Modes of Reproduction in Plants
A mode of reproduction determine the genetic constitution of crop plant and provides the basis for understanding the mechanism of heredity, which are required for handling the desired characters during breeding work. This is the inherent property of the living organism to continue or maintain their races by the mechanism of reproduction. It is the process by which living being propagate or duplicates their own kinds. The modes of reproduction in crop plants are broadly grouped into asexual and sexual.
Asexual:
It does not involve the fusion of male and female gametes. In this new plants may develop from vegetative part of the plant (vegetative reproduction or may develop from embryos without fertilization ( apomixis).
A) Vegetative Reproduction:
In this new plants developed from a portion of the plant body. This may be occurred thorough modified under ground and subarea stems or through bulbs, for example Rhizome –Ginger, Tuber potato, Bulos- onion, corm-gladious, while sub aerial stems gives rise to new plants in strawberry rose etc. Similarly artificial vegetative methods stem cutting ( Sugarcane, Durant a), Root cutting ( Lemon, citrus) ;layering, budding, gooties ( Grapes, lichi) and grafting ( mango) are common methods in propagation of fruits and ornamental horticultural crops.
B) Apomixis:
It is the type of asexual reproduction in which seeds are formed and embryo developed without fertilization. Thus plants developed are dentical in genotype to the parent plant. In apomixis, reproduction is either suppressed or absent. When it occurs, the apomixis is said to be facultative, but when absent it, referred to as obligate. Many crops species show apomixis but it is generally facultative. Apomisix may be of following type:
i) Adventive Embryony:
Embryo directly developed from vegetative cells of the ovule such as integuments and chalaza. Development of embryo sac of embryo does not involve production of embryo sac E.g, Mango, Citrus.
ii) Apospory:
Some vegetative cells of ovule developed into unreduced embryo sacs after meiosis. Embryo may be developed from egg cell or other cell of embryo sac. E.g Crepis.
iii) Displospory:
Embryo sac is developed from the megaspore, which may haploid or diploid.
a) Parthenogenesis:
In this embryo is directly developed from egg cell with fertilization. Depending upon whether the embryo sac is haploid or diploid termed as haploid or diploid parthenogenesis. Haploid parthenogenesis are:
1) Natural Selection:
In nature there is a continuous selection by natural forces. E.g temp, soil, humidity, pest, disease, etc. As a result the genotype more suited to a given environment leaves behind more progeny than the less adapted one, this process is known as natural selection.
2) Artificial Selection:
The selection by man often permit only the selected plants to reproduce, the progeny from the remaining plants are generally discarded. The natural selection considerably retains variability in the species while artificial selection gradually reduces the variability accidentally and reported in solanum, nigrum, Nicotiana and maize, diploid parthenogenesis in many grasses like taraxacum. In many plants like Datura, Rice, Nicotiana, pollen grains are induced in vitro to produce haploids called androgenesis. The phenomenon in which the fruit is developed due to parthenogenesis is called parthinocrpy.
b) Apogamy:
Embryo develops from haploid nuclei other than egg cell i.e synergids or antipodal which may be haploid or diploid. E.g Allium cepa (onion).
Sexual Reproduction:
It involves fusion of male and female gametes to form a zygote, which develops into an embryo. It may be
a) Isogamy: Union of two similar gameties is called Isogamy and uniting undistinguishable gametes are isogametes E.g Mucor spirogyra, etc.
b) Heterogamy: Union of two dissimilar gametes is called fertilization or syngamy and the gametes called heterogametes.
In crops plants, male and female gametes are produced in a specialized structure called flower which consists of four whorls viz calyx, corolla, androecium and gynoecium. A flower containing all whorls’s called complete flower while incomplete flower lacks one or other parts. A flower containing both stamens and pistil is said to be perfect flower or hermaphrodite flower. It may be imperfect flower, when stamens are absent (pistil- late) or carpeles absent ( staminate) flowered. The male gamete is produced in stamen while the pistil produces the female gamete.
Fertilization:
It refers to the fusion of one of the two sperms with egg cell to form a zygote.
Double Fertilization:
One male gametes unite with the eggs cell known as syngamy or fertilization and another male gamete fuse with the pollar nuclei known as triple fusion, when these two processes occurs simultaneously known as double fertilization.