultural crops, especially where readily identifiable qualitative traits are involved. Vegetable crops in which pedigree system is used include tomato, cucumber, pepper, pea, lettuce, snap and field beans, all self-pollinated crops. In general, selection is done on F₂ plants. Yield is important in many horticultural plants, but quality is equally or even more important. Quality may be associated with identifiable traits, making the pedigree system ideal for self-pollinated vegetable crops.

17.1 Introduction

Self-pollinated vegetable, spice and ornamental crops may be heterogeneous. It is exemplified by landrace cultivars. They may have originated from single plant selections but have been grown over a long period of time. Landrace cultivars are a mixture of genotypes, perhaps because of breeders’ lack of appreciation of selection techniques to produce a pure line, spontaneous mutations, chance hybridization, or a combination of factors. Natural selection in landraces has resulted in a cultivar that is well adapted to the local environment. Depending on amount of human selection, differences among genotypes in a landrace cultivar may be small and inconspicuous. On the other hand, a lack of human intervention may mean that differences among plants in a landrace cultivar will be conspicuous. After years of self-pollination, individual plants are homozygous. Selection of such individuals would result in a true-to-type reproduction of their phenotype within the limitations imposed by the environment.

16.1 Introduction

Plant introduction is the process of introducing plants from their growing locality to a new locality. Introduction of plants from a foreign country is called foreign introduction or exotic collection. Introduction from one state to another within a country is called indigenous collection or indigenous introduction. With introduction, the process of acclimatization is associated. Acclimatization is the adaptation or adjustment of an individual plant or population of plants under the changed climate for a number of generations. Plant introduction is an oldest method of plant breeding. It is used in self-pollinated, cross-pollinated and asexually propagated crops. The material of seed-propagated crops is introduced in the form of seed and that of vegetatively propagated crops in the form of cuttings or propagules. The introduced material may be a new crop, a wild relative or a new cultivar of a crop already grown in the area. There are five important sources of plant collection, namely, (1) centres of diversity, (2) gene banks, (3) gene sanctuaries, (4) seed companies, and (5) farmers’ fields.

15.1 Introduction

Plant breeding aims at developing genetically improved crop cultivars with economic benefits for small-scale and commercial farmers. Population growth, dwindling agricultural land and global climate change present increasing risk to crop production. Consequently, plant breeding aims to constantly develop crop cultivars with improved yield and quality and tolerant to drought, diseases and pests. Use of genetically improved crop cultivars and better management practices are among the best strategies to increase food production and meet a projected doubling of food demand in the next 40 years. Both the public and private plant breeding sectors face several constraints, including a shortage of professional plant breeders and breeding technicians, limited budget, inadequate facilities and lack of access to information. Successful plant breeding requires adequate infrastructural investment, in addition to the actual breeding activities. The benefits of plant breeding research and development can only be realized on a long-term basis because of the inherent nature of the crops and the eminent breeding procedures involved. Plant breeding involves two main activities, i.e., pre-breeding/ germplasm enhancement and cultivar development per se. These interdependent activities are the controlling factors that determine the pace at which cultivars are released timeously and consistently to farmers. The two phases, in turn, depend upon various factors, e.g., breeding goals, genetics and agronomy of the crop, breeder’s long-term objectives, availability of testing facilities and national cultivar-registration requirements. Despite these factors, there are certain clear steps and breeding procedures found in any conventional breeding programme. These include choice of parents, making crosses among chosen parents, and selection from recombined parents followed by extensive field testing of the candidate cultivars. Maintenance, multiplication and distribution of the seed are the ultimate stages of a breeding programme. Systematically outlined conventional breeding procedures, timeframes and estimated breeding generations are essential to objectively consider the ultimate timescales required to release new and improved cultivars of all crops. The information may assist plant breeding- students, educators, researchers, project advisors, administrators and funding agencies. Dear students, in this chapter we outline the timeframes of conventional breeding procedures of clonally (also called vegetatively) propagated, self-fertilizing (also called self-pollinated) and cross-fertilizing (also called cross-pollinated) crops.

14.1 Introduction

The mating or crossing of genetically dissimilar individuals is called hybridization. Hybridization results in recombination that in turn widens the spectrum of variation among individuals. So, the reason for genetic diversity in progenies of crosses is the formation of new combinations of genes. This newly created variability is then channelized into development of better cultivars.

