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Tissue culture is a technique used to culture cells, tissues, organs or whole plant under controlled environmental conditions that in vitro condition is known as plant tissue culture. Plant tissue culture exploits the totipotency nature of plant cells, ability to develop in to complete plantlet.

The term ideotype was proposed by Donald in 1968. Ideotype is a biological model which is expected to perform in a predicted manner in a defined environment. A crop ideotype is a plant model that is expected to perform better in a particular environment. The concept of ideotype changes from crop to crop, climate to climate and the concepts and policies of the community concerned. Ideotype breeding is the development of a plant type, expected more quantity of grains, with ideal character. Plant characters are selected from an agronomic point of view.

The prime objective of any plant breeder is to develop high yielding varieties and make the quantity of quality seed of those varieties to farmers for commercial cultivation to realize the benefits of superior varieties. This necessitates, the production of seed of improved varieties in large scale in order to make them available to farmers without losing their genetic purity. Seed production is a continuous process. The seed of improved varieties are produced in several stages, each class producing a special class of seed. The various classes of improved seeds are recognized in order to facilitate seed production and to ensure continuous supply of quality seed. A seed production programme for the production of quality seed has different classes of seeds viz; nucleus seed, breeder seed, foundation seed, registered seed and certified seed. The Association of Official Seed Certifying Agencies (AOSCA) has defined these seed classes as follows:

1. Nucleus Seed

Basic or nucleus seed is the original seed of a released and notified variety. Nucleus seed is multiplied by single plant progeny system, using the concept of maintenance breeding, where purity of the original parental stock is maintained. It is available in very limited quantities and taken from the individual plant of a variety for purification. It is directly under the controlled of concerned crop breeder. This seed has hundred percent genetic and physical purity. Breeder seed is multiplied out of this material and about 500-1000 plants are kept for continued multiplication of nucleus seed.There is no need of any type of tag on nucleus seed but concerned breeder issue a certificate for purity.

Crop breeders are continuously engaged in the improvement and development of crop varieties, in search of a genotype which will prove superior to the existence cultivars. A strain is duly released by appropriate duly empowered legal organization. Only when it is officially released, it is called variety, Before release, it is said to be strain or line. The release of new crop varieties consists of the following major steps:

1. Development of new strains

The new strains are developed by ICAR Crop Research Institutes and State Agricultural Universities for specific purposes. Various breeding methods are used for development of new strains. Some varieties are developed by selection without hybridization such as pure-line variety, mass selected variety and clonal variety. Development of some varieties involves hybridization and selection. This includes varieties that are developed by pedigree, bulk and backcross methods including multiline cultivars and population improvement methods.

2. Evaluation of performance

This trial is conducted at the research station, where new strains has breed. The object is to make sure that new strains are better in performance than the existence variety, so that it can be promoted to the trials where new strains has to be compete with other strains, developed by other scientists/researchers.

The disease , referred as the disorder of crop plants caused by fungi, mycoplasma, bacteria and viruses. Plant resistance is one of the most effective and ideal approach to combat diseases and insect-pest. Resistance is the capacity of plants to resist, withstand and overcome the attack of pathotypes.

Mechanism of Disease Resistance

Different mechanism of disease resistance are as under:

1. Disease escape

The ability of susceptible host plants to avoid attack of disease due to environmental factors like early maturing varieties, shift in the date of planting, change the field, balanced application of NPK etc. Example., early maturing varieties of groundnut and potato may escape from ‘Tikka’ and ‘Late blight’ diseases respectively.

2. Disease tolerance

The ability of the plants to tolerate the attack of the pathogen without loss or little loss in the yield. This endurance is brought about by the influence of external characters. Generally, tolerance is difficult to measure since it is confounded with partial resistance and disease escape. Disease tolerance is of three types:

Abiotic stresses include salinity, drought, flood, extremes in temperature, heavy metals, radiation etc. They are the foremost factors that causes the loss of major crop plants worldwide. This situation is going to be more rigorous due to increasing desertification of world’s terrestrial area, increasing salinization of soil and water, shortage of water resources and environmental pollution.

