Perennial woody plants have a complex annual cycle keyed to the environment. Temperate plants have an annual dormant period commonly broken by exposure to low temperatures, although daily photoperiods of 16 hours or longer may partially substitute for the chilling. Shoot growth in the spring is normally stimulated by rising air and soil temperatures, with photoperiod playing a minor role, if any. In temperate regions, duration of shoot elongation is controlled primarily by endogenous factors, although moisture stress may be more limiting than generally recognized. Shortening photoperiods are the major stimulation inducing dormancy in arctic regions and, probably, in temperate areas that seldom experience a summer drought. Many angiosperms and coniferous species are characterized by ecotypes that sharply differ in thermoperiod or photoperiod requirements for optimum growth and in chilling necessary to break dormancy. The dormant period is an intergrading series of physiological states, each of which has an optimum environment.
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