Did you ever wonder where cabbage or carrot seed comes from?
OK, you might be saying "cabbage and carrot flowers, of course!" But have you
ever seen these plants in flower at the end of the growing season? In some plants, another
environmental factor (besides day length) affects flower initiation. Many plants require a
period of chilling before they will flower. Biennial plants, including cabbage and
carrots, grow only foliage the first year, overwinter, and flower the second season; the
winter chilling is critical to flower initiation. This phenomenon is called vernalization, which can be defined as the promotion
of flowering due to exposure to low temperatures, or chilling. We vegetable gardeners take advantage of plants that require
vernalization when we grow biennial vegetables such as beets, turnips, carrots, kale, and
cabbage. We harvest our crop during the first growing season, when the plants have grown
only foliage and, in the case of root crops, stored large carbohydrate reserves in their
roots. Were we to allow these plants to overwinter, we’d find that they would begin
growth again the following spring, and produce flowers and seed some time during the
growing season.
Not all vernalization responses are absolute. For example,
some types of rye and wheat will form flowers whether or not they have been exposed to a
chilling period. But they will flower in a much shorter time if they have been
exposed to cool temperatures.
What is the purpose of vernalization—what is the
basis for this adaptation that requires some plants to receive a certain amount of
chilling before they will initiate flowers? Again, as with photoperiod, the need for a
certain period of chilling guarantees that plants in temperate regions will flower at the
appropriate time.
Let’s look at some other ways temperature affects a
plant’s life cycle.
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 Chilly mums. Chrysanthemums are short-day perennials that require a chilling
period as well as a certain photoperiod before they will form flowers. Why don’t
greenhouse growers have to chill their greenhouses before regulating day length to
stimulate flowering?
Because most chrysanthemums are propagated
vegetatively by rooting
cuttings. If the original plant (which might be several generations removed) was chilled,
then successive generations propagated from cuttings carry this information with
them—so they don’t require additional chilling. |
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