The interaction of age/size and crown position in regulating photosystem physiology of red spruce and linkages to stem hydraulics

Day, Michael; Autio, Alisha; Greenwood, Michael; Adams, Stephanie

From the abstract: "The foliage of forest trees shows predictable heterophyllic patterns of development due to light environment and heteroblastic patterns regulated by tree age/size. Typically, upper crown, sun‐adapted foliage is morphologically more robust and physiologically allocates more resources to photosynthetic carboxylation capacity. In contrast, lower crown, shade‐adapted foliage has lower leaf mass per area and allocates more resources to light harvesting. Regulation of the morphological and anatomical aspects of sun‐shade adaptation have been associated with light quality through phytochrome sensing pathways and light intensity through sugar metabolism. Previous research, including long‐term reciprocal grafting studies, demonstrated that foliage increased in robustness with tree age/size but also was influenced by meristem‐intrinsic factors. Photosystem physiology, in contrast, showed regulation by factors extrinsic to meristems age. This study compared allocation to photosynthetic pigments in sapling and old‐growth red spruce. While photosynthetic pigments in the younger age class followed patterns similar to that of their sun‐shade morphology, those in old trees showed a distinct lack of plasticity between sun‐ and shade‐adapted foliage. The authors propose a phytohormonal mechanism based on hydraulic flow to needles at shoot apices to explain this incongruity. In this model, the distribution of cytokinins that regulate genes for chlorophyll synthesis are dispersed through longer, more complex hydraulic pathways in old trees, producing similar effects in both upper and lower crown foliage. As cytokinin regulation of chlorophyll has not been verified in conifers, the effect of cytokinins on down‐regulating chlorophyll synthesis in red spruce was ascertained through exogenous applications on foliage from both tree age/size classes."