photoperiodism

(-dĭs'ĭ-tē)
n., pl., -isms, also -i·ties.
The response of an organism to changes in its photoperiod, especially as indicated by vital processes.

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The growth, development, or other responses of organisms to the length of night or day or both. Photoperiodism has been observed in plants and animals, but not in bacteria (prokaryotic organisms), other single-celled organisms, or fungi.

A true photoperiodism response is a response to the changing day or night. Some species respond to increasing day lengths and decreasing night lengths (for example, by forming flowers or developing larger gonads); this is called a long-day response. Other species may exhibit the same response, or the same species may respond in some different way, to decreasing days and increasing nights; this is a short-day response. Sometimes a response is independent or nearly independent of day length, and is said to be day-neutral. There are many plant responses to photoperiod. These include development of reproductive structures in lower plants (mosses) and in flowering plants; rate of flower and fruit development; stem elongation in many herbaceous species as well as coniferous and deciduous trees (usually a long-day response and possibly the most widespread photoperiodism response in higher plants); autumn leaf drop and formation of winter dormant buds (short days); development of frost hardiness (short days); formation of roots on cuttings; formation of many underground storage organs such as bulbs (onions, long days), tubers (potato, short days), and storage roots (radish, short days); runner development (strawberry, short day); balance of male to female flowers or flower parts (especially in cucumbers); aging of leaves and other plant parts; and even such obscure responses as the formation of foliar plantlets (such as the minute plants formed on edges of Bryophyllum leaves), and the quality and quantity of essential oils (such as those produced by jasmine plants). Note that a single plant, for example, the strawberry, might be a short-day plant for one response and a long-day plant for another response.

Animal responses

There are also many responses to photoperiod in animals, including control of several stages in the life cycle of insects (for example, diapause) and the long-day promotion in birds of molting, development of gonads, deposition of body fat, and migratory behavior. Even feather color may be influenced by photoperiod (as in the ptarmigan). In several mammals the induction of estrus and spermatogenic activity is controlled by photoperiod (sheep, goat, snowshoe hare), as is fur color in certain species (snowshoe hare). Growth of antlers in American elk and deer can be controlled by controlling day length. Increasing day length causes antlers to grow, whereas decreasing day length causes them to fall off. By changing day lengths rapidly, a cycle of antler growth can be completed in as little as 4 months; slow changes can extend the cycle to as long as 2 years. When attempts are made to shorten or extend these limits even more, the cycle slips out of photoperiodic control and reverts to a 10–12-month cycle, apparently controlled by an internal annual “clock.”

Seasonal responses

Response to photoperiod means that a given manifestation will occur at some specific time during the year. Response to long days (shortening nights) normally occurs during the spring, and response to short days (lengthening nights) usually occurs in late summer or autumn. Since day length is accurately determined by the Earth's rotation on its tilted axis as it revolves in its orbit around the Sun, detection of day length provides an extremely accurate means of determining the season at a given latitude. Such other environmental factors as temperature and light levels also vary with the seasons but are clearly much less dependable from year to year.

Mechanisms

It has long been the goal of researchers on photoperiodism to understand the plant or animal mechanisms that account for the responses. Light must be detected, the duration of light or darkness must be measured, and this time measurement must be metabolically translated into the observed response: flowering, stem elongation, gonad development, fur color, and so forth. Basic mechanisms differ not only between plants and animals but among different species as well. The roles (synchronization, anticipation, and so on) are similar in all organisms that exhibit photoperiodism, but the mechanisms through which these roles are achieved are apparently quite varied.

Strongest inhibition of flowering in short-day plants comes when the light interruption occurs around the time of the critical night (about 7–9 h for cocklebur plants), but actual effectiveness also depends on the length of the dark period. With short-day cockleburs, the shorter the night, the less the flowering and the longer the time that light inhibits flowering.

Orange-red wavelengths used as a night interruption are by far the most effective part of the spectrum in inhibition of short-day responses and promotion of long-day responses (flowering in most studies), and effects of orange-red light can be completely reversed by subsequent exposure of plants to light of somewhat longer wavelengths, called far-red light. These observations led in the early 1950s to discovery of the phytochrome pigment system, which is apparently the molecular machinery that detects the light effective in photoperiodism of higher plants. See also Phytochrome.

In photoperiodism of short-day plants, an optimum response is usually obtained when phytochrome is in the far-red receptive form during the day and the red-receptive form during the night. Although normal daylight contains a balance of red and far-red wavelengths, the red-receptive form is most sensitive, so the pigment under normal daylight conditions is driven mostly to the far-red receptive form. At dusk this form is changed metabolically, and the red-receptive form builds up. It is apparently this shift in the form of phytochrome that initiates measurement of the dark period. This is how a plant “sees”: when the far-red-sensitive form of the pigment is abundant, the plant “knows” it is in the light; the red-sensitive form (or lack of far-red form) indicates to the plant's biochemistry that it is in the dark.

The measurement of time—the durations of the day or night—is the very essence of photoperiodism. The discovery of a biological clock in living organisms was made in the late 1920s. It was shown that the movement of leaves on a bean plant (from horizontal at noon to vertical at midnight) continued uninterruptedly for several days, even when plants were placed in total darkness and at a constant temperature, and that the time between given points in the cycle (such as the most vertical leaf position) was almost but not exactly 24 h. In the case of bean leaves, it was about 25.4 h. Many other cycles have now been found with similar characteristics in virtually all groups of plants and animals. There is strong evidence that the clocks are internal and not driven by some daily change in the environment. Such rhythms are called circadian.

Circadian rhythms usually have period lengths that are remarkably temperature-insensitive, which is also true of time measurement in photoperiodism. Furthermore, the rhythms are normally highly sensitive to light, which may shift the cycle to some extent. Thus, daily rhythms in nature are normally synchronized with the daily cycle as the Sun rises and sets each day. Their circadian nature appears only when they are allowed to manifest themselves under constant conditions of light (or darkness) and temperature, so that their free-running periods can appear.

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The physiological and behavioral reactions brought about in organisms by changes in the duration of daylight and darkness, e.g. in reproductive activity, shedding of hair. Birds respond to longer daylight hours by increased sexual activity. Use is made of the phenomenon by using artificial light to stimulate egg production.

The response of plants to the length of the day, especially as it affects their blooming. In temperate zones, long-day plants typically bloom in spring and early summer, short-day plants in fall. By manipulating light in greenhouses, growers can induce plants to bloom out of their seasons. Many plants are day-neutral, or not affected by the length of the day.

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