Many biennial and winter annual plants require prolonged exposure to low temperatures before they can flower. Such exposure brings them to the condition they would normally reach in spring, and is therefore known as vernalization.
Like photoperiodism and many other phenomena, vernalization was known long before it was recognized. In an essay written in 1857 Klippart51 wrote:
'To convert winter into spring wheat, nothing more is necessary than that the winter wheat should be allowed to germinate slightly in the fall or winter, but kept from vegetation by a low temperature or freezing, until it can be sown in the spring. This is usually done by soaking and sprouting the seed, and freezing it while in this state and keeping it frozen until the season for spring sowing has arrived. Only two things seem requisite, germination and freezing. It is probable that winter wheat sown in the fall, so late as only to germinate in the earth, without coming up, would produce a grain which would be a spring wheat, if sown in April instead of September. The experiment of converting winter wheat into spring wheat has met with great success. It retains many of its primitive winter wheat qualities, and produces at the rate of 28 bushels per acre.'
|E.g. Allen, 1849|
McKinney65 also cites even earlier writings indicating that the idea of vernalization was well known in the USA in Klippart's time.
Gassner40 was the first to make a systematic analysis of the phenomenon of vernalization. He sowed seeds of both spring and winter forms of Petkus rye on a series of occasions, and held them at 1-2°C, 5-6°C, 12°C or 24°C, until they appeared above the ground, after which they were planted out. He found that whereas spring rye did not have a 'chilling requirement' for flowering, winter rye would not flower unless it was held at low temperatures for a prolonged period.
Gassner's work was extended by Maximov, and by several other Russian investigators, particularly Lysenko, who first coined the name jarovizacija, the anglicized equivalent of which is vernalization. Lysenko contributed more than a dramatic name, however. Several investigators had obtained conflicting results in attempting to repeat Gassner's experiments and it appears to have been Lysenko who first recognized,63 as a result of experiments by Meljnik, that many winter cereals would flower only when vernalization was followed by exposure to long-day conditions. Previously, in 1928, Lysenko had postulated62 that the processes of development in plants are independent of growth, and take place in a series of irreversible steps or phases, which must be completed in a strict sequence. This was his theory of phasic development.
Lysenko's idea that reproductive development was separate from growth was by no means new. MacDougal in 190364 had indicated that they were distinct processes capable of separation. Similarly, Lysenko's idea that development proceeded in a series of steps each with its own environmental requirements had been clearly enunciated by Klebs in 1918.50 What was new was Lysenko's attempt to provide a single all-embracing scheme for the development of all plants. He could hardly have chosen a more perverse field to which to apply his rigid generalizations, and exceptions soon appeared. For example, Gregory and Purvis62 showed that the effects of vernalization could, under some conditions, be reversed by subsequent high temperatures, in the process of devernalization. At the time this finding was of greatest significance in showing that progress in development is not irreversible as Lysenko had postulated. However, the phenomenon of devernalization subsequently proved to be most useful in further analysis of the nature of the process of vernalization.