Light-Induced Leaflet Orientation in Phaseolus vulgaris L
1973
Wien, H. C. | Wallace, D. H.
The effect of light on change of leaflet angle of attached and detached leaves of four cultivars of dry beans (Phaseolus vulgaris L.) was measured. Light receptor and response site were both found to be the pulvlnule at the base of each leaflet. All three pulvinules of the trifoliolate leaf responded. Light impinging on the top of pulvinules of all varieties caused them to bend upward, i.e. toward the light. This rotated attached leaflets upward, orienting them more parallel to the overhead source-direction of the light. Light from the side also caused the pulvinules of all four varieties to bend toward the light. However, the more prominent response to light impinging upon the side of pulvinules was a twisting of the pulvinule which, with reference to the light source, rotated the far side of the leaflet upward and the near side downward. This positioned the leaflets more perpendicular to the horizontal source-direction of the light. Thus, light from the side caused the leaflets to orient themselves so that they would intercept more light energy, whereas light from above oriented them so that they would intercept less. The four varieties differed in amount and type of leaf orientation under a given light intensity. For one variety only, in addition to causing the pulvinule to bend toward the light, light impinging upon the top of the also caused it to both pulvinule twist. With this variety, responses oriented leaflets more parallel to the overhead source-direction of the light, with the result that the leaflets would intercept less light energy. It is hypothesized that leaf characters maximizing both pulvlnule exposure to light and the leaf-orientation response would maximize light penetration into the leaf canopy and lead to higher photosynthetic rates, especially at close spacings with high leaf area index.
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