Some plants won't flower until they've remembered a winter.
Winter wheat, many biennials and fruit trees need weeks of cold before they can bloom. Plants measure it with a molecular memory: prolonged chill gradually switches off a gene that blocks flowering, and it stays off after spring arrives. The effect is called vernalization, and onion growers deliberately reverse it.
Many plants of temperate climates must pass through a long spell of cold, typically between about 1 and 7°C, before they can flower. This ensures they bloom and set seed after winter rather than in autumn. Growers often count the requirement in 'chill hours'. Farmers long distinguished winter cereals, sown in autumn and needing the cold, from spring cereals that flower soon after spring sowing.
Scientists began studying the effect in the 19th century, and the German physiologist Gustav Gassner analysed it systematically in 1918. The name comes from the Soviet agronomist Trofim Lysenko, who in 1928 chilled moistened winter wheat seed so it could be sown in spring and still crop, after harsh winters had destroyed seedlings. Lysenko went on to claim that the vernalized state could be inherited, which the plant biologist Richard Amasino describes as inaccurate, though a few researchers argue that some epigenetic inheritance is plausible.
Molecular biology has revealed the mechanism, best studied in thale cress, Arabidopsis thaliana. A protein called Flowering Locus C acts as a brake on flowering. Prolonged cold gradually silences the FLC gene by remodelling the chromatin around it, the packaging of DNA, with the help of cold-induced RNA molecules. The silencing persists through later cell divisions, so the plant 'remembers' winter; it is reset in each new seed so that the next generation must experience its own winter. Cold alone is often not enough: day length signals then trigger the flowers.
Gardeners meet this every season. Biennials flower in their second year after a winter; many fruit trees need enough winter chill to bloom well. It can also be undone. Commercial onion growers store onion sets cold but then 'devernalize' them with warmth before planting, so the plant puts its energy into swelling a bulb rather than bolting into flower.
Source: Wikipedia — Vernalization · Text summarised from Wikipedia (CC BY-SA 4.0)
