Short-day plant

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 light and dark periods.

Many flowering plants (angiosperms) use a photoreceptor protein, such as or , 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 one condition.

Phytochromes are signaling proteins that promote photomorphogenesis in response to red light and far-red light. There are two forms of phytochromes: red light absorbing, Pr, and far-red light absorbing, Pfr. Pfr, which is the active form of phytochromes, can be reverted to Pr, which is the inactive form, slowly by inducing darkness or by inducing far-red light which makes the process rapid. There are five genes that encode phytochromes in the Arabidopsis thaliana genetic model, PHYA-PHYE. PHYA is the gene that is involved in the regulation of photomorphogenesis in response to far-red light. PHYB is the gene that is involved in regulating photoreversible seed germination in response to red light. PHYC mediates the response between PHYA and PHYB. PHYD and PHYE mediate elongation of the internode and control the time in which the plant flowers.

Cryptochromes are blue-light photoreceptors that regulate responses to blue light. There are two different forms of crytochromes, CRY1 and CRY2, which regulate the inhibition of hypocotyl elongation in response to blue light.

The combination of phytochromes and cryptochromes mediate growth and the flowering of plants in response to red light, far-red light, and blue light.

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. Along with long-day plants and short-day plants, there are plants that fall into a "dual-day length category". These plants are either long-short-day plants (LSDP) or short-long-day plants (SLDP). LSDPs flower after a series of long days followed by short days whereas SLDPs flower after a series of short days followed by long days. Each plant has a different length critical photoperiod, or critical night length.

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Wikipedia