Photoperiodism

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

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.

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.

A long day plant is a plant that requires less than a certain number of hours of night in each 24 hour period to induce flowering. Long day plants typically flower in the late spring or early summer.

A short day plant is a plant that cannot flower under the long days of summer. Short day plants typically flower in the fall of the year. These plants require a certain number of hours of darkness in each 24 hour period (a short daylength) before floral development can begin. Plants use the phytochrome system to sense day length or photoperiod.

A short day plant (long night) will not flower if a flash of light is used on the plant during the night. This does not occur from natural light such as lightning, fire flies, etc.

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.