Explain the factors that control flowering in short day plants.

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Plants, specifically flowering ones (angiosperms), use photoreceptor proteins eg phytochrome or cryptochrome to flower. These proteins help the plants respond to seasonal change in the photoperiod (night-time length), some angiosperms needing long or short nights to induce flowers (obligate plants), but some, though flowering more readily given the right light conditions, eventually flowering regardless of photoperiod (facultative). The latter then, essentially, are less 'fussy' about the photoperiod.

Photoperiodism causes flowering by inducing the shoots of these plants to produce floral buds instead of leaves and lateral buds. This can be encouraged artifically by a term known to horticulturalists as forcing plants on.

Whether obligate or facultative, photoperiodic flowering plants are classified as either long-day plants or short-day plants (though in fact it is, however, the night-time length that is critical and not the day-time). Particular species of these plants have different length critical photoperiods.

The combination of active photoreceptor proteins (created by the plant collecting light during the daytime, which can be artificial light), with the circadian clock of the plant (essentially 'body clock', which is independent of environmental stimulae) enables the plant to register the length of night-time.

Short-day plants in particular flower when the night is longer than their critical photoperiod. They cannot flower when the night-time is too short or if artificial light is shone on them for more than a few minutes at night. Indeed they need continuous darkness of a critical duration before flowers can bloom. Natural lights at night, eg moonlight or lightning do not affect ability to flower as naturally (by design of the plant) they aren't bright enough to cause a problem.

Generally speaking, short-day (or more accurately long-night) plants flower as days grow shorter (and nights grow longer) after 21 June (summer solstice) in the northern hemisphere, ie in summer or fall. Different species have different critical photoperiods, and even different varieties of the same species, making the flowering process in these plants a complex phenomena to catalogue. Forcing plants on (inducing them to flower by artificially increasing their perceived night-time) is hence an art that requires honing and practice.

Some examples of short-day (facultative) plants include hemp, cotton, rice, jowar and green gram.

In summary, short-day flowering plants need the usual requirements plants need (light - natural or articificial, water, nutrients), in particular to produce the photoreceptor proteins needed to respond to night-time length, and also a long enough night-time length (beyond a critical length), either natural or (under the right treatment) artificial.