Course 11.3Lesson 3 of 3
Vernalisation is the promotion of flowering by a period of low temperature. Winter varieties of wheat, barley and rye, and biennials such as sugar beet, cabbage and carrot, need it. The cold is sensed by the shoot apex, where cells divide. It stops a plant from flowering in autumn, and it can be undone by a warm spell. Gibberellic acid can take the place of cold in some biennials.
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A farmer has two bags of wheat grain. One is sown in spring and gives grain by the end of summer. The other is a winter wheat. If the farmer sows it in spring, the plants grow green and tall, but they do not flower, and there is no grain at all. The same winter wheat, sown in autumn, flowers in the next spring and gives a good crop.
The difference is a cold winter. The winter wheat needs one before it will flower. This is vernalisation.
Vernalisation is the promotion of flowering by a period of low temperature. The word comes from the Latin for spring, since the cold treatment makes a plant act like a spring plant. The Russian scientist Lysenko used the term, and a German scientist, Gassner, had shown the effect in 1918 with winter rye. Gassner found that grains of winter rye flowered when they were chilled while germinating.
Wheat, barley and rye each have two kinds of variety.
So winter varieties are sown in autumn. The seeds germinate, and the plant spends the winter as a small seedling. It resumes its growth in the spring, flowers, and is harvested around the middle of summer.
A biennial is a plant that takes two seasons to complete its life. It grows leaves and stores food in the first season. In the second season it flowers, makes seeds, and dies. Because it flowers once and dies, it is a monocarpic plant.
Sugar beet, cabbage and carrot are biennials. In the first season the plant makes a rosette of leaves and a fat root or head. A cold winter follows. In the next season it sends up a flowering stem. Kept in a warm place all through, a biennial may keep making leaves and never flower. A cold treatment given to a growing biennial makes it flower afterwards, as it responds to the day length.
Vernalisation acts on the shoot apex, where cells are dividing actively. A seed that has begun to germinate can be vernalised, and so can a growing plant.
The cold must be real cold, but not freezing. A few degrees above freezing is the most effective, and the plant must stay at that low temperature for some weeks. A short cold spell is not enough. The effect adds up, week by week.
A warm spell soon after the cold can wipe out the effect. This is devernalisation. If the chilled seeds are kept warm and dry for a time, the plant returns to the state of a plant that has never been cold.
Some scientists believe that the cold makes a flowering substance, and they call it vernalin. It has not been identified. In some biennials, gibberellic acid can take the place of the cold. A plant that has been sprayed with GA flowers in a warm room, without winter.
Vernalisation stops a plant from flowering too early. Suppose a winter wheat flowered in autumn. Its flowers would meet the frost and die. The need for winter tells the plant that spring has come, and that the cold season is over. It also gives the plant time to grow before it flowers, so it is a strong plant at maturity.
Farmers use the idea. Seeds of a winter variety are soaked, kept moist in the cold for some weeks, and then sown in spring. Such a crop flowers in the same season, even though it was sown in spring. This treatment is used in cold countries.
A table of results usually gives weeks of cold and the time to flowering, or the share of plants that flowered. Read it as follows.
You have now met three ways in which a plant waits or tells time. Keep them apart.
For flowering, vernalisation often comes first, in winter, and photoperiodism comes after it, in the spring. A winter wheat is a good example: cold in winter, then the long days of spring.
You should find that nothing flowers when the store is too warm or below freezing. In the right range, more weeks of cold raise the share of plants that flower, up to every plant.
The program prints the share of plants that flower after different weeks of cold.
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