In plants, stem elongation at high density in response to vegetation shade is hypothesized to be an example of adaptive phenotypic plasticity. Elongated stems may increase the light capture for plants in dense stands, while nonelongated stems may be favored for plants in low density. We tested the adaptive value of plastic stem elongation in Impatiens capensis by manipulating the controlling light cue, red to far red ratio, to produce elongated and nonelongated plants. These plants were then transplanted into high and low densities in a natural population. The results supported the adaptive plasticity hypothesis; elongated plants were more fit at high density, and suppressed plants were more fit at low density. Phenotypic selection analysis revealed selection for increased height in high density and for decreased height relative to leaf length in low density. Elongated plants showed less growth of the second internode at 2 wk after transplantation in both densities, which suggests a cost of elongation. Di...

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Testing the Adaptive Plasticity Hypothesis: Density-Dependent Selection on Manipulated Stem Length in Impatiens capensis

Understanding maize–weed competition: resource competition, light quality and the whole plant

The light environment experienced by plants in natural vegetation is strongly dependent upon interactions with neighbors. For plants in dense stands, reduced irradiance can lead to reductions in growth and fitness. Spectral light quality is also altered beneath a leaf canopy, and can serve as an important signal of competition for light. Recent physiological studies indicate that plants can perceive the quality of light reflected from neighbors as an accurate predictor of future competition, and respond morphologically even before they are directly shaded. These findings have important implications for plant population biology, and provide a valuable opportunity for the study of adaptive plasticity.

Light spectral quality, phytochrome and plant competition.

Many plants display characteristic phytochrome-mediated stem elongation responses to crowding and vegetation shade, commonly referred to as the shade avoidance syndrome We tested the hypothesis that this elongation is a form of adaptive plasticity by comparing the relative performance at high and low density of wild-type plants and transgenic and mutant plants in which the shade avoidance response was disabled Transgenic tobacco plants in which elongation in response to neighbors was blocked by expression of the oat PHYA gene had decreased relative fitness when grown in competition with elongated wild-type plants In contrast, constitutively elongated Brassica ein mutant plants, deficient in light-stable phytochrome, had lower fitness relative to nonelongated wild type at low density than in competition with elongated wild type at high density The observation that phytochrome-mediated elongation is advantageous in dense stands, but disadvantageous for uncrowded plants, indicates that a response to foli

A Test of the Adaptive Plasticity Hypothesis Using Transgenic and Mutant Plants Disabled in Phytochrome-Mediated Elongation Responses to Neighbors

(...) This article reviews the regulation of development by phytochrome and discusses evidence from physiological studies of wild type, mutant and transgenic plants consistent with the proposal that different members of the phytochrome family have different photosensory roles. (...)

Phytochrome, a family of photoreceptors with multiple physiological roles

. Spectral photon distributions, red:far-red ratios (i.e. R:FR) and phytochrome photoequilibria (i.e. Pfr/Ptotal) were measured at various distances from artificial canopies composed of mustard or tobacco plants. Measurements were compared for radiation propagated predominantly vertically downwards and radiation propagated predominantly horizontally. Reflection signals from the artificial canopies were computed and shown to consist of a depletion of radiation over the 400–690 nm wavelength range, and an enhancement of radiation over the 690–800 nm range. R:FR and Pfr/Ptotal increased gradually with distance from the canopies, with significant depressions of both parameters evident at least as far as 30 cm from the vegetation stands. It is concluded that, in principle at least, detection of spectral quality differences by phytochrome would allow not only the presence but also the proximity of neighbouring plants to be perceived. Proximity perception is proposed as an ecologically valuable mechanism through which plants may be able to gauge their anticipatory responses to incipient shading according to the challenge posed by the nearness of neighbouring plants.

G. M. Jackson

Papers

Reflection signals and the perception by phytochrome of the proximity of neighbouring vegetation