Sexual Propagation in Plants
Multiplication of plants by using seed is called as sexual propagation.
Advantages:
1. The plant raised by seed is planted lived.
2. They are hardy with deep root system. So they are vigorous in growth.
3. The possibility is there to obtain change in seedling, the performance of which are better than their parents. E.g. mango variety like Alpahanso Dasharia.
4. The polyembryony. The phenomenon of propation of more than one seedling from a single seed, produce true to type, nuclear embryonic seedling which could be used as rootstock for uniform performance. E.g. Mango, varieties. Olour and bappakal. It is also common in citrus and jamun.
5. Seed propagation is necessary when vegetative propagation is unsuccessful or expenses e.g. papaya, coconut and Areca nut.
6.Exoplotaion of hybrid, vigor is possible only when when the hybrid s are multiplied in the first instance through sexual propagation although subsequent fixing of heterocyst is effected through vegetative propagation, e.g. Sapota ( Cricket Ball X oval ) and ratna Mango ( Alpahanso *Neelum ).
7. Roots stocks are usually raised by seed e.g. Rangpur lime and Jamberi for citrus.
8. When seedling is required in large number, seed propagation is the only easy mean e.g. Dry land fruit, and Forest spp.
Disadvantages:
1. When progenies are not true type an so they become inferior because in the commercial orchard, it is necessary to have uniform quality, growth and yielding capacities.
2. Choice tree or any hybrid trees cannot be perpetuated true to type by seed. (except in Apomixes )
3. Seedling has a long juvenile period. In crops like citrus, coca, and rubber. The seeds must be sown afresh. i.e. immediately after extraction. Many varieties are seedless.
4. Seeds loose its viability in short period
.
Seed Germination and Seed Propagation:
Seed propagation is necessary in the following cases:
i) Where vegetative propagation is unsuccessful or difficult or expensive.
ii) It is necessary for raising rootstock for grafting and budding. E. g Rangpur lime and Jamberi for citrus, khirni for Sapota.
In all such cases rootstock plants have to be raised through seeds mostly.
Seed Formation and Maturity:
Seed develops along with the fruit and reaches, full size and maturity when the fruit ripens. Hence seed should be extracted only from ripe fruit. Seeds gathered from immature fruit may not germinate to under favorable conditions and may loose viability more quickly than fully matured seeds.
Seed Storing:
Seeds should be normally stored in relatively dry condition at low temperature. Some seeds should be sown immediately after extraction. (Citrus).Other must be kept for sometimes depending upon the kind of plant. Seeds of most plant retain their viability longer when stored at a relatively low then at a high temperature. Hence store the seeds in dry, cool place. If exported to damp atmosphere, even after through drying seeds absorb moisture and rapidly deteriorate. Some seeds like close of apple, stone fruit loose viability if dried after harvest. They require to be kept moist and exposed o cold or freezing temperature, known as stratification to after ripen dormant and to modify seed coverings.
Dormancy:
It is term used to describe a seed that will not germinate because of any condition associated either with the seed itself or with existing environmental factors such as temperature and moisture.
Some seeds may even germinate within the fruit, e.g. Jack, avocado, chow- chow, papaya, called as Viviparous germination.
Best Period:
Some seeds will not germinate immediately after harvest even if conditions are favorable. This failure to germinate is due to physiological condition. This is said to be the seeds are in the rest period or they require rest period after ripening period.
Seed viability and longevity:
Viability means the presence of life in the seed. Longevity refers to the length of time that seeds will retain their viability viability. Some seeds are short lived. (Citrus).
Pre – germination Seed Treatment:
1. Chemical (Acid scarification):
The purpose is to modify hard or important or impermeable seed covering generally soaking seed in concentrated sulphuric acid is an effective method. The time of treatment may vary from 10 minutes to 6 hour according to species. After treatment seeds are thoroughly washed in clean water to make them free of acid and then re sown immediately. i.e. the seeds of ber, cotton, Asparagus are treated with 50% concentrated H2so4. The seeds are soaked in acid for 3 to 5 minutes.
2. Mechanical (Scarification):
Seeds of a few species with impermeable seed coat. i.e. hard seed coat can be rendered permeable to water and gases their germination is greatly improved by mechanical scarification in taking care that seeds should be injured not be injured heavily. This can be achieved by
i) Placing the seeds between two sand paper doses, one station and other revolving.
ii) Passing seeds through machine that scratches the surface.
iii) Filling and notching to make the seed coat permeable to water.
iv) By use of harmer. E.g. Lotus Anna.
3. Seedling (Boiled Water Treatment):
Pouring boiling water over seeds and getting it to cool gradually for about 12 to
Hour to soften dry and hard shelled seeds. E.g. Coffee, chicku, canna , Babul, Chillar,etc.This will lasten the process of germination.
4. Soaking in Water:
The purpose of soaking seeds in water is to modify hard seed coats, to remove inhibitors to soften seed and to reduce the time of germination. The time of soaking seeds in cold water depend upon the hardness of the seed coat. E. g Gulmohar, peas, beans, cassia tree etc.
5. Stratification (Moist Chilling):
Seed of many woody trees or shrubs are exposed to low temperature to bring about prompt and uniform germination. Stratification. Stratification has some benefit in softening the seed coats. The seeds are arranged in alternate layers of sand in shallow boxes for pits or trenches. This condition helps in rapid germination peach cherry, plum, oat, grapes.