Plant Propagation and Nursery Management (1+1)

Germination and Dormancy

Giberellic acid is the most potent germination promoter, by breaking seed dormancy in a wide range of species like peach, wild mustard, citrus, rough lemon, trifoliate orange, sweet orange, beans, peach, chinese cabbage etc. Giberellin occurs at a relatively high concentration in developing seeds but usually drop to a lower level in mature dormant seeds. Giberellins play an important role in the initial enzyme induction, activation of reserve food and certain types of dormancy, including physiological dormancy, photo dormancy and thermo dormancy.
Cytokinin is believed to offset the effect of inhibitors like ABA in germination. It allows giberellic acid to function. Applied cytokinin can also be effective in overcoming thermo dormancy.
Ethylene has been effective in overcoming seed dormancy in snowberry (Symphoricarops), honey suckle (Lonicera), corn and other cereals, germinating bean and pea seeds produce ethylene. Ethylene is a natural germination promoting agent for certain kinds of seeds, but has limited role in germination of seeds.
Other Nutrients: Use of potassium nitrate has been an important seed treatment in seed testing laboratories. Thiourea overcomes certain types of dormancy, such as the seed coat deep embryo-dormancy. Prunus seeds as well as high temperature inhibition of lettuce seeds. The effect of thiourea may be due to its cytokinin activity in overcoming inhibition.
Hormonal changes during stratification
A triphasic change in endogenous hormones is typical for many seeds; generally there is:

I. A reduction of ABA
II. Increased synthesis of cytokinin and gibberellins
III. Reduction in hormone synthesis in preparation for germination.




Fig.12.1. Triphasic changes in hormone production during chilling stratification

In general, gibberellins promote germination in dormant seeds, while ABA inhibits germination (Fig.12.1). Pre-sowing treatments with certain seeds not only reduce the stratification requirement and improve the seed germination but also enhances seedling growth in a number of temperate fruits.