| s leaves form, they develop morphological and physiological characteristics that appear to be adaptive for the light conditions under which they formed. However, for evergreen conifers, foliage can experience large decreases in light after leaf growth is complete and leaf morphology is set. Can foliage that is structurally determined as sun foliage change physiologically when it is shaded to function as shade foliage? Is this acclimation independent of the aging process and source-sink relations? These questions were investigated and the photosynthetic function of Abies amabilis foliage within a canopy was examined and related to three different processes that affect foliage function: foliage aging, initial expansion phase sun-shade acclimation, and subsequent re-acclimation.;The results of an artificial shading study indicate that mature foliage can physiologically acclimate to shading within 100 days. The primary response was a reallocation of resources (nitrogen) within the leaf to optimize the ratio of dark and light reactants for a given light environment. A change in the total resources allocated to the foliage was a secondary response and appears to function on a slower time scale. In a second study examining whole canopy patterns of photosynthetic function, most physiological indicators were more strongly correlated with the current light environment than with foliage age or leaf thickness with two exceptions: chlorophyll content and the chlorophyll a:b ratio.;The mechanism for photosynthetic acclimation of mature foliage to its current light environment was mainly a redistribution of nitrogen both within and between leaves and was driven by changing light. Within these Abies amabilis canopies, both leaf nitrogen content and the chlorophyll:nitrogen ratio were strongly correlated with the current light environment, but only weakly with leaf age. These results support the idea that changing light is the driving force for the redistribution of nitrogen both within and between leaves. However,... |