The inadequacy of water availability, including precipitation and soil moisture storage capacity, in quantity and distribution during the life cycle of a crop to restrict the expression of its full genetic yield potential”. Generally drought stress occurs when the available water in the soil is reduced and atmospheric conditions cause continuous loss of water by transpiration or evaporation. Drought stress tolerance is seen in almost all plants but its extent varies from species to species and even within species.

Polyploidy, the condition in which a normal diploid cell or organism acquires one or more additional sets of chromosomes. In other words, the polyploid cell or organism has three or more times the haploid chromosome number. Polyploidy arises as the result of total nondisjunction of chromosomes during mitosis or meiosis. The somatic chromosome number of any species, whether diploid or polyploidy is designated as 2n. The somatic cells of plants are diploid that they posses two complete set of chromosomes, one received from seed parent and another from pollen parent. The gametic chromosome number is said to be haploid and denoted by n. Haploid is defined as cell with gametic chromosome number. While monoploid has the basic chromosome number and represented by x.

Classification of Polyploidy

Polyploids are classified in to two groups, euploidy and aneuploidy. Euploidy have chromosome number which is an exact multiple of basic number. Euploidy includes monoploids, diploids and polyploids. In aneuplod individual with other than exact multiple of basic chromosome number or change in chromosome number which posses one or few chromosomes of the genome.

Mutagenesis is the process whereby sudden heritable changes occur in the genetic information of an organism not caused by genetic segregation or genetic recombination, but induced by chemical, physical or biological agents.

Mutations were first identified by Hugo de Vries in the late nineteenth century, while experimenting on the ‘rediscovery’ of Mendel’s laws of inheritance. He coined the term ‘mutation’. Radiation-induced mutations as a tool for generating novel genetic variability in plants, advanced as a field after the discovery of the mutagenic action of X-rays demonstrated in maize, barley and wheat by Stadler. The discovery of X-rays by Rontgen in 1895 led to the application of X-rays for inducing mutations in fruit fly by Muller (1927). Mutation is of two types

Reproduction that does not involve the union of male and female gametes is known as asexual reproduction. Reproduction in which sexual organs or related structures take part but fertilization does not occur, or crops which are propagated asexually or by vegetative means are known as asexually propagated crops. Most of the fruit plants are propagated asexually which consist of large number of clones. Example: sugarcane, potato, sweet potato, banana, mango, citrus, pears, peaches, litchi, crops that propagates by asexual means. The procedure of selection used for asexually propagated crops is known as clonal selection, since the selected plants are used to produce new clones or superior clones can be isolated from local variety, introduced variety or inter crossed population.

Hybrids are produced by crossing two genetically diverse parents variety. Hybrid vigour is at its maximum in the F₁ generation and hence highest emphasis is placed on the F₁ hybrids. Hybrid varieties leads to maximum exploition of heterosis as compared to any other varieties. Beal in 1880 recommended the use of F₁ varietal crosses of maize for commercial cultivation.

The commercial exploitation of heterosis started only after the rediscovery of Mendel’s law of inheritance and consequent understanding of the theoretical basis of hybrid vigour. Heterosis is considered as a major revolution in classical plant breeding era. Utilization of heterosis has substantial influence in genetics to provide hybrid vigour, consequently high yielding hybrids in different crops and plants. Yield increase from heterotic hybrids due to the expression of heterosis is up to 30% superior to conventional varieties. The term heterosis was first used by Shull (1914) and represents the increase or decrease in fitness and vigour of the F₁ hybrid over the mean or mid parent values, produced by crossing two genetically different individuals for one or more traits. Generally, positive heterosis is desirable but in some cases like maturity duration and presence of toxic substances, negative heterosis is condiderable as desirable. Heterosis can be estimated in three different ways. When the heterosis is estimated over mid parent, it is called as relative heterosis or mid parent heterosis. Estimation of heterosis over better parent out of the two parents involved in the cross is referred as “Heterobeltiosis”, the term which was coined by Fonseca and Patterson (1968). “Standard Heterosis”, often called as useful heterosis which was coined by Meredith and Bridge (1972) and denotes the superiority of F₁ over the standard commercial check.

