Rhythms of Plant Life

Plants exhibit regular, cyclic physiological changes of many sorts. For ex­ample, leaflets of wood sorrel, Oxalis, fold downward to a vertical position at night and return to their normal horizontal orientation during the day. Because this rhythm takes place over a period of approximately twenty-four hours, it is called a circadian rhythm.

Forces within the plant control the movements of wood sorrel and many other circadian rhythms. This is easily demonstrated by placing wood sor­rel in total darkness for several days. In the absence of external light stim­uli, the movements continue in an approximate circadian rhythm, but the period tends to lengthen slightly to approximately twenty-six hours. A rhythm that is controlled by an internal timing mechanism, the so-called “biological clock,” is an endogenous rhythm.

Environment can affect endogenous rhythms. By changing the pho­toperiod scientists can alter the period of a circadian rhythm as long as the imposed photoperiod does not differ significantly from the normal circa­dian rhythm. Altering circadian rhythms by varying one or more environ­mental parameters is called entrainment. Phytochrome, an important plant pigment, and blue light receptors regulate entrainment.

Why should a plant like wood sorrel fold its leaves at night? German botanist Erwin Bunning suggests that the folding of leaves at night hides them from moonlight, which could disrupt the plant’s ability to measure night length accurately. Thus, wood sorrel can use its internal biological clock to anticipate the middle of the night, when a moon might be full. Circadian rhythms are advantageous to Gnetum gnemon, a plant found in tropical rainforests, which secretes nectar in the evening, the most ac­tive time for the moths that carry its pollen. Other endogenous circadian rhythms of plants include opening and closing of stomata, growth rate of stems and roots, opening and closing of flowers, production of floral scent, and carbon dioxide uptake.

An old-growth forest with an understory of wood sorrel in Mt. Hood National Forest in Oregon. Plants exhibit regular, cyclic physiological changes of many sorts.

References

Sweeney, Beatrice. Rhythmic Phenomena in Plants. San Diego, CA: Academic Press, 1987.

Thomas, Brian, and Daphne Vince-Prue. Photoperiodism in Plants. San Diego, CA: Academic Press, 1996.