Categories of mode of reproduction

A. Asexual Reproduction:

• A sexual 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).

I. Vegetative Reproduction:

• In nature, a new plant develops from a portion of the plant body.
• This may occur through modified underground and sub-aerial stems, and through bulbils.

II. 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. Examples of such modifications are given below.

Tuber: Potato

Bulb: Onion, Garlic

Rhizome: Ginger, turmeric

Corm: Bunda, arwi

III. Sub-aerial Stems:

• These modifications include runner, stolon, sucker etc.
• Sub-aerial stems are used for the propagation of mint, date palm etc.

IV. 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.

V. Artificial Vegetative Reproduction:

• 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.

Significance of Vegetative Reproduction

• 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.

VI. Apomixis:

• In apomixis, seeds are formed but the embryos develop without fertilization.
• 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.

Classification of Apomixis

a) Adventive Embryony:

• In this case, embryos develop directly from vegetative cells of the ovule, such as nucellus, integument, and chalaza.
• Development of embryo does not involve production for embryo sac.
• Adventive embryony occurs in mango, citrus, etc.

b) Apospory:

• Some vegetative cells of the ovule develop into unreduced embryo sacs after meiosis.
• 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.

c) Diplospory:

• Embryo sac is produced from the megaspore, which may be haploid or, more generally, diploid.
• Generally, the meiosis is so modified that the megaspore remains diploid.
• Diplospory leads to parthenogenesis or apogamy.

VII. Parthenogenesis:

• The embryo develops from embryo sac without pollination. It is of two types:

a) Gonial parthenogenesis – embryos develop from egg cell.

b) Somatic parthenogenesis – embryos develop from any cell of the embryo sac other than the egg cell.

VIII. 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.

Significance of Apomixis

• It is of great help when the breeder desires to maintain varieties.
• Once a desirable genotype has been selected, it can be multiplied and maintained through apomictic progeny.

B. SEXUAL REPRODUCTION

• Sexual reproduction involves fusion of male and female gametes to form a zygote, which develops in to an embryo.
• In crop plants, male and female gametes are produced in specialized structures known as flowers.

I. Flower:

• A flower usually consists of sepals, petals (or their modifications), stamens and/or pistil.
• A flower containing both stamens and pistil is a perfect or hermaphrodite flower.
• If it contains stamens, but not pistil, it is known as staminate, while a pistillate flower contains pistil, but not stamens.
• Staminate and pistillate flowers occur on the same plant in a monoecious species, such as maize, Colocasia, castor (Ricinus communis), coconut, etc.
• But in dioecious species, staminate and pistillate flowers occur on different plants, e.g., papaya, date palm (Phoenix dactylifera), pistachio (Pistacia vera), etc.

II. Sporogenesis:

• Productions of microspores and megaspores is known as sporogenesis.
• Microspores are produced in anthers (microsporogenesis), while megaspores are produced in ovules (megasporogenesis).

a) Microsporogenesis:

• Each anther has four pollen sacs, which contain numerous pollen mother
• cells (PMCs).
• Each PMC undergoes meiosis to produce four haploid cells or microspores.
• This process is known as microsporogenesis.
• Microspores mature into pollen grains mainly by a thickening of their walls.

b) Megasporogenesis:

• Megasporogenesis occurs in ovules, which are present inside the ovary.
• A single cell in each ovule differentiates into a megaspore mother cell.
• The megaspore mother cell undergoes meiosis to produce four haploid megaspores.
• Three of the megaspores degenerates leaving one functional megaspore per ovule.
• This completes megasporogenesis.

III. Gametogenesis:

• The production of male and female gametes in the microspores and the megaspores, respectively, is known as gametogenesis.

a) Microgametogenesis:

• This refers to the production of male gamete or sperm.
• During the maturation of pollen, the microspore nucleus divides mitotically to produce a generative and a vegetative or tube nucleus.
• The pollen is generally released in this binucleate stage.
• When the pollen lands onto the stigma of a flower, it is known as pollination.
• Shortly after pollination, the pollen germinates, the pollen tube enters the stigma and grows through the style.
• The generative nucleus now undergoes a mitotic division to produce two male gametes or sperms.
• The pollen, along with the pollen tube, is known as microgametophyte.
• The pollen tube finally enters the ovule through a small pore, micropyle, and discharges the two sperms into the embryo sac.

b) Megagametogenesis:

• The nucleus of a functional megaspore divides mitotically to produce four or more nuclei.
• In most of the crop plants, megaspore nucleus undergoes three mitotic divisions to produce eight nuclei.
• Three of these nuclei move to one pole and produce a central egg cell and two synergid cells; one synergid is situated on either side of the egg cell.
• Another three nuclei migrate to the opposite pole to give rise to antipodal cells.
• The two nuclei remaining in the center, the polar nuclei, fuse to form a secondary nucleus.
• The megaspore thus develops into a mature megagametophyte or embryo sac.
• The development of embryo sac from a megaspore is known as megagametogenesis.
• The embryo sac generally contains one egg cell, two synergids, three antipodal cells (all haploid), and one diploid secondary nucleus.

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 through hybridization.