1. Bread Wheat (Triticum aestivum)

Wheat has been traditionally grouped into the genera Triticum in the tribe Tritiaceae of the family Poaceae. Bread wheat, 2n=6x=42 is a hexaploid consisting of three distinct genomic complementary, namely, A, B and D, with the genome structure AABBDD. In the past there were 4 species, viz. Triticum aestivum, T. durum, T. dicoccum and T. sphaerococcum, under cultivation in India. T. sphaerococcum has now practically gone out of cultivation because of its low productivity and high susceptibility to diseases. The major wheat producing countries are China, India, USA, the Russian Federation, and Australia. These five countries together contribute more than half of the global wheat production. In India, Uttar Pradesh has registered the highest production followed by Punjab, Madhya Pradesh, Haryana, Rajasthan and Bihar. These top six states together contributed around 90 per cent of the total wheat production in the country

Floral Biology

The inflorescence of wheat is known as spikelets. Spikelets are alternately arranged on the rachis; each spikelet’s has a small joint axis which bear two to nine flower known as florets. Close to the base of rachilla are two empty boat shaped bracts known as glumes. Above these inserted alternately on opposite side of the rachilla known as lemma. Opposite it’s a thin membrane bracts known as palea enclosed within the lemma and palea are a single pistil and three steamens. The pistil consists of an ovary bearing two short styles each carrying a feathery stigma. At the base of the ovary are two membranous scales known as lodicules. Each stamen consists of a thread like filament carrying a pollen containing anther.

The genetic organization of cross pollinated crops are different from that of self pollinated crops, because of differences in the reproductive system. Open pollinated populations are heterozygous with free inter mating individuals, heterozygous at most of the loci. Heterozygosity is an essential feature of commercial varieties of out breeders. It is maintained during breeding or is restored at the end. Population improvement involves selection, generation after generation, with recombination of selected individuals. Its aim to increase the frequency of favorable alleles in the population. The improved forms may be used for commercial cultivation or as a base germplasm for the derivation of inbred lines. The list of cross pollinated plants is presented in Table 10.1. The factors that promote the cross pollination are dicliny, dichogamy, herkogamy, self-incompatibility and male sterility. The breeding methods which are commonly used in cross-pollinated crops are population improvement, hybrid and synthetic varieties.

Population Improvement

In cross pollinated crops heterozygosity must be restored in the end product of any breeding programme. It is classified into the following types:

The prime object of plant breeder is to develop high yielding variety with superior characteristics. Selection is based on the phenotypes of the plants. Consequently the effectiveness of selection primarily depends on the degree to which the phenotype of plants reflects their genotype. These are all self fertilizing species. In these species development of seed take place by self fertilization. Hence self pollinated species are also known as autogamous species. Various plant characters such as homogamy, cleistogamy, chasmogamy, bisexuality etc. promotes self pollination. The self pollinated crops with their description are given in Table 9.1. The breeding methods used in self and often-self pollinated crops are discussed below:

Plant Introduction

Plant introduction consists of taking a genotype or a group of genotypes of plants into a new area or region or environment, where they have not being grown earlier. It is very simplest and fastest method of developing new variety. It is of two types:

The mating or crossing between two genetically dissimilar plants or lines is known as hybridization. In other words, hybridization is an artificial controlled pollination in which pollen grains of desired male parent is transferred on the stigma of the desired female parent.

Objectives of Hybridization

To create genetic variability, which is pre-requisite for crop improvement programme.

To transfer one or more characters into single plant.

To know the inheritance of the characters.

To estimate the general combining ability of the parents and specific combining ability of the crosses.

To exploit and utilize the hybrid vigour.

To produce hybrid/synthetic varieties.

Pollination can be defined as the pre-fertilization event or process where pollen grains from anther are transferred to the stigma of a flower. Pollination is the process that helps to unite the male and female gametes and thus helps in fertilization. It can be broadly classified into two, cross-pollination and self-pollination.

