Dormant Vegetative Structures
You have learnt that light a period of chilling or application of gibberellin often breaks seed dormancy and stimulates seed germination. Vegetative organs such as tubers, corms, bulb and winter buds can also enter a period of dormancy. What kind of stimulus is required for these dormant structures to resume growth? It is observed that light can stimulate bud break in certain trees and a period of chilling is necessary for triggering the development of dormant buds, and other organs in plants of temperate region. The period of chilling required for the bud break is usually a week or more.
Similarly, lengthening of days is necessary for breaking dormancy of winter buds exposed to light. Dormant buds can also be stimulated by the application of gibberellins. As you know gibberellins promote growth and Abscisic acid inhibits the growth. Accordingly, on breaking of bud dormancy increase in the level of GA and fall in the level of Abscisic acid has been noted. It needs to emphasised here that though all the buds of a plant receive the same dormancy breaking stimulus, however, it is only the apical bud that grows and the growth of other buds is inhibited by it because of apical dominance. Plants reproduce vegetatively through rhizomes, runners or stolons. Tuber formation in potatoes can be induced by exposing leaves to day length less than the critical value. Artificial application of hormones show that in tuber formation gibberellic acid decreases while Abscisic acid increases. The results show that the level of a particular hormone can increase growth in some organ and decrease in other.