Vernalization is a term coined by the Russian agronomist Lysenko, to refer to the method of acceleration of the flowering ability of biennials or winter annuals, by exposing their soaked seeds to low temperatures. However, the term has been extended also to include the promotion of flowering in plants by exposing them to low temperatures at any stage in the life-cycle.
The two species on which classic experiments in vernalization were performed are Hyoscyamus niger and Secale cereale. Both these plants are LDPs and have winter and spring varieties. The winter varieties are sown in winter and harvested in spring, thus they act as biennials, whereas their spring varieties act as annuals, that is, they are sown in spring and harvested in the same season. When the winter varieties of both these species are sown in spring, they do not flower in the spring, but if they are kept alive by providing green house conditions, they bloom only in the next spring after they have passed through a winter season. However, if these spring-sown winter varieties are exposed to low temperature (vernalised) they start blooming in the spring itself. Thus winter varieties of these species of plants, when vernalised resemble the spring varieties in every way.
Some perennials have either a qualitative or quantitative vernalization response. Many grasses bloom luxuriantly after vernalization. In Chyrysanthemum (SDP), if the vernalised plants are vegetatively propagated, they do not need vernalization any more. Most temperate zone trees and shrubs require low temperature, as a precondition breaking dormancy and opening in the spring. Once a plant is vernalized the changed response usually persists for a long time.
Flower induction in unvernalized plants when grafted to vernalised plants, suggests that a hormonal mechanism is involved in the process of vernalization. It has been suggested that a hormone vernalin may be involved in vernalization. However, vernalin has not been isolated so far and nothing is known about its chemical nature. Suggestions have also been made that vernalin may be one of the gibberellins. This also seems to be doubtful.
Abscission is shedding of leaves, fruits or flowers by a plant, generally due to a change in the hormonal balance. A separation (abscission) layer is developed within the region of attachment. The middle lamella between certain cells in this layer, is often digested by polysaccharide-hydrolysing enzymes, such as cellulase and pectinases. Other degenerative changes also occur, making the region soft and weak. The organ from the plant is then easily detached whenever there is heavy rainfall or wind, etc. Abscisic acid plays an important role in this process.