Abstract. It has previously been proposed that the fundamental function of phytochrome in the natural environment is the perception of the relative proportions of red and far-red light, i.e. the red: far-red ratio. This paper re-evaluates this hypothesis, for vegetative green plants, in the light of recent findings. Essentially, three issues are considered: (a) the modulation of the response to red: far-red by fluence rate: (b) the anticipation of competition for light by perception of changes in red: far-red that precede actual shading: and (c) characteristics of phytochrome that may be important in the mechanism of photoperception (i.e. the accumulation of photoconversion intermediates, and the stability of Pfr). We conclude: (a) the red: far-red ratio provides a reliable signal of plant density, even before shading by neighbours occurs: (b) plants are able to perceive and respond to these signals, and that possible ambiguities due to low red: far-red at low solar angles may be avoided by modulation of the perception process by fluence-rate dependent mechanisms; (c) although direct experimental evidence does not yet exist, circumstantial evidence suggests that the perception of red: far-red may confer positive adaptive advantage; and (d) plants of certain species perceive and respond to fluence rate changes, mediated perhaps by a blue-light absorbing photoreceptor or by phytochrome, but that these responses do not necessarily lead to shade avoidance reactions and their ecological relevance is not fully understood.
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