The importance of pure line selection as a method of breeding started decreasing when plant breeders felt that the genetic variability existing in the land races of self-pollinated crops had been almost completely exploited and there was little chance for further improvement in these crops by using pure line breeding. Therefore, no option was left for plant breeders except to cross diverse individuals and combine together desirable characters in a single genotype from two or more parents. The planned hybridization which could avoid self-pollination and chance cross-pollination, soon became the main feature of almost all plant breeding programmes.

13.1 Introduction

F₁ plants produced by crossing two parents with contrasting characters are called hybrids. Hybrids generally exhibit increased vigour and fitness over the parents. This phenomenon is called hybrid vigour or heterosis. Thus, heterosis is the phenomenon in which the hybrid of two genetically dissimilar parents shows increased vigour at least over the mid-parental value. Heterosis is just the reverse of inbreeding depression, which results after selfing the plants of the cross-pollinated crops. Kolreuter (1761-66) was the first to report hybrid vigour in the hybrids of tobacco, Datura, etc. Most of the early plant hybridizers including Mendel (1865) had noticed this phenomenon. Shull (1914) coined the term heterosis from two Greek words, hetero (different) and osis (condition), to replace his own earlier word ‘stimulus of heterozygosis’. In current usage, heterosis and hybrid vigour are used as synonyms and interchangeable.

13.2 Types of Heterosis

Depending on the nature, origin, adaptability, reproducibility and non-reproducibility, heterosis can be categorized into two: (1) euheterosis (true heterosis) and (2) pseudo-heterosis (false heterosis, i.e., luxuriance).

12.1 Introduction

Allowing certain plants from the population to reproduce, while barring others, comprises selection. This permission for reproduction may be granted by nature or by the plant breeder. In the other words, both nature and breeder exert selection pressure. Natural selection operates through differences of fertility among the adult individuals of parent generation as well as through differences of viability among the individuals of offspring generation. Artificial selection, i.e., the selection from action of the plant breeder in the choice of parents, produces its change of gene frequency by separating the adult individuals of parent generation into two groups, viz., the selected and the discarded, that differ in gene frequency. So, selection simply implies the retaining of desirable types or culling of unwanted or undesirable types.

11.1 Introduction

In chapter 10, we have discussed the different types of variation, namely, genetic, environmental and genotype × environment interaction variations. We have also learnt how to estimate the phenotypic variance and its components including genotypic (also called genetic), environmental and g × e interaction variances from trial data. Now in this chapter, we shall discuss on the genetic components of polygenic (quantitative) variation and its implications on breeding strategies.

11.2 Genetic Components of Polygenic Variation

In this chapter whenever we call genetic variation, we mean polygenic (quantitative) variation, and we know that variation is usually measured in term of variance. However, variation, variability and variance are almost used synonymously. The genetic variance has been classified into different components by various workers. Fisher (1918) divided genetic variance into three components: additive, dominance and epistatic (interaction) variances. Wright (1935) classified genetic variance into two components: additive variance and non-additive variance (dominance and epistatic variances). Mather (1949) classified genetic variance into two components: heritable fixable variance (additive variance and additive × additive epistatic variance) and heritable non-fixable variance (dominance variance, and additive × dominance and dominance × dominance epistatic variances). Later, Hayman and Mather further divided the epistatic variance into three categories: additive × additive, additive × dominance and dominance × dominance.

10.1 Introduction

Plant breeders come across two categories of characters (traits) for improvement, namely, qualitative characters and quantitative characters. Red vs white flower, round vs wrinkled seed, determinate plant growth vs indeterminate plant growth etc. are examples of qualitative trait differences. There is no overlapping of characters and no confusion during classification. Such trait differences are usually governed by one or two gene pairs, each with large effect that can be measured even in the presence of segregation at other loci and even in the presence of effects of environmental factors. These characters are very less influenced by the environmental factors.

On the other hand, most of the traits of economic importance like yield of fruits of vegetable and fruit crops, yield of vegetative parts in fodder crops, yield of lint in cotton, yield of grains in cereals, pulses, oilseeds etc., plant height, days to maturity etc. are examples of quantitative characters. Variation observed for such traits is continuous. Again, there is overlapping of trait differences. So, one cannot assign individuals to a specific class unambiguously. Such characters are influenced by environmental factors, such that phenotypes are not same when grown over locations and/or seasons.

