BACKGROUND The effects of light on plants can be divided into three broad categories: a. Photosynthesis: The process whereby light energy is captured and converted into chemical energy. The action spectrum for photosynthesis has absorption maxima in the red and blue region of the spectrum and the principle pigments are the chlorophylls. b. Phototropism: The bending growth of plants to unilateral illumination having an action spectrum in the blue region of the spectrum. For this response, the photoreceptor is not known. c. Photomorphogenesis: The effects of light on the growth and development of plants. The number and variety of responses elicited by plants in light are listed in Table I. All of the responses listed in this table possess an action spectrum in either the red (660 nm) or far-red (730 nm) regions of the spectrum and they are mediated via the phytochrome pigment system. The range of intensities of light to which plants respond is shown h 0 in Fig. 1. In plants the variation is great ranging from 10 pW/cm for photosynthesis to 10'° pW/cm for photomorphogenesis in grass seedlings. It should be noted that the responses of plants in the lower range of intensities listed in Figure 1 were obtained from plants grown under strictly dark conditions. Since etiolated plants are not found in natural environments the importance of these responses must be interpreted with caution. Furthermore, the responses of plants to very low light intensities are measured over long time periods. Thus, for some of the responses listed in Fig. 1, exposure times vary from 12 days for photomorphogenesis in Avena
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