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Curr Opin Plant Biol. Abstract Plants utilize circadian clocks to synchronize their physiological and developmental events with daily and yearly changes in the environment. This process is known as '''vernalisation'''.
However, it has actually been discovered that it's the '''night length''', not the day length that controls flowering. The period of uninterrupted darkness critical night length is what actually controls flowering. The diagram left shows how a short day plant will only flower of the period of uninterrupted darkness exceeds the critical night length.
Conversely, the long day plant only flowers when the period of uninterrupted darkness is less than the critical night length. They are classified under three groups according to the photoperiods: short-day plants, long-day plants, and day-neutral 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 one condition.
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Phytochrome comes in two forms: P r and P fr. 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.
Photoperiodism in the Plant Kingdom
P r is the inactive form of phytochrome and will not allow for plant growth. This system of P fr to P r conversion allows the plant to sense when it is night and when it is day. Experiments by Halliday et al.
They discovered that plants tend to flower later when exposed to more red light, proving that red light is inhibitory to flowering. 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. This shows how important both of the photoreceptors are in regards to determining day-length. In , W. Garner and H. 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.
Similarities in the circadian clock and photoperiodism in plants.
Along with long-day plants and short-day plants, there are plants that fall into a "dual-day length category". 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. 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.
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Other than flowering, photoperiodism in plants includes the growth of stems or roots during certain seasons and the loss of leaves. Artificial lighting can be used to induce extra-long days. Long-day plants flower when the night length falls below their critical photoperiod. In the northern hemisphere, the longest day of the year summer solstice is on or about 21 June. This situation is reversed in the southern hemisphere i. Short-day plants flower when the night lengths exceed their critical photoperiod.