Re: Plant Breeding Principles And Methods By Bd Singh Pdf Download
Katerine Aldrige
Africa produces a diversity of crops including cereal, pulse, oilseed, root, and tuber species (Table 1), but contributes less than a quarter of the world production of root and tuber crops (Ngopya, 2003). In East Africa, bananas (especially cooking bananas) are an extremely important crop. In Uganda, for example, bananas serve as the largest only source of calories. Even though crop production is mostly for subsistence needs, there is tremendous business potential in local and international markets.
Breeding efforts only address the first two constraints of biotic factors and abiotic factors. The ability to store a crop after harvest is also affected by biotic and abiotic factors. In order to improve food security and provide income to farmers, continuous efforts by plant breeders are needed to increase production per unit area of land while maintaining crop quality.
Plant reproductive systems (or mating systems) fall into three main categories: asexual, autogamous (self-fertilizing), and allogamous (cross-fertilizing). These topics are covered in greater detail in Crop Genetics on Reproduction in Crop Plants.
Asexual reproduction generates individuals that are genetically identical to the mother parent plant and are referred to as clones. The two main forms of asexual reproduction/propagation are vegetative and apomictic. Vegetative propagation is the creation of clones from stem cuttings, suckers (similar to tillers), tubers, runners (stolons), rhizomes, bulbs, scions, and other plant parts. Cassava, sweet potato, and sugarcane are propagated via stem cuttings. Bananas are typically propagated by suckers, while potatoes are propagated by tubers. Elephant grass (Napier grass, Pennisetum purpureum) is propagated by rhizomes, sets (suckers), and stem cuttings. Apomictic reproduction is the asexual propagation of a plant via clonal seeds formed by one of several means that either bypass meiosis or result in a failure of meiosis. Examples of apomictic crops include Citrus and many perennial forage species.
Sexual reproduction involves the union of a male sperm with a female egg cell or ovary. This process is called fertilization. There are two types of pollination: self-pollination and cross-pollination. When the pollen of a plant pollinates a flower on the same plant the process is called self-pollination (Fig. 2A). Pollination is the transfer of male sperm carried in pollen to the female part of a flower called the stigma (Fig. 2B). When the pollen of a plant pollinates a flower on another plant of the same species the process is called cross-pollination. In nature, cross-pollination requires wind, water, insects, birds, or other animals to transfer the pollen. Most cultivars (i.e., cultivated varieties) are created by a process that involves at least one generation of cross-pollination by plant breeders; including both self- and cross-pollinated species. (See Reproduction in Crop Genetics).
Both the sperm and the egg are haploid, meaning they contain a single set of chromosomes from the male or female parent, respectively. Fertilization unites the single set of chromosomes in the sperm nucleus with the single set of chromosomes in the egg nucleus to produce a complete pair of chromosomes (diploid) in the zygote. Several crop species, for example, banana, sweet potato, potato, and many grasses have three or more sets of chromosomes and therefore they are, in general, called polyploids (e.g., triploids, tetraploids, pentaploids, hexaploids, octaploids).
Flowering plants have a unique process called double-fertilization in which the embryo and the endosperm are fertilized separately. Each pollen grain contains two pollen nuclei; one pollen nucleus fuses with the egg cell to form a diploid zygote and the second pollen nucleus fuses with the two polar nuclei in the ovule, eventually developing into a triploid endosperm. The zygote begins to divide by mitosis forming a multicellular embryo within the ovule. The endosperm provides the energy source that is used by the embryo prior to formation of true leaves that begin photosynthesis. Following fertilization the ovule (with embryo and endosperm) develops into a seed.
Genetic information (i.e., DNA) from both the male and female parents is present in a seed produced by fertilization. It is this union of sperm and egg cell that results in the creation of genetic variation (if the male and female gametes possess different genetic information). Offspring that result from the union of gametes from male and female plants with dissimilar genotypes are known as hybrids. Plant breeders select genotypes (i.e., male and female parents) that complement each other to combine the positive (or desirable) traits from each parent in the hybrid offspring.
Genetic information is transferred from generation to generation through seed, which typically consists of an embryo, an endosperm, and a seed coat. The endosperm includes the beginnings of a radicle, a hypocotyl, and one or two cotyledons (seed leaves), which support the formation of roots, stems, and true leaves, respectively. Monocot plant species have a single cotyledon (i.e., mono = one) and dicot plant species have two cotyledons (di = two). The seed coat serves as a protective coat around the seed.
