In crop plants, the mode of reproduction is of two types, viz.:
A. Asexual reproduction and
B. Sexual reproduction.
These are briefly described below:
A. Asexual Reproduction:
Multiplication of plants without the fusion of male and female gametes is known as asexual reproduction. Asexual reproduction can occur either by vegetative plant parts or by vegetative embryos which develop without sexual fusion (apomixis).
Thus asexual reproduction is of two types, viz.:
(1) Vegetative reproduction, and
(2) Apomixis.
(1) Vegetative Reproduction:
Vegetative reproduction refers to multiplication of plants by means of various vegetative plant parts.
Vegetative reproduction is again of two types, viz.:
(i) Natural vegetative reproduction, and
(ii) Artificial vegetative reproduction.
(i) Natural Vegetative Reproduction:
In nature, multiplication of certain plants occurs by underground stems, sub-aerial stems, root and bulbils. In some crop species, underground stems (a modified group of stems) give rise to new plants. Underground stems are of four types: viz. rhizome, tuber, corm and bulb.
The examples of plants which reproduce by means of underground stems are given below:
a. Rhizome:
Turmeric (Curcuma domestica), Ginger (Zingiber officinale).
b. Tuber:
Potato (Solarium tuberosum).
c. Corm:
Arvi (Colocasia esculenta), Bunda (C. antiquorum).
d. Bulb:
Garlic (Allium sativum), Onion (A. cepa).
Sub aerial stems include runner, sucker, stolon, etc. These stems lead to vegetative reproduction in mint (Mentha sp.) rose, strawberry, banana, etc. Bulbils are modified forms of flower. They develop into plants when fall on the ground. Bulbils are found in garlic.
In some plants, reproduction takes place even by roots. In citrus, normal roots give rise to new plants and in sweet potato tuberous roots give rise to new plants.
(ii) Artificial Vegetative Reproduction:
Multiplication of plants by vegetative parts through artificial method is known as artificial vegetative reproduction. Such reproduction occurs by cuttings of stem and roots, and by layering, grafting and gootee.
Examples of such reproduction are given below:
a. Stem Cuttings:
Sugarcane (Saccharum sp.), grapes (Vitis vinifera) roses, etc.
b. Root Cuttings:
Sweet potato, citrus, lemon, etc.
Layering, grafting and gootee are used in fruit and ornamental crops.
Significance:
Vegetative reproduction has several advantages.
The main advantages are given below:
a. It leads to continuity of same genotype with great precision, because all the progeny of asexually reproduced plant will have similar genotype and phenotype.
b. It is useful in obtaining large number of genetically identical individuals of a genotype, irrespective of the degree of heterozygosity.
c. Promising individuals occurring at any stage in a breeding programme can be easily picked up and maintained by asexual reproduction.
d. It makes use of desirable bud mutations. Mutants can be directly released as varieties.
(2) Apomixis:
Apomixis refers to the development of seed without sexual fusion (fertilization). In apomixis embryo develops without fertilization. Thus apomixis is an asexual means of reproduction. Apomixis is found in many crop species. Reproduction in some species occurs only by apomixis.
This apomixis is termed as obligate apomixis. But in some species sexual reproduction also occurs in addition to apomixis. Such apomixis is known as facultative apomixis.
There are four types of apomixis , viz.:
(i) Parthenogenesis,
(ii) Apogamy,
(iii) Apospory and
(iv) Adventive embryony.
These are briefly discussed as follows:
(i) Parthenogenesis:
Parthenogenesis refers to development of embryo from the egg cell without fertilization.
It is of two types, viz.:
(a) Haploid and
(b) Diploid.
When the embryo develops from a haploid egg cell, it is known as haploid parthenogenesis. The plants which develop from such embryos are haploid and sterile. Haploid parthenogenesis is found in Solanum nigrum.
Sometimes, embryo-sac develops without reduction division. Such embryo-sac and all cells within it are diploid. It gives rise to diploid embryos. Such parthenogenesis is known as diploid parthenogenesis and has been reported in grasses like Taraxacum. In plant species like tobacco and rice, pollen grains may be induced to develop into embryos. This development of embryos from pollens or anthers is termed as androgenesis.
There are several causes of parthenogenesis.
The main causes include:
(a) Inability of the pollen tube to discharge the contents inside the embryo-sac,
(b) Insufficient attraction between male and female gametes,
(c) Early degeneration of the sperm,
(d) Very long style,
(e) Schlerenchymatous style.