When different plants/lines of a species exhibit trait differences, it is called variation. Variation is essential for any improvement in a crop. It may be present in the population to be improved or else it may be created by different means such as hybridization, mutation, polyploidy, domestication, plant introduction, somaclonal variation, genetic engineering etc. Now, we shall discuss the types and estimation of variation.

9.1 Introduction

The sum total of genes in a plant species is called as plant genetic resources (also called germplasm, gene pool, or genetic stock). In other words, plant genetic resources refer to a whole library of different alleles of a plant (crop) species. Germplasm is the lifeblood of plant breeding without which breeding is impossible to conduct. It is the genetic material that can be used to perpetuate a species or population. It not only has reproductive value, but through genetic manipulation (plant breeding), germplasm can be improved for better performance of the crop. Germplasm provides the materials (parents) used to initiate breeding programme. Sometimes, what a plant breeder does is to evaluate plant genetic resources and make a selection out of existing biological variation. Promising genotypes that are adapted to the production region are then released for cultivation by the farmers. Other times, a plant breeder generates new variability by using different methods such as hybridization, mutagenesis and more recently gene transfer. This base population is then subjected to appropriate selection methods, leading to the identification and further evaluation of promising genotypes for release as cultivars. When a plant breeder needs to improve crop plants, he has to find a source of germplasm that would supply the genes needed to undertake the plant breeding programme. To facilitate the use of genetic resources (germplasm), germplasm (gene) banks are given the responsibility of collecting, evaluating, cataloguing, storing and managing large numbers of germplasm. This strategy allows a plant breeder ready and quick access to genetic resources when he needs it. The fact that Sir Otto Frankel coined the term ‘genetic resources’ only in 1968 shows that the plant breeders, though aware of the gradual loss of germplasm, failed to recognize the urgency of protecting the genetic resources of crop plants prior to a point of no return.

8.1 Introduction

In pre-historic times, people lived as hunters and gatherers. Then, gradually they began to produce the crops they ate on piece of land, and farming was born. Wild plants have certain traits that keep them protected and productive without human intervention. However, most of them are not desired for food or other use by humans. By spontaneous mutagenesis and hybridization, occasionally variant plants were found that lacked some of the undesired traits, such as bitter taste, thorns, or had particularly attractive features, such as larger fruits and more regular tuber shape. Consequently, as time passes such variation was picked up by humans by selection of such variants, keeping and advancing them. Thus, plant domestication is the process of bringing wild species under human management. In other words, plant domestication is the process by which genetic changes (shifts) in wild plants are brought about through a selection process imposed by humans. It is a method of plant breeding in the sense that, when successful, it provides domestic types that are superior to ones previously available.

Pre-historic man domesticated virtually all of our present-day crops. But, domestication of wild species continues even today and is likely to continue for a long time in the future. For example, in many countries there is an increasing emphasis on bringing forest trees producing timber, etc. under human management. A similar trend is seen for medicinal plants and plants producing specific compounds of interest, e.g., hydrocarbons, oils, etc. Kala jeera (Bunium persicum), a perennial spice, was domesticated during 1990s in Himachal Pradesh; it is being cultivated as an orchard crop. One important aspect of domestication at present is the use of particular genes from wild relatives in the improvement of cultivated species. When a plant breeder transfers one or a few desirable genes from a wild relative to a cultivated type, he is, in a sense, domesticating the wild species in part.

7.1 Introduction

Breeding plans are based on controlled mating (i.e., mating between selected strains) and selfing of selected strains or a combination of both. Continued selfing is practised to isolate purelines and inbreds. On the other hand, crossing (hybridization) is necessary to produce hybrids and synthetics. In crossing, self-pollination is prevented in the female parent. Male sterility and self-incompatibility easily exclude self-pollination, and so they are used in hybrid seed production.

7.2 Male Sterility

Koelreuter (1763) for the first time reported male sterility in flowering plants. Male sterility is a reproductive deficiency of some plants in which male organs in bisexual flowers become defunct. It relates particularly to nonviable pollen grains that are formed through a chain of vital processes during microsporogenesis. These processes are so delicately balanced under genetic control of many loci that mutation of any one locus may lead to the formation of non-functional microspores, and hence male sterility. In male sterility, pollen grains are non-functional (inactive); but the female gametes (egg cells) are normally functional.

