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Since plants are sedentary organisms, they are much more at the mercy of their environment than are animals. It is therefore not surprising that environmental factors, such as light and temperature, have a profound effect on the growth and development of plants. In fact, the greater regulatory importance of external factors is one of the ways in which plant development differs fundamentally from that of animals (Goldberg, 1988). Environmental stimuli frequently act as cues to initiate developmental transitions in plants, a good example being the induction of leaf expansion and chloroplast biogenesis when plants are first exposed to light. Indeed light probably has a more extensive regulatory influence on plant development than any other environmental factor. In addition to detecting the presence or absence of light, plants are able to detect subtle changes in spectral quality and light intensity (Smith, 1982), and they can measure the length of the photoperiod, which is used by many species to initiate flowering (Vince-Prue, 1983). These responses are mediated by photoreceptors that are quite distinct from those that are used to harvest light energy in photosynthesis. Some examples of light-mediated responses in plants are given in Table 1.1. |
