The term "photoperiodism" was coined to describe a plant's ability to flower in response to changes in the photoperiod: the relative lengths of day and night. Because flowers produce seeds, flowering is crucially important for the plant to complete its life cycle. Although people had long known that plants such as tulips flower in the spring and chrysanthemums flower in the fall, until the early 1900s little was known about what actually caused flowering.
Beginning in 1910, Wightman Garner and Henry Allard conducted experiments to test the effect of day length on flowering. They discovered that plants such as barley flowered when the day length was longer than a certain critical length. These plants, which they named long-day plants (LDPs), flower mainly in the summer as the days are getting longer. Others, such as soybeans, flower when the day length is shorter than a certain critical length. These short-day plants (SDPs) flower in the fall as the days are getting shorter. Still others are not sensitive to the photoperiod and are called day-neutral plants.
Photoperiodism is responsible for the distribution of many plants worldwide. For example, ragweed (a SDP) is not found in northern Maine because the plant flowers only when the day length is shorter than 14.5 hours. In northern Maine, days do not shorten to this length until August. This is so late in the growing season that the first frost arrives before the resulting seeds are mature enough to resist the low temperatures, and so the species cannot survive there. By contrast, spinach (a LDP) is not found in the tropics because there the days are never long enough to stimulate the flowering process.
To investigate photoperiodism, plants can be grown in growth chambers, in which timers are used to control the length of the light and dark periods. Such research has shown that the dark period is more important than the light period. For example, if SDPs are grown under short-day conditions but the dark period is interrupted by a flash of light, the SDPs will not flower. The long night that normally accompanies a short day is interrupted by the flash. An interruption of the light period with dark has no effect. Thus, SDPs should more accurately be called long-night plants; and LDPs should be called short-night plants to emphasize the key role played by darkness in photoperiodism. Most plants require several weeks of the appropriate long-night or short-night cycle before they will flower.
Red light having a wavelength of 660 nanometers was found to be the most effective for interrupting the dark period, and this effect can be reversed by a subsequent exposure to far-red light (730 nanometers). These observations led to the discovery of phytochrome, the pigment responsible for absorbing those wavelengths and apparently the light sensor in photoperiodism. It has been suggested that photoperiodism results from an interaction between phytochrome and the plant's biological clock, which measures the time between successive dawns (rich in red light) and successive dusks (rich in far-red light). Under the appropriate conditions, these interactions are thought to activate the genes for flowering.
Many other processes in plants and animals are now known to be affected by the photoperiod.
Robert C. Evans
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