Nyctinasty is the circadian rhythmic nastic movement of higher plants in response to the onset of darkness. Examples are the closing of the petals of a flower at dusk and the sleep movements of the leaves of many legumes. The earliest recorded observation of this behavior in plants dates back to 324 BC when Androsthenes, a companion to Alexander the Great, noted the opening and closing of tamarind tree leaves from day to night.
Nyctinastic movements are associated with diurnal light and temperature changes and controlled by the circadian clock and the light receptor phytochrome. Plants use phytochrome to detect red and far red light. Depending on which kind of light is absorbed, the protein can switch between a Pr state that absorbs red light and a Pfr state that absorbs far red light. Red light converts Pr to Pfr and far red light converts Pfr to Pr. Many plants use phytochrome to establish circadian cycles which influence the opening and closing of leaves associated with nyctastic movements. Anatomically, the movements are mediated by pulvini. Pulvinus cells are located at the base or apex of the petiole and the flux of water from the dorsal to ventral motor cells regulates leaf closure. This flux is in response to movement of potassium ions between pulvinus and surrounding tissue. Movement of potassium ions is connected to the concentration of Pfr or Pr. In Albizzoa julibrissin longer darker periods, leading to low Pfr leads to a faster leaf opening. In the SLEEPLESS mutation of Lotus japonicus, the pulvini are changed into petiole-like structures, rendering the plant incapable of closing its leaflets at night.
Leaf movement is also controlled by bioactive substances known as leaf opening or leaf closing factors. Several leaf-opening and leaf-closing factors have been characterized biochemically. These factors differ among plants. Leaf closure and opening is mediated by the relative concentrations of leaf opening and closing factors in a plant. Either the leaf opening or closing factor is a glycoside, which is inactivated by hydrolysis of the glycosidic bond via beta glucosidase. In Lespedeza cuneata G. Don the leaf opening factor, potassium lespedezate is hydrolyzed to 4 hydroxy phenyl pyruvic acid. In Phyllanthus urinaria L., leaf closing factor Phyllanthurinolactone is hydrolyzed to its aglycon during the day. Beta glucosidase activity is regulated via circadian rhythms.
Fluorescence studies have shown that the binding sites of leaf opening and closing factors are located on the surface of the motor cell. Shrinking and expansion of the motor cell in response to this chemical signal allows for leaf opening and closure. The binding of leaf opening and closing factors is specific to related plants. The leaf movement factor of C. mimosoides was found to not bind to the motor cell of A. julibrissin. The leaf movement factor of A. julibrissin similarly didn’t bind to the motor cell of C. mimosoides, but did bind to A. saman and A. lebbeck.
The purpose of, and conferred benefits of nyctinastic movement have yet to be identified. Studies using mutant plants with a loss of function gene that results in petiole growth instead of pulvini found that these plants has less biomass and smaller leaf area than the wild type. This indicates nyctnastic movement may be beneficial toward plant growth.
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