The function, action and adaptive significance of phytochrome in light‐grown plants

Abstract

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.