• The mode of reproduction determines the genetic constitution of crop plants — whether plants are normally homozygous or heterozygous. This in turn determines the goal of a breeding programme. 
• If crop plants are naturally homozygous (self-pollinators like wheat, rice, barley), then a homozygous pure line is the most suitable varietal type. 
• If plants are naturally heterozygous (cross-pollinators like maize, bajra, sunflower), then a heterozygous population — typically an F1 hybrid or a synthetic variety — is the more appropriate variety type.
• Consequently, breeding methods are vastly different for self-pollinated and cross-pollinated crops. A thorough knowledge of the mode of reproduction is also indispensable for making artificial crosses — the ability to emasculate, pollinate, and control mating is the foundation of hybridization-based breeding.

ASEXUAL REPRODUCTION

Asexual reproduction does not involve fusion of male and female gametes. New plants may develop from vegetative parts of the plant (vegetative reproduction) or from embryos that develop without fertilization (apomixis). Both forms produce offspring that are genetically identical to the parent plant.

2.1  Vegetative Reproduction

In nature, a new plant develops from a portion of the plant body — stems, roots, leaves, or modified structures. This may occur through modified underground stems, sub-aerial stems, or bulbils.

A. Natural Vegetative Reproduction — Underground Stems

Underground modifications of the stem serve as storage organs and contain many buds. These buds develop into shoots and produce plants after rooting. The following types are recognised in the source materials:

• Tuber: A swollen underground stem storing food reserves with prominent eyes (buds). Example: Potato (Solanum tuberosum). Tubers are planted directly as 'seed potato'. Each eye can develop into a new plant.
• Bulb: A short, disc-like underground stem with fleshy leaf bases (scales) enclosing the terminal bud. Examples: Onion (Allium cepa), Garlic (Allium sativum). In garlic, the inflorescence also produces bulbils (see below).
• Rhizome: A horizontal underground stem with nodes and internodes; grows below soil surface. Examples: Ginger (Zingiber officinale), Turmeric (Curcuma longa), Banana (through underground rhizomes producing suckers). Rhizomes have nodes bearing scale leaves and axillary buds from which new shoots arise.
• Corm: A solid, swollen, vertical underground stem without fleshy leaves. Examples: Bunda (Colocasia esculenta, also called arvi or taro), Elephant foot yam. Corms differ from bulbs in being solid rather than composed of leaf scales.

B. Natural Vegetative Reproduction — Sub-aerial Stems

These modified stems run along the soil surface or just above it. They include:

• Runner: A slender, elongated stem growing horizontally along the soil surface, rooting at nodes. Example: Strawberry (Fragaria). Runners can travel considerable distances from the parent plant before rooting.
• Stolon: Similar to a runner but may grow above or below ground. Example: Mint (Mentha). Stolons root at the tip or at nodes.
• Sucker: A shoot arising from a bud near the base of the stem, at or below the soil surface. Examples: Banana, pineapple, date palm. The sucker becomes a new plant when detached.

C. Bulbils

• Bulbils are modified flowers or modified axillary buds that develop directly into plants without formation of seeds. 
• They are vegetative bodies — their development does not involve fertilization. 
• The lower flowers in the inflorescence of garlic (Allium sativum) naturally develop into bulbils instead of normal flowers. 
• Scientists are trying to induce bulbil development in plantation crops by culturing young inflorescences on tissue culture media; it has been successfully done in the case of cardamom (Elettaria cardamomum) 

D. Artificial Vegetative Reproduction

Artificial vegetative propagation is commonly used for many crop species where natural vegetative reproduction does not occur or is insufficient. The following methods are in common use:

