Phytochrome mediated effects on leaves of white clover:: Consequences for light interception by the plant under competition for light

Competition for light is one of the main factors affecting the persistence of clover in plant communities. As a signal of plant competition, lowering the red to far-red (R:FR) ratio of incident light is known to elicit morphological responses in white clover shoots, such as reducing the branching rate and increasing organ size. The objectives of this study were to examine the influence of the R:FR ratio on leaf development and to investigate the consequences for the ability of leaves to intercept light. Two short-term experiments were conducted on isolated plants using either selective light filters in a glasshouse (expt 1) or monochromatic light-emitting diodes in a climate chamber (expt 2) to induce changes in the R:FR ratio. In each experiment, morphological data and biomass were recorded. Four weeks after the establishment of the light treatments, plants were labelled with (CO2)-C-14 (expt 1). The optical properties (absorptance, transmittance and reflectance) of main stolon leaves were measured. Rates of leaf initiation and emergence were not influenced by light treatment in either experiment and no significant effect of the R:FR ratio was observed on mean leaf area of individual leaves. However, light quality acted indirectly on total leaf area mainly through branch development, as previously demonstrated in white clover. Lowering the R:FR ratio stimulated petiole elongation and this was sustained by the allocation of more biomass to growing petioles and more C-14 assimilates to growing stolon apices. It is suggested that light interception by white clover leaves could be influenced through FR-promoted petiole growth only, as there was no direct modification of individual leaf area, or of the capacity of leaves to absorb light and assimilate carbon. (C) 2001 Annals of Botany Company.