6.1 Introduction

The process by which living beings give rise to the progeny of similar kind is called reproduction. Reproduction allows both multiplication and maintenance of a genotype/line/cultivar/species. So, the process of reproduction is also called propagation, and the plant part used for this purpose is called propagule. The way progeny originate is known as mode of reproduction (also called breeding system or reproductive system). The mode of reproduction of a crop determines its genetic composition which, in turn, is the deciding factor to develop suitable breeding methods. Knowledge of mode of reproduction is also essential for its artificial manipulation to breed improved types. The methods to be adopted for multiplication and maintenance of improved cultivars are also governed by the mode of reproduction. Especially in undertaking breeding investigations with new plants the nature of reproduction must be known before efficient breeding plans can be developed. The mode of reproduction has been well established for most of our important crop species.

6.2 Mode of Reproduction

Mode reproduction in crop plants may be sexual or asexual. In sexual reproduction (also called amphimixis), specialized reproductive cells called gametes are formed by the process of gametogenesis. Fusion of the male and female gametes leads to the development of an embryo and eventually the seed. In asexual reproduction, fusion of the male and female gametes does not occur, and new plants arise from unfertilized gametes (but not from zygote), or from specialized vegetative organs such as rhizome, tuber, bulb, corm, runner, sucker, stolon, bulbil etc., or by various artificial means of propagation such as rooting of stem and root cuttings, layering, grafting, gootee, etc., or tissue culturing.

5.1 Introduction

The development of quantitative trait loci (QTL) mapping methods to identify genes or QTLs controlling quantitative traits was a landmark achievement in plant genetics research in the late 1980s. Since then literally thousands of research papers have reported the identification of genes or QTLs for important traits across a diverse range of plant species. This wealth of genetic information was built on the foundation of decades of research in crop molecular genetics and genomics.

The selection and enrichment of QTLs within breeding material was the inevitable next step from QTL discovery to application, and there have been numerous reports of successful tracking of genes and QTLs within breeding programmes. DNA (or molecular) markers have enabled selection of major genes or QTLs for critical or important traits in a process called marker-assisted selection (MAS), which has revolutionized plant breeding. DNA markers are used as tools by breeders to improve the accuracy or efficiency of selection as well cost and time efficiency. Apart from MAS, DNA markers have numerous applications including DNA fingerprinting, genetic diversity analysis and parental characterization.

Recent developments in genotyping platforms and systems to implement molecular breeding schemes offer new tools for modern plant breeders. Single nucleotide polymorphism (SNP) markers are biallelic, co-dominant markers which are abundant in the genome. Insertion-deletion (Indel) markers are also prevalent and can be screened using high-throughput genotyping platforms. These platforms are cost-efficient, can handle large sample sizes and provide fast data turn-around time. The availability of genome sequences and genomic resources continues to provide a wealth of SNP and Indel markers which will undoubtedly be the marker type of choice for decades to come.

The availability of high-throughput SNP platforms in conjunction with advances in computational methods has also led to the molecular dissection of traits using genome-wide association studies (GWASs) across diversity panels and the development of genomics-based molecular breeding strategies. Association mapping (AM) has now emerged as a standard method for gene and QTL discovery for major crops.

4.1 Introduction

Many important traits such as yield, quality and some forms of disease resistance are controlled by many genes and are known as quantitative traits or characters (also called ‘metric’,‘polygenic,’ ‘multifactorial’ or ‘complex’ traits). The regions within genomes that contain genes associated with a particular quantitative trait are known as quantitative trait loci (QTLs). The identification of QTLs based only on conventional phenotypic evaluation is not possible. A major breakthrough in the characterization of quantitative traits that created opportunities to select for QTLs was initiated by the development of DNA (also called molecular) markers in the 1980s.

One of the main uses of DNA markers in agri-horticultural research has been in the construction of linkage maps for diverse crop species. Linkage maps have been utilised for identifying chromosomal regions that contain genes controlling simple traits (controlled by a single gene) and quantitative traits using QTL analysis. The process of constructing linkage maps and conducting QTL analysis–to identify genomic regions associated with traits–is known as QTL mapping (also called ‘genetic,’ ‘gene’ or ‘genome’ mapping). DNA markers that are tightly linked to agronomically important genes (called gene ‘tagging’) may be used as molecular tools for marker-assisted selection (MAS) in plant breeding. MAS involves using the presence/absence of a marker as a substitute for or to assist in phenotypic selection, in a way which may make it more efficient, effective, reliable and cost-effective compared to the more conventional plant breeding methods. The use of DNA markers in plant (and animal) breeding has opened a new realm in agriculture and horticulture called ‘molecular breeding’.