Crops are capable of both self- and cross-pollination. These crops are classified as either autogamous (self-pollinated) or allogamous (cross-pollinated) depending on the relative frequency of self- or cross-pollination that is observed in the species.
Self-fertilization occurs if male and female gametes derived from the same plant unite. Self-fertilization also refers to the union of gametes from the same genotype. One trait, i.e., plant characteristic, that virtually ensures self-pollination is cleistogamy, where pollen shed occurs before the flower opens (i.e., anthesis). Cleistogamy promotes self-pollination and severely limits cross-pollination. Cleistogamy is observed in some legumes (e.g., groundnut, peas, some beans, soybean). In some cereals (e.g., rice, wheat, and barley) the majority of self-pollination occurs before flowers open, but some cross-pollination can occur after the flowers open, even if only partially. This allows for some cross-pollination compared to relatively little or no cross-pollination in cleistogamous species. Most self-fertilizing species undergo a small amount of outcrossing: for example in soybean natural outcrossing of 0.03% to 2% or higher has been observed in some conditions (Caviness, 1966; Ray et al., 2003). Thus, it is critical to understand the mating system of the crops you are working with and how it is affected by different environmental conditions.
Monoecious plants have separate male (i.e., staminate) and female (i.e., pistillate) flowers, although they occur on the same plant. In some crops, the male and female flowers are present in the same inflorescence such as in banana (Fig. 4). In some cases, they are on separate inflorescences, as in maize (Fig. 5).
Dioecious plants have separate staminate and pistillate flowers present on different plants. Dioecious plants are diclinous (i.e., having flowers of only one sex). Examples of dioecious crops include papaya, date palm, and spinach.
Breeders must understand the reproductive system of the crop they are working on to make knowledgeable decisions about which breeding methods (i.e., crossing techniques, population maintenance, isolation distances, line and population development) are suitable and which type of cultivar (i.e., hybrid, pure-line, synthetic, clone) is appropriate. The modes of pollination and reproduction of some major crops are shown in Table 2. More information on the reproductive systems of crops is found in Allard (1960; pp. 40-41).
Genetically speaking, a population is a group of individuals that share a common gene pool. If all individuals within the population have the same genotype the population is homogeneous; if the individuals have different genotypes the population is heterogeneous. For example, gene A has alleles A1 and A2 (assuming a diploid). If a population is homogenous then all individuals are the same; all are A1A1 (homozygous), or all are A1A2 (heterozygous), or all are A2A2 (homozygous). If a population is heterogeneous then some individuals have different genotypes; a combination of A1A1, A1A2, and/or A2A2.
The genotype of a population and individuals within a population varies depending on the reproductive system of a species. A natural population of a cross-pollinated species consists of a heterogeneous mixture of individuals some or most of which will be heterozygous (A1A2) for individual loci. A natural population of a self-pollinated species will usually also consist of a heterogeneous mixture of individuals, but each individual will be mostly homozygous (A1A1and/or A2A2) at individual loci. Populations of an asexually reproducing species may be homogeneous or heterogeneous and individuals will likely be heterozygous (A1A2) at many loci.
Clonal, synthetic, and hybrid cultivars are heterozygous. Pure-line cultivars are homozygous. Self-pollination is used to achieve homozygosity in an autogamous species. In allogamous species self-pollination is used to develop inbred lines that are used as parents to create hybrids
Synthetic cultivars: Synthetic cultivars are produced by intermating a population of purposefully selected inbred lines, clones, hybrids, strains, or other populations of cross-pollinated plants. Synthetic cultivars are highly heterozygous and heterogeneous. Inbreeding depression is severe and plants that develop from self-pollinated seed lack the vigor of those obtained by cross-pollination. In a heterogeneous population, each plant is genetically different from another.
The components (clones, inbred lines, etc.) of a synthetic cultivar are maintained in their original form so that the cultivar can be reconstituted as needed. A synthetic cultivar is different from an open-pollinated variety because the components are maintained in their original form while with an open-pollinated variety the components are not maintained. Clonally propagated plants or inbred lines with desirable characteristics (traits) are selected and then isolated and allowed to cross-pollinate, randomly or in a structured format in a polycross nursery (Fig. 8). Seed is harvested from the clones or inbred lines and planted in progeny rows for evaluation. The best clones or inbred lines are then selected both for superior plant traits and on the performance of their progeny rows, which measures their general combining ability with the rest of the population. These selected parents are then replanted and permitted to cross pollinate in isolation. Open-pollinated seed harvested from these parental clones or inbred lines (after one or more cycles of intermating) is then sold as a synthetic cultivar.
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