Circadian rhythms are entrained by light reception
The timing and duration of biological activities in living organisms are governed in all eukaryotes and some prokaryotes by what is commonly called a “biological clock”—an oscillator within cells that alternates back and forth between two states at roughly 12-hour intervals. The major outward manifestations of this clock are known as circadian rhythms (Latin circa, “about,” + dies, “day”). Think of your own life: in all probability you sleep at night and are awake during the day. The circadian rhythms of animals are discussed in Key Concept 52.5. In plants, circadian rhythms influence, for example, the opening (during the day) and closing (at night) of stomata in Arabidopsis, and the raising toward the sun (during the day) and lowering (at night) of leaves in bean plants. From these two examples, it is obvious that circadian rhythms are ecologically useful adaptations, in that they relate the plant’s physiology to its environment.
Two qualities characterize circadian rhythms, as well as other regular biological cycles: the period is the length of one cycle, and the amplitude is the magnitude of the change over the course of a cycle. The circadian rhythms of plants have several noteworthy characteristics:
The period of a circadian rhythm is remarkably insensitive to temperature, although lowering the temperature may drastically reduce the amplitude.
Circadian rhythms are highly persistent; they may continue for days, even in the absence of environmental cues, such as light–dark periods.
Circadian rhythms can be entrained, within limits, by light–dark cycles that do not exactly correspond to 24 hours. That is, the period of a rhythm can be made to coincide (within limits) with that of the light–dark cycle to which the organism is exposed.
Consider what happens when a person abruptly moves across many time zones: what was the night becomes the day, and gradually the person’s sleep–wakefulness circadian rhythm entrains to the new environmental cues. Similar entrainment occurs in plants adapting to day length as the seasons progress during the year. The action spectrum for plant entrainment indicates that phytochrome (and to a lesser extent, blue-light receptors) is very likely involved. At sundown phytochrome is mostly in the active Pfr form. But as the night progresses, Pfr gradually gets converted back to the inactive Pr form. By dawn phytochrome is mostly in the Pr state, but as daylight begins, it rapidly converts to Pfr. The switch to the Pfr state resets the plant’s biological clock. However long the night, the clock is still reset at dawn every day. Thus while the total period measured by the clock is consistent, the clock adjusts to changes in day length over the course of the year.