• Stem cuttings: A portion of the stem is cut and induced to root by treatment with rooting hormone (IBA — Indole-3-butyric acid). Commercially used for sugarcane (cane setts — each sett has 2-3 nodes), grapes, roses, cassava, sweet potato.
• Layering: A stem is bent to the ground, covered partially with soil, and allowed to develop roots while still attached to the parent plant. When well-rooted, it is separated. Used in jasmine, strawberry, guava.
• Air layering (Gootee / Marcotting): A ring of bark is removed from a branch, wrapped with moist sphagnum moss or mud covered with polythene. Roots develop at the wound. Used in mango, litchi, guava, rubber.
• Budding: A bud from one plant is inserted under the bark of another (rootstock). Widely used in rose, citrus, mango.
• Grafting: A shoot portion (scion) is united with a rootstock. Scion provides the aerial variety while rootstock provides root system and some disease resistance. Used in mango, citrus, grapevine, tomato.
• Tissue culture micropropagation: Meristem, shoot tip, or other explants cultured on MS medium to regenerate many plants. Used commercially in banana, sugarcane, potato (virus-free material), orchids, carnation, chrysanthemum, strawberry, potato. Discussed in detail in the Biotechnology chapter.

E. Significance of Vegetative Reproduction in Plant Breeding

Vegetatively reproducing species offer unique breeding possibilities that sexually reproducing species do not. 

• A desirable plant may be used directly as a variety regardless of whether it is homozygous or heterozygous. This means the breeder does not need to fix homozygosity through repeated selfing — a heterozygous genotype with high performance can be directly multiplied and used as a variety.
• Mutant buds, branches, or seedlings that show desirable phenotypic changes can be identified, multiplied vegetatively, and used directly as varieties. This is the basis of clonal selection in vegetatively propagated crops.
• Vegetative propagation allows indefinite maintenance of a specific genotype, including heterozygous genotypes that would segregate and change if sexually reproduced.
• F1 hybrids in vegetatively propagated crops (potato, sugarcane, sweet potato) can be maintained clonally across generations without the loss of heterozygosity that would occur through seed.

2.2  Apomixis

In apomixis, seeds are formed but the embryos develop without fertilization. Consequently, the plants arising from them are genetically identical to the parent plant. This is essentially seed-mediated clonal reproduction.

A. Obligate vs Facultative Apomixis

• Obligate apomixis: Sexual reproduction is completely absent. All offspring are produced apomictically. The plants are genetically identical clones of the mother. Examples: most Taraxacum (dandelion) species.
• Facultative apomixis: Sexual reproduction can occur alongside apomixis. The plant can produce both apomictic (clonal) seeds and sexually produced seeds in the same or different seasons. Most crop species showing apomixis are facultative apomicts. This is important because it means hybridization is still possible, but apomictic seeds in the progeny must be distinguished from sexually produced seeds.

B. Types of Apomixis 

1. Adventive Embryony (Nucellar Embryony):

• Embryos develop directly from vegetative cells of the ovule — specifically from the nucellus, integument, or chalaza cells — without involving the formation of an embryo sac. This is the most commercially important form of apomixis and is distinct from the other types in that no embryo sac is formed.
• Crops affected: Mango (Mangifera indica), Citrus species (all cultivated citrus), Garcinia mangostana (mangosteen), Opuntia (prickly pear). In mango, seeds regularly produce multiple embryos — one from fertilization and one or more from nucellar cells. The nucellar embryos are genetically identical to the mother tree.
• Breeding significance: In citrus and mango rootstock production, nucellar embryony is used to produce true-to-type rootstock seedlings that are genetically uniform and free from viruses (which are not transmitted through nucellus). However, it makes variety development difficult because all progeny are identical — hybridization produces apomictic progeny that do not segregate.