1. Self-Pollination

Self-pollination is the process by transferring the pollen grains directly from anther into the stigma of the same flower. It is called as self-pollination. This form of pollination is common in hermaphrodite or dioecious plants which have both male and female sexual organs on the same flower. In self-pollinating flowers, the anthers and stigma are of similar lengths to facilitate the transfer of pollen. Self – pollination can be further divided into two types:

Self incompatibility in plants is a mechanism preventing the seed set by self pollination, and is probably way in which out crossing is enforced. Other factors may cause selfed seed may not to set seed due to embryo abortion, but SI is prezygotic and prevents embryo formation. SI is, therefore prevention of fusion of fertile male and female gametes after fertilization. Koelreuter first time in 18^th century reported S.I in Verbascum phoeniceum plants. In self-incompatible pollen grains fail to germinate on the stigma of the flower that produced them. Self incompatibility has been reported in more than 300 plant species belonging to 70 family of angiosperm. It is important out breeding device (cross pollination) for normal fruit set. It is maintained high degree of heterozygosity and can take place at any stage between pollination and fertilization.

Male sterility refers to a condition in which pollen is either absent or non functional or unable to produce functionally viable pollen in flowering plants. Since it was first reported by Kolreuter as early in 1763 that anther abortion occurs naturally in some plant species or in their inter specific crosses, its occurrence was observed in several species. In the 19th century, Gartner (1844) and Herbert (1847) reported the occasional recurrence of anthers rudimentariness or abortion in many ornamental and vegetable crops in normal populations. During the early 20th century, a large number of species were shown to possess male sterility, for example, Suturela hortensis, Lathyrus odoratus, Merculalis annua, Solanum tuberosum, Vitis vinifera, Linnum usitatissimum, Zea mays, Antirrhinum majus, Prunus persicaer and Rubus idaeus. Following the rediscovery of Mendel’s Laws of Inheritance, study of the genetic main basis of inheritance of male sterility attracted the attention of several workers. The main impetus for the study of inheritance of male sterility came from the elucidation of heterosis in maize by G. H. Shull, in 1908 and its exploitation through the use of male sterility by Jones and Clark (1943). The male sterility based scheme of hybrid seed production in corn opened up a new era for the hybrid corn industry as it could overcome the necessity of detasseling of female (parent) lines in the hybrid seed production process.

The male sterility may arise due to malformation of male sex organs, lack of normal anther sac, and inability of anthers to release mature pollens, lack of development of normal microspore.

Apomixis is an asexual method of reproduction through the seed. Apomictic reproduction is widespread and found in over 300 species of plants in 30 families. Interest in apomixis as a reproductive mechanism has increased as its potential and role are being recognized in genome preservation, polyploid build up, propagation of derivatives of wide crosses, permanent fixation of hybrid vigour, evolution, speciation and plant breeding .

Winkler (1906) defined apomixis as “the substitution for sexual reproduction of another asexual reproductive process that does not involve nuclear or cellular fusion.” One of the chief barriers to an understanding of the apomictic phenomenon has been the highly complex terminology as well as the differences of opinion among specialists as to the correct usage of these terms, probably because the phenomenon by itself is complex and in many ways quite different from that of amphimixis. Convincing evidence for the presence of apomixis can be obtained by tracing the steps that are involved in embryonic development. It should be amply clear that as the prerequisite for apomixis, fertilization does not take place at all in any form. Thus there is no normal development of embryo from the egg cell. However, the embryo may develop from an unfertilized egg cell other than the egg cell within the embryo sac.

Apomixis is widely distributed among higher plants. More than 300 species belonging to 35 families are apomictic. It is most common in Gramineae, Compositae, Rosaceae and Rutaceae. Among the major cereals like maize, wheat and pearl millet have apomictic relatives.

Germplasm is the sum total of all the genes and their alleles present in a crop and its related species. Germplasm is also termed as genetic resources. It may be release as a variety, subjected to selection for developing a variety, and may be used as a parent in hybridization programme. They may be obtained from gene banks, center of diversity, gene sanctuary, farmer's field and seed companies.

Types of Germplasm

1. Land races

These are primitive varieties/cultivars. They are storehouse of genetic variability. Land races were first collected and studies by N.I.Vavilov in rice. They are the source of development of varieties in many valuable crops.

2. Obsolete varieties

The improved varieties of recent past are called as obsolete varieties. These varieties were breed by systematic breeding methods, were once commercially cultivated, but are no more grown. For example, wheat varieties K65, Ph591, many NP series etc.