Chilling and Flower Induction
Some plants flower only after passing a winter season. For example, winter wheat is sown in the autumn for harvest in the following summer. It needs exposure to cold. If winter is mild, plants do not flower and the crop fails. It has been shown that winter wheat and many other plants require a period of chilling 0° -2° C for about a week for flower formation. The cold treatment given for flower induction is called vernalisation. The technique of vernalisation was developed in Russia where winter crop required chilling for successful cultivation. The seeds are soaked for small period to initiate germination and then they are buried in the snow. Later, they are planted in the spring when severely cold conditions are over.
Thus chilling is also a stimulus for flower induction. Which part of the plant requires chilling stimulus for floral induction? Experiments show that only the shoot apex receives the stimulus (needs vernalisation) which then is passed on to the other parts of the plant. When the shoot apex that has received the stimulus is pinched off, the lateral shoots flower and if their apices are also pinched off, the side shoots develop and flower. Moreover when extracts of a vernalised plant are applied to a long-day plant growing under short-days the recipient plant flowers. Like light induction, the stimulus can also pass through a graft to a non-vernalised plant. These results show that some kind of flowering stimulus is transmitted from shoot apex to other tissues. The chilling stimulus was named vernalin. The nature of this compound has yet not been identified.