PHOTOPERIODISM

Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals. Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of the light and dark periods. Here it should be emphasized that photoperiodic effects relate directly to the timing of both the light and dark periods. In plants Many flowering plants use a photoreceptor protein , such as phytochrome or cryptochrome , to sense seasonal changes in night length, or photoperiod, which they take as signals to flower. In a further subdivision, obligate photoperiodic plants absolutely require a long or short enough night before flowering, whereas facultative photoperiodic plants are more likely to flower under the appropriate light conditions, but will eventually flower regardless of night length. Photoperiodic flowering plants are classified as long- day plants or short-day plants, though the regulatory mechanism is actually governed by hours of darkness, not the length of the day. Modern biologists believe [citation needed] that it is the coincidence of the active forms of phytochrome or cryptochrome, created by light during the daytime, with the rhythms of the circadian clock that allows plants to measure the length of the night. Other than flowering, photoperiodism in plants includes the growth of stems or roots during certain seasons, or the loss of leaves. Long-day plants A long-day plant requires fewer than a certain number of hours of darkness in each 24-hour period to induce flowering. These plants typically flower in the northern hemisphere during late spring or early summer as days are getting longer. In the northern hemisphere, the longest day of the year is on or about 21 June (solstice). After that date, days grow shorter (i.e. nights grow longer) until 21 December (solstice). This situation is reversed in the southern hemisphere (i.e. longest day is 21 December and shortest day is 21 June). In some parts of the world, however, "winter" or "summer" might refer to rainy versus dry seasons, respectively, rather than the coolest or warmest time of year. Some long-day obligate plants are:Carnation (Dianthus) Henbane (Hyoscyamus) Oat (Avena) Ryegrass (Lolium) Clover (Trifolium) Bellflower (Campanula carpatica) Some long-day facultative plants are: Pea (Pisum sativum) Barley (Hordeum vulgare) Lettuce (Lactuca sativa) Wheat (Triticum aestivum, spring wheat cultivars) Turnip (Brassica rapa) Arabidopsis thaliana (model organism) Short-day plants Short-day plants flower when the night is longer than a critical length. They cannot flower under long days or if a pulse of artificial light is shone on the plant for several minutes during the middle of the night; they require a consolidated period of darkness before floral development can begin. Natural nighttime light, such as moonlight or lightning, is not of sufficient brightness or duration to interrupt flowering. In general, short-day (i.e. long-night) plants flower as days grow shorter (and nights grow longer) after 21 June in the northern hemisphere, which is during summer or fall. The length of the dark period required to induce flowering differs among species and varieties of a species. Photoperiod affects flowering when the shoot is induced to produce floral buds instead of leaves and lateral buds. Note that some species must pass through a "juvenile" period during which they cannot be induced to flower—common cocklebur is an example of a plant species with a remarkably short period of juvenility and plants can be induced to flower when quite small. Some short-day obligate plants are: Chrysanthemum Coffee Poinsettia Strawberry Tobacco, var. Maryland Mammouth Common duckweed , (Lemna minor) Cocklebur (Xanthium) Maize – tropical cultivars only[citation needed] Some short-day facultative plants are: Hemp (Cannabis) Cotton (Gossypium) Rice Sugar cane Day-neutral plants Day-neutral plants, such as cucumbers, roses and tomatoes, do not initiate flowering based on photoperiodism at all; they flower regardless of the night length. They may initiate flowering after attaining a certain overall developmental stage or age, or in response to alternative environmental stimuli, such as vernalisation (a period of low temperature), rather than in response to photoperiod.