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 (angiosperms) 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.
In 1920, W. W. Garner and H. A. Allard published their discoveries on photoperiodism and felt it was the length of daylight that was critical, but it was later discovered that the length of the night was the controlling factor. Photoperiodic flowering plants are classified as long-day plants or short-day plants, even though night is the critical factor, because of the initial misunderstanding about daylight being the controlling factor. Each plant has a different length critical photoperiod, or critical night length.
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. Artificial lighting can be used to induce extra-long days.
 Long-day plants
Long-day plants flower when the day length exceeds their critical photoperiod. 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 day lengths are less than their critical photoperiod. 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
Some short-day facultative plants are:
 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.
 In 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.
Birds', such as the canary, singing frequency depends on the photoperiod. In the spring when the photoperiod increases (more daylight), the male canary's testes grow. As the testes grow, more androgens are secreted and song frequency increases. During autumn when the photoperiod decreases (less daylight), the male canary's testes regress and androgen levels dramatically drop resulting in decreased singing frequency. Not only is singing frequency dependent on the photoperiod but also song repertoire. The long photoperiod of spring results in a greater song repertoire. Autumn's shorter photoperiod results in a reduction in song repertoire. These behavioral photoperiod changes in male canaries are caused by changes in the song center of the brain. As the photoperiod increases so does the high vocal center (HVC) and the robust nucleus of the archistriatum (RA). When the photoperiod decreases these areas of the brain regress.
In mammals 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). Some mammals are highly seasonal while humans' seasonality is largely evolutionary baggage.
 See also
- Mauseth, James D. (2003). Botany : An Introduction to Plant Biology (3rd ed.). Sudbury, MA: Jones and Bartlett Learning. pp. 422–427. ISBN 0-7637-2134-4.
- Capon, Brian (2005). Botany for Gardeners (2nd ed.). Portland, OR: Timber Publishing. pp. 148–151. ISBN 0-88192-655-8.
- Hamner, K.C.; Bonner, J. (1938). "Photoperiodism in relation to hormones as factors in floral initiation and development". Botanical Gazette 100 (2): 388–431. doi:10.1086/334793. JSTOR 2471641.
- Hamner, K.C. (1940). "Interrelation of light and darkness in photoperiodic induction". Botanical Gazette 101 (3): 658–687. doi:10.1086/334903. JSTOR 2472399.
- Nelson Randy J. (2005) An Introduction to Behavioral Endocrinology (p.189). Sunderland, MA: Sinauer Associates.
- Foster, Russell; Williams, Robyn (5 December 2009). "Extra-retinal photo receptors" (Interview). Science Show. ABC Radio National. Retrieved 2010-05-28. "...we have the evolutionary baggage of showing seasonality but we're not entirely sure what the mechanism is."
 Related reading
- 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.