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 day length, which they take as signals to flower.

Broadly, flowering plants can be classified as long day plants, short day plants, or day neutral plants.

Long day plants are plants that flower when the day is longer than a critical length (i.e. the night is shorter than a critical length). These plants generally flower in the spring or early summer, as days are getting longer.

Short day plants are plants that flower when the day is shorter than a critical length, or the night is longer than a critical length. These plants generally flower in late summer or fall, as days are getting shorter.

It is actually the night length rather than day length that controls flowering, so flowering in a long day plant is triggered by a short night (which, of course, also means a long day). Conversely, short day plants will flower when nights get longer than a critical length. This is known by using night break experiments. For example, a short day plant (long night) will not flower if a pulse (say 5 minutes) of artificial light is shone on the plant during the middle of the night. This generally does not occur from natural light such as moonlight, lightning, fire flies, etc, since the light from these sources is not sufficiently strong to trigger the response.

Day neutral plants do not initiate flowering based on photoperiodism i.e. they can flower regardless of the night length; some may use temperature (vernalization) instead. Quantitative long day or short day plants will have their flowering advanced or retarded by short or long days, but will eventually flower in sub-optimal day lengths. Again, temperature is likely to also influence flowering time in these plants.

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 instances of photoperiodism in plants include the growth of stems or roots during certain seasons, or the loss of leaves.

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 SCN which is informed by retinal light-sensitive ganglion cells which are not involved in vision. The information travels through the retinohypothalamic tract (RHT).