2. Apospory:

• Some vegetative cells of the ovule (usually nucellar cells) develop into unreduced (diploid) embryo sacs, bypassing the normal meiotic process of megasporogenesis. The embryo may then develop from the egg cell or from some other cell of this embryo sac. Since the embryo sac is unreduced, the embryo is diploid.
•  Examples: Hieraceum (hawkweed — the plant Mendel studied next after peas, but which gave confusing results because of apomixis!), Malus (apple relatives), Crepis (hawk's beard), Ranunculus (buttercup), Panicum, Poa, Paspalum.
• Note on Hieraceum: Mendel's attempts to repeat his pea experiments with Hieraceum failed because the plants showed aposporous apomixis — the offspring did not segregate as Mendelian ratios predict, confusing Mendel considerably. This is a fascinating historical footnote. I believe this account is well-established in genetics history but recommend verifying from a genetics history textbook.

3. Diplospory:

• The embryo sac is produced from the megaspore mother cell itself, which may fail to complete meiosis normally (or meiosis is so modified that the megaspore remains diploid). The resulting embryo sac is unreduced. Diplospory leads to parthenogenesis or apogamy.
• Examples: Taraxacum (dandelion), Eragrostis (a grass), Antennaria.

4. Parthenogenesis:

The embryo develops from the embryo sac — specifically from the egg cell — without fertilization. Two types are described:

• Gonial (Generative) parthenogenesis: Embryo develops from the egg cell (which is the female gamete). The resulting plant may be haploid (if the egg cell was produced by normal meiosis — non-recurrent apomixis) or diploid (if meiosis was suppressed and the egg cell is unreduced — recurrent apomixis).
• Somatic parthenogenesis: Embryo develops from any cell of the embryo sac other than the egg cell — for example, from synergids or, in some definitions, from other cells. 

5. Apogamy:

• In apogamy, synergids or antipodal cells of the embryo sac develop into an embryo. Like parthenogenesis, apogamy may be haploid or diploid depending on whether the embryo sac was reduced or unreduced. Diploid apogamy occurs in Antennaria, Alchemilla, Allium, and many other plant species.

C. Recurrent vs Non-Recurrent Apomixis

• Recurrent apomixis: The embryo sac is unreduced (diploid). The resulting embryo is therefore diploid and has the same chromosome number as the mother plant. The offspring is genetically identical to the mother. All the above types (adventive embryony, apospory, diplospory leading to diploid parthenogenesis, diploid apogamy) are forms of recurrent apomixis.
• Non-recurrent apomixis: The embryo sac is reduced (haploid, produced by normal meiosis). The embryo develops from the egg or other cells of the haploid embryo sac without fertilization. The resulting plant is haploid. Non-recurrent apomixis is of no practical significance in variety development but may be useful in haploid plant production. 

D. Significance of Apomixis in Plant Breeding

Apomixis has a dual significance in plant breeding — it creates both difficulties and opportunities :

Difficulties created by apomixis:

• When the breeder wants to obtain sexual progeny (selfs for inbred line development, or hybrids for F1 heterosis exploitation), apomixis produces apomictic progeny that do not reflect true sexual recombination. The breeder must distinguish apomictic from sexual seedlings, which is laborious.
• Multiple embryony in mango and citrus seeds means several seedlings emerge from one seed — the breeder must identify the sexually produced seedling (which is usually the most vigorous, most different from the mother) among the nucellar embryos.

Benefits of apomixis:

• Variety maintenance: Once a desirable genotype is selected, it can be multiplied and maintained indefinitely through apomictic progeny. This keeps the genotype of a variety completely intact across generations without genetic drift or segregation.
• Fixation of heterosis: This is the most revolutionary potential application. If apomixis could be transferred into F1 hybrid varieties of major cereals (rice, maize, wheat), farmers could save and resow the seeds of F1 hybrids and still recover full hybrid vigour in subsequent generations. This would eliminate the need for commercial hybrid seed each season, dramatically reducing the cost of hybrid crop production. This remains a major research objective globally.  for example, CIMMYT has had programmes on apomixis in Tripsacum and maize 
• Rootstock uniformity in citrus and mango: Nucellar embryony produces true-to-type rootstock seedlings that are genetically uniform, disease-free (nucellus is virus-free), and represent the mother plant exactly. This is commercially valuable in citrus nursery production.