modes of reproduction in crops
- 1. Prepared By: Prof. T.V.Rathod
- 2. MODE OF REPRODUCTION • Mode of reproduction determines the genetic constitution of crop plants, that is, whether the plants are normally homozygous or heterozygous. • A knowledge of the mode of reproduction of crop plants is also important for making artificial hybrids. • Production of hybrids between diverse and desirable parents is the basis for almost all the modern breeding programmes. • The modes of reproduction in crop plants may be broadly grouped into two categories, asexual and sexual.
- 3. Asexual Reproduction Vegetative Reproduction • In nature, a new plant develops from a portion of the plant body. This may occur through following modified forms, 1. Natural vegetative • Underground stem • Sub-aerial stems • Bulbils 2. Artificial vegetative • Stem cuttings Asexual reproduction does not involve fusion of male and female gametes. New plants may develop from vegetative parts of the plant (vegetative reproduction) or may arise from embryos that develop without fertilization (apomixis). Apomixis • seeds are formed but the embryos develop without fertilization. 1. Parthenogenesis 2. Apospory 3. Apogamy 4. Adventive Embryony
- 6. Natural vegetative • Underground Stems • The underground modifications of stem generally serve as storage organs and contain many buds. These buds develop into shoots and produce plants after rooting. Eg., Tuber : Potato Bulb : Onion, Garlic Rhizome : Ginger, turmeric Corm : Bunda, arwi. • Sub-aerial Stems • These modifications include runner, stolon, sucker etc.,. • Sub-aerial stems are used for the propagation of mint, date plam etc. • Bulbils • Bulbils are modified flowers that develop into plants directly without formation of seeds. • These are vegetative bodies; their development does not involve fertilization and seed formation. • The lower flowers in the inflorescence of garlic naturally develop into bulbils. • Scientists are trying to induce bulbil development in plantation crops by culturing young inflorescence on tissue culture media ; it has been successfully done in the case of cardamom.
- 7. Artificial vegetative • It is commonly used for the propagation of many crop species, although it may not occur naturally in those species. • Stem cuttings are commercially used for the propagation of sugarcane, grapes, roses, etc. • Layering, budding, grafting and gootee are in common use for the propagation of fruit trees and ornamental shrubs. • Techniques are available for vegetative multiplication through tissue culture in case of many plant species, and attempts are being made to develop the techniques for many others. • In many of these species sexual reproduction occurs naturally but for certain reasons vegetative reproduction is more desirable.
- 8. Significance of Vegetative Reproduction • Vegetatively reproducing species offer unique possibilities in breeding. • A desirable plant may be used as a variety directly regardless of whether it is homozygous or heterozygous. • Further, mutant buds, branches or seedlings, if desirable, can be multiplied and directly used as varieties.
- 9. Apomixis • In apomixis, seeds are formed but the embryos develop without fertilization. (without the fusion of male & female gametes). • Consequently, the plants resulting from them are identical in genotype to the parent plant. • In apomictic species, sexual reproduction is either suppressed or absent. • When sexual reproduction does occur, the apomixis is termed as facultative. But when sexual reproduction is absent, it is referred to as obligate. • Many crop species show apomixis, but it is generally facultative. • The details of apomictic reproduction vary so widely that a confusing terminology has resulted.
- 12. • Parthenogenesis • The embryo develops from embryo sac without pollination. It is of two types Gonial parthenogenesis – embryos develop from egg cell, Somatic parthenogenesis – embryos develop from any cell of the embryo sac other than the egg cell. • Apospory • The embryo may develop from egg cell or some other cell of this embryo sac. • Apospory occurs in some species of Hieraceum, Malus, Crepis, Ranunculus, etc. o Diplospory • Embryo sac is produced from the megaspore, which may be haploid or, more generally, diploid. • Apogamy • In apogamy, synergids or antipodal cells develop into an embryo. • Like parthenogenesis, apogamy may be haploid or diploid depending upon the haploid or diploid state of the embryo sac. • Diploid apogamy occurs in Antennaria, Alchemilla, Allium and many other plant species. • Adventive Embryony • In this case, embryos develop directly from vegerative cells of the ovule, such as nucellus, integument, and chalaza. • Development of embryo does not involve production of embryo sac. • Adventive embryony occurs in mango, citrus, etc.
- 15. Significance of Apomixis • Apomixis is a nuisance when the breeder desires to obtain sexual progeny, i.e., selfs or hybrids. • It is of great help when the breeder desires to maintain varieties. • The breeder has to avoid apomictic progeny when he is making crosses or producing inbred lines. • But once a desirable genotype has been selected, it can be multiplied and maintained through apomictic progeny. • Asexually reproducing crop species are highly heterozygous and show severe inbreeding depression. • Therefore, breeding methods in such species must avoid inbreeding.
- 16. SEXUAL REPRODUCTION • Sexual reproduction involves fusion of male and female gametes to form a zygote, which develops in to an embryo. Significance of Sexual Reproduction • Sexual reproduction makes it possible to combine genes from two parents into a single hybrid plant. • Recombination of these genes produces a large number of genotypes. • This is an essential step in creating variation thr ough hybridization. • Almost the entire plant breeding is based on sexual reproduction. • Even in asexually reproducing species, sexual reproduction, if it occurs, is used to advantage, e.g., in sugarcane, potato, sweet potato etc.
- 17. MODES OF POLLINATION • Self-pollination • Bisexuality • Cleistogamy. • Homogamy • Chasmogamy • Position of anthers in relation to stigma. • Genetic Consequences of Self- Pollination • Self-pollination leads to a very rapid increase in homozygosity. • Therefore, populations of self- pollinated species are highly homozygous. • Self-pollinated species do not show inbreeding depression, but may exhibit considerable heterosis. • Therefore, the aim of breeding methods generally is to develop homozygous varieties.
- 19. • Cross-Pollination • Unisexuality (Dicliny) monoecy dioecy • Dichogamy protogyny Protandry • Heterostyly • Herkogamy • Self incompatibility • Male sterility • Genetic Consequences of Cross- Pollination. • promotes heterozygosity in a population. • highly heterozygous and show mild to severe inbreeding depression and considerable amount of heterosis. • Usually, hybrid or synthetic varieties are the aim of breeder wherever the seed production of such varieties is economically feasible. # Often Cross Pollination #