Morphogenetic Responses of Plants to Mechanical Stimuli or Stress

Abstract

Although it has long been part of the botanist's lore that plants will respond to environmental stimuli such as light, gravity, temperature, or chemicals, most observers have felt that, except for certain extraordinary cases (e.g., sensitive mimosa, tendrils, Venus' flytrap), vascular plants neither sense nor respond to mechanical perturbation. If these plant systems are the only kind that respond to mechanical cues, we might say that reaction to does not represent a general kind of response to environmental mechanical perturbation by ordinary growing plants. To investigate the possibility of such a phenomenon, a systematic study of the effects of mechanical perturbation on plants was begun in my laboratory. The results, which have been confirmed by other investigators, demonstrate that growth of all of the species that have been tested is markedly retarded by very slight rubbing or bending of their stems. I have called this phenomenon thigmomorphogenesis -the prefix thigmo meaning touch (Jaffe 1973). In nature, wind is probably the environmental factor most responsible for this phenomenon; indeed, this supposition has been supported in the laboratory (see Figure 1) (Jaffe 1976a). However, rubbing by animals, machinery, or other plants and the torsional and leverage stresses set up by the weight of branches or even leaves are also undoubtedly involved. Much of the literature on thigmomorphogenesis is anecdotal; only recently has a direct effort been made to elucidate the phenomenon experimentally. Usually the observations dealing with the responses of plant growth and development to mechanical stimulation were made accidentally while an investigator was studying something else and are parenthetically buried in the report of the primary study. Nevertheless, in the literature there are a number of papers that report thigmomorphogenetic responses in a wide variety of vascular plants (46 species from 19 families). The primary thigmomorphogenetic response is one of growth retardation coupled with increased radial growth (Jaffe 1973) (Figure 1). The ubiquity of this response is significant. Thigmomorphogenesis is as common a response as other, much more widely studied plant growth responses, such as geotropism or photomorphogenesis.