SEXUAL PROPAGATION OF PLANTS:-
Sexual propagation of plants involve the union of male and female sex cells, the formation of seeds and therefore, creation of population of seedlings with new and different genotypes.
In sexual reproduction, meiosis cell division takes place. In meiosis, the cells produced are gametes or spores. Gametes unite together to form new offspring cells. The offspring produced may resemble either or none of the parents. The inheritance takes place due to gene present on the chromosomes of the sex cells. If produced by self-pollination, population is called Homozygous. If produced by cross-pollination of plants, population is called Heterozygous.
Process of Sexual propagation of plants:
The process of sexual reproduction in plants involves four basic steps which are seed production, seed formation, seed dormancy treatment and germination.
Seed certification programs recognize four classes of seeds in agronomic crops, which are as follows.
- Breeder Seed: produced in small amounts under the control of breeder.
- Foundation seed: Produced in limited amount to produce registered seed.#
- Registered seed: it is seed source for growers of registered seed.
- Certified seed: It is available in large quantities and sold to farmers for general crop production.#
Pollination is the first step in reproduction. Pollen grams are shed from the Anthers and fall onto the feathery stigmas.
Fertilization is the second step in seed formation. The pollen that reaches the stigma germinates and forms a pollen tube that carries the male cells to the ovary. A male cell unites with the female cell in the Ovule. The fertilized ovule is a seed.
A seed has three essential parts;
- The embryo which develops into new plant.
- Food storage material for survival.
- Seed covering for protection.
Seed dormancy treatment:-
Dormancy is the survival mechanism not only of seed but also of plants. If seed germinal in adverse conditions, it would not be able to survive. Dormancy can persist indefinitely until and unless favourble conditions are provided for the germination.
Types & treatment of Seed Dormancy:
Seed Coat Dormancy:-
Seed coating is impermeable to water and nutrition, which therefore resists germination. In other case, the seed coating is so hard that it devoid the embryo to break coating and germinate. In such conditions, softening of seed coal is done through following methods.
Scarification is the mechanical screening or rubbing of seed coat. It is done through sheets of sandpaper, abrasive conundrum boards, and rotating and tumbling the seeds in drums. In scarification, utmost care is needed because insufficient scarification won’t improve germination and over-scarification will damage the embryo.
Seeds soaked in cone, H2 SO4 for 15 mints to three hours, depending upon the type of seeds. After washing with water for 15 minutes, dry them and plant them.
Soaking the seeds in boiling water for 24 hours and then drying them before plantation.
Embryo dormancy is due to presence of physiological germination blocks in the embryo itself which hinders growth even in favourable conditions.
In stratification, boxes filled with alternate layers of soil and seeds are kept under controlled conditions of temperature, moisture and lights for 20-90 days, depending upon the type of seed. During stratification, level of growth promoting hormones increase and level of growth inhibiting hormones decrease, permitting germination. This change that allows germination is called after-ripening.
Seed coat dormancy as well as Embryo dormancy:
Seeds having viable embryo, absence of germination blocks, and proper environment conditions like light, moisture and temperature resumes germination. Initially, the nutrients stored in the endosperm of the seed nourish the developing embryo. Later, when new shoots rise above grounds and develops leaves; plant starts producing its own food through photosynthesis.
The sequence of events during seed germination is as follows:
Absorption of water by Seeds:-
Seeds absorb water and sell. The seed covering becomes soft and then undergo rupture, permitting easy passage covering becomes soft and then undergo rupture, permitting easy passage of gases and nutrients into the seed.
Activation of Hormones and Enzymes:-
Absorption of water and pressure of nutrients and gases activate certain hormones and enzymes already present inside the seed. Enzymes convert the complex food storage molecules into simple chemicals which are used for growth. These chemicals are trans-located to every part of the plant, including roots and shoots.
Inside the seed, embryo develops and grows into a full plant with roots, shoots and leaves etc. The photosynthesis tissues make food for survival and become an independent plant.
Factors influencing seed germination:-
Adequate amount of water is inevitable for embryo growth and seed survival. Soaking of seed before sowing facilitates germinations but prolonged soaking is harmful.
Seeds of cool season (peas etc) germinate well at low t (O – 10c).
Seeds of worm season (soybean etc.) germinate well at high T (15-26c)
Low aeration retards germination (rice is exception which grows well in anaerobic conditions). Overwatering, compaction of soil and other cultural activities retard aeration of soil.
Fungi pathogens most destructive.
Continuous evaporation of irrigation water and water logging affects seed germination.
Seedlings are injured by tools during intercultural operations, which may also results in poor germination of seeds.
Advantages of Sexual Propagations:
- Seeds is cheaper, abundant and easily available.
- Sexually propagated plants have long life than vegetative propagated plants.
- Seeds have long shelf life; not perishable.
- It leads to the production of chance seedlings which are sometime superior to the parent plant.
- Relatively more resistant to pests/insects diseases than asexually propagated plants.
- Seedlings can bear adverse environmental conditions in a better way than vegetative propagated plants.
- They are easily transportable.
- No special skills/techniques required for sowing the seed or raising the plant.
Disadvantages of Sexual Propagations:
- Promotes heterozygosity.
- Seeds of fruit plants lose viability after extraction and show low germination rate.
- Seeding plants spread and are tall; requiring intensive care and resources.
- Seed-borne viruses exist in a number of fruit plants. Thus, multiplication of plants through sexual propagation is not feasible.
- It is not possible to perpetuate the exact character of any superior selection through seeds.