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ABSTRACT Plants sense neighbours even before these cause a decrease in photosynthetic light availability. Light reflected by proximate neighbours signals a plant to adjust growth and development, in order to avoid suppression by neighbour plants. These phenotypic changes are known as the shade-avoidance syndrome and include enhanced shoot elongation and more upright-positioned leaves. In the present study it was shown that these shade-avoidance traits in tobacco (Nicotiana tabacum ) are also induced by low concentrations of ethylene. Furthermore, it was shown that transgenic plants, insensitive to ethylene, have a delayed appearance of shade-avoidance traits. The increase in both leaf angles and stem elongation in response to neighbours are delayed in ethylene-insensitive plants. These data show that ethylene is an important component in the regulation of neighbour-induced, shade-avoidance responses. Consequently, ethylene-insensitive plants lose competition with wild-type neighbours, demonstrating that sensing of ethylene is required for a plant to successfully compete for light. [ABSTRACT FROM AUTHOR]

Abstract Modelling is used to predict long-term forest responses to increased atmospheric CO2 concentrations. Although productivity models are based on light intercepted by the canopy, very little experimental data are available for closed forest stands. Nevertheless, the relationships between light inside a canopy, leaf area, canopy structure, and individual leaf characteristics may be affected by elevated CO2 , affecting in turn carbon gain. Using a free-air CO2 enrichment (FACE) design in a high-density plantation of Populus spp., we studied the effects of increased CO2 concentrations on transmittance (τ) of photosynthetic photon flux density (Q p ), on ratios of red/far-red light (R/FR), on leaf area index (LAI), on leaf inclination, on leaf chlorophyll (chl) and nitrogen (N) concentrations, and on specific leaf area (SLA) in the 2nd and 3rd years of treatment. Continuous measurements of τ were made in addition to canopy height profiles of light and leaf characteristics. Two years of Q p measurements showed an average decrease of canopy transmittance in the FACE treatment, with very small differences at canopy closure. Results were explained by an unaffected LAI in closed canopies, without a FACE-induced stimulation of relative crown depth. In agreement, leaf inclination and extinction coefficients for light were similar in control and FACE conditions. Ratios of R/FR were not significantly affected by the FACE treatment, neither were leaf characteristics, with the exception of leaf N, which allows speculation about N limitation. In general, treatment differences in canopy profiles resulted from an initial stimulation of height growth in the FACE treatment. P. × euramericana differed from P. alba and P. nigra , but species did not differ significantly in their response to the FACE treatment. By the time fast-growing high-density forest plantations have passed the exponential... [ABSTRACT FROM AUTHOR]

Measurements of the photosynthetic photon flux density ( 400-700 nm) and of the spectral distribution of photon flux density across that 370-800 nm waveband, were made under both clear and overcast sky conditions above and at various positions within two Pinus radiata canopies of different stocking but similar leaf area indices. The spectra obtained for the daylight conditions (i.e. above forest canopy) were generally similar to those published previously. The spectra for shadelight within the forest canopy showed no blue peak which was characteristic of previously reported measurements which were restricted to the diffuse radiation component. There was almost neutral absorption within the 400-700 nm waveband, and typical lower attenuation in the 700-800nm waveband. The blue: red ratio was largely unchanged by either canopy type or sky conditions and varied between 0.57 and 0.81. The red: far-red ratio in shadelight was between 0.22 and 0.41 under clear sky and between 0.68 and 0.95 under overcast sky and between 0.68 and 0.95 under overcast sky conditions. Values for daylight were between 1.16-1.22, Calculated phytochrome photoequilibrium values in shadelight were approximately 0.35 under clear sky and 0.46 under overcast sky conditions. In each case there appeared to be no differences between the two canopies with respect to these minimum values. [ABSTRACT FROM AUTHOR]