Vernalization

Vernalization (from Latin vernus, "of the spring") is the induction of a plant's flowering process by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually flower. Vernalization is sometimes used to refer to herbal (non-woody) plants requiring a cold dormancy to produce new shoots and leaves but this usage is discouraged.

Many plants grown in temperate climates require vernalization and must experience a period of low winter temperature to initiate or accelerate the flowering process. This ensures that reproductive development and seed production occurs in spring and summer, rather than in autumn. The needed cold is often expressed in chill hours. Typical vernalization temperatures are between 5 and 10 degrees Celsius (40 and 50 degrees Fahrenheit).

For many perennial plants, such as fruit tree species, a period of cold is needed first to induce dormancy and then later, after the requisite period of time, re-emerge from that dormancy prior to flowering. Many monocarpic winter annuals and biennials, including some ecotypes of Arabidopsis thaliana and winter cereals such as wheat, must go through a prolonged period of cold before flowering occurs.

In the history of agriculture, farmers observed a traditional distinction between "winter cereals," whose seeds require chilling (to trigger their subsequent emergence and growth), and "spring cereals," whose seeds can be sown in spring, and germinate, and then flower soon thereafter. Scientists in the early 19th century had discussed how some plants needed cold temperatures to flower. In 1857 an American agriculturist John Hancock Klippart, Secretary of the Ohio Board of Agriculture, reported the importance and effect of winter temperature on the germination of wheat. One of the most significant works was by a German plant physiologist Gustav Gassner who made a detailed discussion in his 1918 paper. Gassner was the first to systematically differentiate the specific requirements of winter plants from those of summer plants, and also that early swollen germinating seeds of winter cereals are sentitive to cold. In 1928 a Russian geneticist Trofim Lysenko published his works on the effects of cold on cereal seeds, and coind the term "яровизация" ("jarovization") to describe a chilling process he used to make the seeds of winter cereals behave like spring cereals (Jarovoe in Russian, originally from – from jar meaning fire or the god of spring). Lysenko himself translated the term into "vernalization" (from the Latin vernum meaning Spring). After Lysenko the term was used to explain the ability of flowering in some plants after a period of chilling due to physiological changes and external factors. The formal definition was given in 1960 by a French botanist P. Chouard, as "the acquisition or acceleration of the ability to flower by a chilling treatment."

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