Photoperiodism
Photoperiodicity is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals.
Photoperiodism, 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, while 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 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 late spring or early summer as days are getting longer.
Some long day obligate plants are:
 Carnation (Dianthus)
Henbane (Hyoscyamus)
Oat (Avena)
Ryegrass (Lolium)
Clover (Trifolium)
Bellflower (botany) (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)
Short day plants
Short day plants flower when the night is longer than a critical length. They cannot flower under the long days of summer. These plants generally flower in late summer or fall, as days are getting shorter. Short day plants will not flower if a pulse of artificial light is shined 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.
Some short day obligate plants are:
Chrysanthemum
Coffee
Poinsettia
Strawberry
Tobacco, var. Maryland Mammouth
Common duckweed, (Lemna minor)
Cocklebur (Xanthium)
Maize - tropical cultivars only
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 vernalization (a period of low temperature), rather than in response to photoperiod.
Photoperiodism, animals
Daylength, and thus knowledge of the season of the year, is vital to many animals. A number of biological and behavioural changes are dependent on this knowledge. Together with temperature changes, photoperiod provokes changes in the colour of fur and feathers, migration, entry into hibernation, sexual behaviour and even the resizing of sexual organs.
In mammals, for example, daylength is registered in the Suprachiasmatic nucleus (SCN) which is informed by retinal light-sensitive ganglion cells which are not involved in vision. The information travels through the retinohypothalamic tract (RHT).
See also
Chronobiology
Florigen
Scotobiology
References
D.E. Fosket, Plant Growth & Development, A Molecular Approach. Academic Press, san Diego, 1994, p. 495.
B. Thomas and D. Vince-Prue, Photoperiodism in plants (2nd ed). Academic Press, 1997.
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