Rhythmic nature of thigmomorphogenesis and thermal stress of Phaseolus vulgaris L. shoots

Effects of mechanical stress (or stimulus) on the length of Phaseolus vulgaris L. (cv. Kentucky Wonder pole bean) shoots displayed a 24h rhythm, relative to the time of day that plants were subjected to stress. Shoots were shorter when subjected to mechanical stress near the end of the dark span and the beginning of the light span. Mechanical stress also affected the ultradian shoot movement rhythm (circumnutation) of P. vulgaris. Generally, horizontal shoot movements (monitored at 10 min intervals) displayed a mean period of about 1.4 h. After a brief exposure to mechanical stress (e.g., 10 rubs), the mean period increased to about 2.0 h. Effects of thermal stress were similar to those of mechanical stress. When plants were exposed briefly (5 min) to either high (45 °C) or low (0 °C) temperatures, the period increased to about 2.1 h. Regardless of whether plants were subjected to mechanical or cold stress, ultradian periodicity was restored to its original period length after one cycle.