Circadian Rhythmicity in Leaf Pulvini

Abstract

Plants, sometimes considered static and unresponsive, may really be quite dynamic. Roots and shoots bend in response to light and gravity, flowers unfold, stomata open and close, and tendrils curl. Leaves exhibit several types of movements. Some track the sun, always exposing their broad surface to the light and thereby improving their photosynthetic rates. Some plants, growing under conditions of high temperature and/or water stress, cannot dissipate all of the absorbed solar energy through photosynthesis and avoid photodamage by turning their leaves edgewise to the sun. Others are sensitive to touch, folding their leaflets together at the slightest disturbance. Still other plants show sleep movements, or nyctinasty, such that their leaves fold together into a vertical position during the night and reopen horizontally when the next dawn approaches. These movements are especially common in leguminous plants, including Phaseolus, Samanea, Albizzia, Mimosa, and Trifolium. Although nyctinastic leaf closure superficially resembles wilting, it is not a recurring response to daily water stress. It may, however, prevent other stresses during the night. The discovery of nyctinastic leaf movements sparked much speculation and some experimentation regarding their adaptive significance. Charles Darwin (1881) hypothesized that nyctinasty might protect plants from chilling or from frost damage, since vertical leaves would have less radiative