Photoperiodism

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Photoperiodicity is the physiological reaction of organisms to the length of day or night. It occurs in animals and plants.

Many flowering plants use the pigment phytochrome 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. They were classified with these names, now known to be fallacious, because they observed the following:

Long day plants require a certain amount of daylight to initiate flowering, so these plants flower in the spring or summer. Conversely, short day plants will flower when the length of daylight falls below a certain amount. Day neutral plants do not initiate flowering based on photoperiodism; some may use temperature (vernalization) instead.

However, it was later found that long day plants are in fact really "short night plants", and short day plants are really "long night plants". This was experimentally determined in the following way: Two short day plants were placed in regulated conditions of light and dark. One plant received a short pulse of red light in the middle of the night, while the other was left undisturbed during the dark. The one which received the light did not flower. There was a set of similar experiments performed on long day plants, and these proved to also be regulated by the dark, not the light.

Also, the regulation by dark makes more logical sense. The dark is unlikely to be interrupted in nature--the only things that will do this are lightning strikes, fire, or fireflies. However, almost anything can cast a shadow over the flower, which would break its day period up.

Phytochrome is converted to its active form by red light (660 nm), and its inactive form by far-red light (730 nm). Moonlight produces a greater percentage of far-red light than sunlight, so during the night the phytochrome is slowly converted to its inactive form. More phytochrome is converted in a longer night, allowing the plant 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.

Some birds use photoperiodism to prepare for a migration or for the cold of winter. In some mammals, the time of estrus is regulated by photoperiod.