'Short-day' plants are misnamed. What they actually measure is the night.
Many plants flower only when the season is right, and they judge it by light. Scientists first thought day length was the trigger. It turned out to be the length of unbroken darkness: switch on a light for a few minutes at midnight and a short-day plant may refuse to bloom.
In 1920 the scientists W. W. Garner and H. A. Allard published their discovery that many plants time their flowering by the length of the day. They classified plants as long-day, flowering as days lengthen in late spring and early summer, or short-day, flowering as days shorten after midsummer. Later work showed the crucial factor was actually the length of the night, but the original names stuck. Each species or variety has its own critical night length, and some absolutely require it, while others are merely more likely to flower under the right conditions.
The proof is simple and striking. A short-day plant, better called a long-night plant, needs a continuous stretch of darkness. Interrupt it with a pulse of artificial light for several minutes in the middle of the night and it may not flower at all, while a long-day plant given the same midnight light may be pushed into flowering. Natural night light such as moonlight or lightning is not bright or long enough to interfere.
Plants sense the dark with light-detecting proteins. Phytochrome switches between two forms: red daylight converts it to an active form, while far-red light, abundant in shade, and long periods of darkness turn it back. Cryptochromes, which absorb blue light, help set the plant's internal circadian clock. Biologists think plants measure the night by the coincidence of these light-activated signals with the rhythm of that clock.
For gardeners and growers, the categories are useful. Long-day plants include carnations, oats, peas, barley, lettuce and wheat. Rice, cotton and soybeans lean towards short days. Day-neutral plants such as cucumbers, roses and tomatoes flower when they reach a certain age or in response to other cues, like a spell of cold. Growers can also use artificial lighting to create extra-long days.
Source: Wikipedia — Photoperiodism · Text summarised from Wikipedia (CC BY-SA 4.0)
