Regulation of Leaf Traits in Canopy Gradients

The gradient of leaf traits in a canopy from sunlit upper regions to shaded lower ones is regulated in response to the density of its leaf area. The gradients of environmental factors act as signals for the regulation. The result is improved resource use efficiency for carbon gain at the whole plant level. Herbaceous species with relatively fast leaf turnover typically grow new leaves at the top in high light that are subsequently progressively shaded in developing dense canopies. Export of resources associated with photosynthetic capacity accompanies the progressive shading, later on followed by degradation of light harvesting components when senescence is induced. The red:far-red ratio of the light gradient is involved in the reallocation of resources and the induction of leaf senescence, but the irradiance component of the light gradient dominates the canopy effect. It impacts a multitude of physiological processes. Their effect can operate locally such as perception by photoreceptors and excitation pressure implicated in chloroplast organization. Other effects impact processes operating at the whole plant level such as the distribution of signaling compounds in the transpiration stream and the supply of assimilates to developing young leaves. These systemically operating pathways are at the basis of a coordinated response of plants to the shading effect in a canopy gradient, which is different from whole plant shading. The available evidence for mechanisms involved in the regulation of leaf traits in canopies is discussed.