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

Abstract

It has previously been proposed that the fundamental function of phylochrotne in the natural environtnent is the perception of the relative proportions of t-ed and far-ted light, i.e, the red; far-red ratio. This paper t-e-evaluates this hypothesis, for vegetative green plants, in the light of recent findings. Essentially, thtee issues ate considered: (a) the tnodulation of the response to red : far-red by fiuence rate; (b) the anticipation of competition for light by perception of changes in red : far-red that precede actual shading; and (c) chatacteristics of phytochrome thai tnay be important in the mcchanistn of photoperception (i.e. the accumulation of photoconversion intet-tnediates, 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 at-e able to pet-ceive and respond to these signals, and that possible atnbiguities due to low red : far-red at low solar angles may be avoided by modulation of the pet-ception process by fluence-rate dependent tnechanisms; (c) although direct experitnental evidence does not yet exist, circumstantial evidence suggests that the perception of red : far-ted tnay confer positive adaptive advantage; and (d) plants of certain species perceive and respond to lluence rate changes, mediated perhaps by a blue-light absorbing photot-eceptor or by phytochrome, bul thai these responses do not necessarily lead to shade avoidance reactions and their ecological relevance is not fully understood.

Antagonistic but Complementary Actions of Phytochromes A and B Allow Optimum Seedling De-Etiolation

“…Shade avoidance is an extremely important competitive strategy that confers a strong, adaptive advantage (Schmitt et al, 1995), but for seedlings with little reserves can be counterproductive. lt has been well established that shade-avoidance reactions are only effective when the plant receives sufficient PAR to provide the resources for the accelerated growth (Smith and Hayward, 1985;Casal and Smith, 1989). This may seem somewhat of a paradox, since it suggests that shade avoidance is only effective when the plants are not shaded!…”

Plasticity to Light Cues and Resources in Arabidopsis Thaliana: Testing for Adaptive Value and Costs

“…In dense stands or beneath a foliage canopy, plants experience a reduction in the ratio of red to far red light (R:FR), a specific cue perceived by the phytochrome family of photoreceptors (Smith 1995). Phytochrome-mediated, shade-avoidance responses are hypothesized to be a form of adaptive plasticity that allows plants to avoid competition for light in dense stands (Casal and Smith 1989;Schmitt and Wulff 1993). Recent experimental evidence supports this hypothesis Dudley and Schmitt 1996;Schmitt 1997).…”

“…Shade-avoidance responses are usually hypothesized to confer a fitness advantage by enhancing light capture in dense stands (Casal and Smith 1989;Schmitt and Wulff 1993;Smith 1995). However, stem elongation is predicted to be maladaptive if the plant cannot overtop the foliage canopy, due to opportunity costs of allocating resources to stem at the expense of resourcecapturing organs (Morgan and Smith 1979;Schmitt and Wulff 1993).…”

Morphological variation in Plantago lanceolata L.: effects of light quality and growth regulators on sun and shade populations

“…The plant canopy absorbs much of the red light while transmitting or reflecting much of the far-red light. It is thought that elongation responses, triggered by the ratio of red to far-red light (R/FR-ratio) and mediated by the phytochrome photoreceptor, are of prime importance for plants to respond adequately to shading (Holmes and Smith, 1977;Smith, 1982;Kaul and Kasperbauer, 1988;Casal and Smith, 1989;Schmitt and Wulff, 1993;Schmitt et al, 1995). Elongation in dense stands may reduce the negative fitness consequences of competition for light, by positioning the photosynthetic active plant parts higher in the vegetation.…”

“…Elongation in dense stands may reduce the negative fitness consequences of competition for light, by positioning the photosynthetic active plant parts higher in the vegetation. In low, open vegetations an erect phenotype may be maladaptive, because of resource allocation to height at the expense of photosynthesis and because of greater risk or breakage (Casal and Smith, 1989;Schmitt and Wulff, 1993). Selection for plastic responses to shading may eventually lead to generalist genotypes that can adapt to a wide range of light conditions (Lewontin, 1957;Bradshaw, 1965).…”