Long-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.

Phytochrome comes in two forms: pr and pfr. Red light (which is present during the day) converts phytochrome to its active form (pfr). This then triggers the plant to grow. In turn, far-red light is present in the shade or in the dark and this converts phytochrome from pfr to pr. Pr is the inactive form of phytochrome and will not allow for plant growth. This system of pfr to pr conversion allows the plant to sense when it is night and when it is day. Pfr can also be convereted back to Pr by a process known as dark reversion, where long periods of darkness trigger the conversion of Pfr. This is important in regards to plant flowering. Experiments by Halliday et al. showed that manipulations of the red-to far-red ratio in Arabidopsis can alter flowering. They discovered that plants tend to flower later when exposed to more red light, proving that red light is inhibitory to flowering. Other experiments have proven this by exposing plants to extra red-light in the middle of the night. A short-day plant will not flower if light is turned on for a few minutes in the middle of the night and a long-day plant can flower if exposed to more red-light in the middle of the night.

Cryptochromes are another type of photoreceptor that is important in photoperiodism. Cryptochromes absorb blue light and UV-A. Cryptochromes entrain the circadian clock to light. It has been found that both cryptochrome and phytochrome abundance relies on light and the amount of cryptochrome can change depending on day-length. This shows how important both of the photoreceptors are in regards to determining day-length.

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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