Photoperiodism - in Plants

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 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. Using artificial lighting to induce both extra-long days and nights, short-day plants will bloom but long-day plants will not.