Vernalization (Yarovizatsiya)

The Phenomenon and Its Mechanism

Vernalization is the induction or acceleration of a plant's flowering by prolonged exposure to winter cold, or by an artificial equivalent. Even after treatment a plant may still need additional seasonal cues or weeks of growth before it actually flowers. Many plants of temperate climates must pass through cold in order to initiate or advance flowering, which ensures that reproduction and seed production occur in spring rather than autumn. The required cold is expressed in chill hours, and typical treatment temperatures are between 1 and 7 degrees Celsius. If the cold period is too short or temperatures stay above 15 degrees, vernalization is delayed or does not occur.

The mechanism has been studied most fully in Arabidopsis thaliana. Flowering is repressed by the protein FLC, which blocks the genes that promote flowering; prolonged cold induces VIN3 expression, which through a polycomb-like complex lowers FLC expression by chromatin remodeling. This inactive state is registered as stable silencing of individual FLC loci, and because the silent chromatin marks are removed during embryogenesis, the vernalized state is not inherited. Exposure to high temperatures of 25 to 40 degrees can reverse the effect (devernalization).

Research before 1929

The link between cold and flowering was already observed in the 19th century. In 1857 the American agriculturist John Hancock Klippart reported on the effect of winter temperature on the germination of wheat, and in 1918 the German plant physiologist Gustav Gassner systematically distinguished the requirements of winter plants from those of summer plants and showed that the swollen germinating seeds of winter cereals are sensitive to cold. In the USSR the effect of low temperature on plant development was studied in the 1920s, for example by Maksimov, and once the genetic basis was established the difference between spring and winter crops was explained by genes controlling flowering time.

In 1928 the Soviet agronomist Trofim Lysenko published his work on the effect of cold on cereal seeds and coined the Russian term yarovizatsiya for the chilling process that made winter cereal seeds behave like spring cereals; he himself rendered it as vernalization, from the Latin for spring. The Russian term came to denote the phenomenon generally.

Mass Introduction and Abandonment in the USSR

Lysenko introduced the technique into Soviet agriculture on a mass scale. The vernalized area grew year by year and in 1941 reached 13 percent of the total grain sowing area of the USSR. His method, however, gave inconsistent results and was labor-intensive. Under laboratory conditions vernalizing spring crop seeds worked best at 0 to 5 degrees, while his instructions specified 5 to 12 degrees, and turning the moistened seed damaged the sprouts, so that sowing rates had to be doubled. Vernalization did not raise the yield of winter cereals, and treating them with cold weakened the plants. Around the Great Patriotic War and afterward mass introduction was stopped because of the labor intensity, the uncertainty of the effect and the risk of damaging germinated seed if sowing was delayed; only millet and potato cultivation retained it.

Criticism and 1948

Defects were pointed out from November 1929, when the Agricultural Gazette, the organ of the People's Commissariat of Agriculture, presented Lysenko's experiments as of "enormous practical importance" and opened a debate among specialists. Academician P. N. Konstantinov surveyed 54 variety-testing stations and 35 wheat cultivars over five years and found an average gain of only 0.04 centners per hectare, one-twentieth of the figure Lysenko reported; he noted that under unfavorable weather vernalized sowings suffered more than untreated ones and sometimes perished, and A. A. Lyubishchev criticized Lysenko for moving from small plots to large fields without adequate verification. This criticism was politically blocked. From 1935 Lysenko dominated Soviet biology, attacking Mendelian genetics as bourgeois pseudoscience, and after the August 1948 session of the All-Union Academy of Agricultural Sciences banned classical genetics, the debate remained controlled: in 1958, when a journal criticized vernalization, Pravda answered with a rebuttal article.

Vernalization was abandoned as a mass field technique but retained its legitimacy and utility as a breeding tool and became an established concept of modern plant physiology. Nikolai Vavilov supported it as a breeding method that allowed collection seeds from southern regions to be preserved without freezing damage, flowering times to be controlled and crosses to be made; for him the technique was compatible with genetic research, unlike Lysenko's doctrine. Lysenko claimed that the vernalized state was inherited and used the technique as evidence for the inheritance of acquired characteristics and as a foothold for attacking Mendelian genetics, and on this basis the persecution of geneticists was justified. Vavilov was arrested in 1940 and died in prison in 1943.

Modern Uses

Vernalization is used today in crop cultivation and breeding. Artificial vernalization is widely practiced to obtain additional plant generations in greenhouses, climatic chambers and phytotrons during winter, to preserve collection seeds from southern areas, and to regulate the growing period so that plants flower at the time needed for crossing. For potatoes the term separately denotes keeping tubers in the light.