Photoperiodism

Photoperiodism is the phenomenon by which a plant's physiological responses -- most conspicuously, the induction of flowering -- are regulated by the relative duration of light and darkness it experiences in each daily cycle, that is, by the photoperiod. The term itself, and much of the early experimental work establishing the phenomenon, is credited to the American researchers W. W. Garner and H. A. Allard, who were puzzled by a variety of tobacco called Maryland Mammoth that grew vigorously through the summer in open field conditions but never flowered there, yet flowered readily when kept over winter in a greenhouse; systematically testing different day lengths, they established that it was specifically the shorter day length (equivalently, the longer uninterrupted night) of winter, and not temperature or any other seasonal factor, that was triggering flowering in this variety.

On the basis of how their flowering responds to day length, plants are classified into three broad categories. Short-day plants (SDPs) flower only when the day length falls below a species-specific threshold called the critical photoperiod (chrysanthemum, tobacco variety Maryland Mammoth, cocklebur, rice and soybean are familiar examples). Long-day plants (LDPs) flower only when the day length exceeds their own critical photoperiod (spinach, radish, henbane and wheat are familiar examples). Day-neutral plants (DNPs), such as tomato, cotton, sunflower, cucumber and maize, flower on a schedule essentially independent of day length altogether, relying instead chiefly on the plant's own developmental age or size. …