Define Photoperiod..
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It is the the period of time each day during which an organism receives illumination; day length.
Photoperiodism refers to the response of plants and animals to the length of day or night. Photoperiod has an important role in agriculture as it determines fundamental physiological developments in some plants.
This is crucial as plants are able to program themselves to develop in line with the right season.
The term "photoperiodism" was coined to describe a plant's ability to flower in response to changes in the photoperiod: the relative lengths of day and night. Because flowers produce seeds, flowering is crucially important for the plant to complete its life cycle.
Many models have been suggested over the years, but today, most biologists think photoperiodism—at least, in many species—is the result of interactions between a plant's "body clock" and light cues from its environment. Only when the light cues and the body clock line up in the right way will the plant flower.
Photoperiod affects reproduction in seasonal breeders, both plants and animals. In animals photoperiod affects the seasonal breeder through changes in melatonin secretion by the pineal gland. The reproductive axis is controlled by melatonin which is secreted at night only and therefore relays day length to the body.