(f) Short pollen tube,
(g) slow rate of pollen tube growth,
(h) Stimulation of pollination in the absence of pollen tube, and
(i) Incompatibility.
Parthenogenesis can be artificially induced by four main ways:
(a) By the stimulation of widely related pollen or foreign pollen,
(b) By low temperature,
(c) By pollinating with X-ray irradiated pollens and
(d) By treatment with certain chemicals like belviton.
All these help in inducing parthenogenetic development of egg cell.
(ii) Apogamy:
The origin of embryo from either synergids or antipodal cells of the embryo-sac is called apogamy.
It is of two types, viz.:
(a) Haploid apogamy and
(b) Diploid apogamy.
The synergids or antipodal cells may be haploid or diploid.
If embryo develops from haploid synergids or antipodal cells, it is known as haploid apogamy. When the embryo develops from diploid synergids or antipodal cells, it is called as diploid apogamy. Diploid apogamy has been reported in Allium, Iris and many other species.
(iii) Apospory:
In apospory, first diploid cell of ovule lying outside the embryosac develops into another embryo-sac without reduction. The embryo then develops directly from the diploid egg cell without fertilization.
Apospory is of two types: viz.:
(a) Generative apospory and
(b) Somatic apospory.
Origin of embryo from the embryo-sac that has developed from the cell of archesporium is known as generative apospory as in Parthenium. When the embryo originates from the embryo-sac that has developed from the cell of either nucellus or integument, it is known as somatic apospory. This is found in Malus, Crepis, Poa and many other crop species.
(iv) Adventive Embryony:
The development of embryo directly from the diploid cells of ovule lying outside the embryo-sac belonging to either nucellus or integuments is referred to as adventive embryony. There is no production of another embryo-sac like apospory. This is a type of sporophytic budding which is very common in Citrus and Mango.
Apomixis is also classified as recurrent (2n) and non-recurrent (n). In recurrent apomixis the embryo-sac develops from diploid cells. There is no reduction in the chromosome number and all the cells of embryo-sac are diploid. This process is repeated from generation to generation and hence it is called recurrent apomixis.
It includes:
(a) Diploid parthenogenesis,
(b) Diploid apogamy and
(c) Apospory.
In non-recurrent apomixis, the embryo-sac consists of usual haploid cells. The plants produced by this method contain haploid set of chromosomes and are usually sterile. This process is not repeated from one generation to another and hence is called non recurrent apomixis. It includes haploid parthenogenesis and haploid apogamy.
Role in Plant Breeding:
Apomixis has several useful applications in plant breeding.
The important applications are given below:
1. Rapid Production of Pure Lines:
Apomixis is an effective means for rapid production of pure lines. Haploid parthenogenesis gives rise to haploid plants which upon colchicine treatment will produce diploid pure lines that can be used in plant breeding programmes.
2. Maintenance of Superior Genotypes:
A superior plant type which produces seed by apomictic means will usually bread true for the characteristics of mother plant. Thus apomixis is useful in maintaining the characteristics of mother plant from generation to generation.
3. Conservation of Heterosis:
In some cases, hybrid vigour may be conserved for many generations by using recurrent apomixis.
B. Sexual Reproduction:
Multiplication of plants through embryos which have developed by fusion of male and female gametes is known as sexual reproduction. All the seed propagating species belong to this group. The new plants arise from the embryos which have developed from the fusion of male and female gametes.
The main features of sexual reproduction are given below:
i. The embryo develops by the fusion of male and female gametes.
ii. The male and female gametes are formed in the flower. First microspores and megaspores are formed in anthers and ovules respectively by meiotic cell division. This process is known as sporogenesis. Then sperms and egg cells are formed in microspores and megaspores by mitotic cell division. This process is called gametogenesis.
iii. The union of male and female gametes leads to fertilization and zygote formation. The zygote develops into diploid embryo which is used for sexual reproduction.
iv. Majority of crop plants propagate by sexual reproduction.
Sexual reproduction plays an important role in creating and maintaining vast genetic variability in crop plants, which is essential for crop improvement.
In such species, genetic variability can be created by – (1) crossing over, (2) hybridization, and (3) polyploidy. Moreover, sexual reproduction offers opportunities to combine desirable genes together from different sources. Crossing over takes place only during meiosis and leads to new gene combinations. Crossing over does not take place in asexually reproducing species. In asexually propagating species variability is created only through somatic mutations.
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