Vernalisation

Day length is not the only environmental cue that can control the timing of flowering. In a number of plants, particularly those from temperate regions, flowering is instead promoted or hastened by a period of exposure to low temperature. This phenomenon is called vernalisation (from the Latin vernus, "of spring").

Why some plants need a cold period

Many biennial and winter-annual plants — species that would otherwise take an unusually long time to flower, or fail to flower at all in a single growing season — flower rapidly and reliably once they have first experienced a period of low temperature, usually during winter. The cold exposure does not by itself cause flowering; rather, it removes a block and makes the plant competent to respond to the other conditions (such as the right photoperiod) that trigger flowering once warmer weather returns.

Winter and spring varieties of wheat

A classic illustration of vernalisation is seen in different varieties of wheat:

• Spring varieties of wheat are sown in spring and complete their life cycle — including flowering and grain formation — within that same growing season, since they have no cold requirement.
• Winter varieties of wheat, by contrast, are sown in autumn. They germinate and grow through the cooler months, are naturally exposed to the low temperatures of winter, and then flower and are harvested the following summer. If a winter variety is instead sown in spring, it fails to receive the cold exposure it needs, and so it does not flower (or flowers only very poorly) within that growing season.

This is why winter wheat is deliberately sown in autumn rather than spring — the winter months themselves supply the cold treatment the plant needs before it can flower.

Substituting the cold requirement

The vernalisation requirement can, in many species, be artificially satisfied without a genuine winter — for example, by treating moistened, germinating seeds with a period of low temperature under controlled conditions before sowing. This treated seed will then flower on schedule even if sown later, without needing to pass through an actual cold season.

Experimentally, the plant growth regulator gibberellin has been shown to be able to substitute for the cold treatment in a number of vernalisation-requiring plants — applying gibberellin can induce flowering in some biennials even without any exposure to low temperature at all, which is one of the pieces of evidence linking the vernalisation response to the plant's hormonal growth-regulation system described earlier in this chapter.

Vernalin — a proposed but unconfirmed messenger …