Interactive effects of leaf age and self-shading on leaf structure, photosynthetic capacity and chlorophyll fluorescence in the rain forest tree, Dryobalanops aromatica

In the tropical canopy tree, Dryobalanops aromatica Gaertn. f., upper-canop y leaves (UL) develop under sunlit conditions but are subjected to self-sha ding within the crown as they age. In contrast, lower-canopy leaves (LL) ar e exposed to uniform dim light conditions throughout their life span. By co mparing leaf morphology and physiology of UL and LL, variations in leaf cha racteristics were related to leaf age and self-shading, Mass-based chloroph yll (chl) concentration and the chlorophyll/nitrogen (chl/N) ratio were low er and the chi a/b ratio was higher in UL than in LL, In UL, the chl/N rati o gradually increased and the chi a/b ratio gradually decreased with leaf a ging, whereas these ratios remained unchanged with leaf age in LL. The effe ctive quantum yield of photosystem II (PSII) (Delta F/F-m') at a given irra diance remained unchanged with leaf age in LL, whereas Delta F/F-m' changed with leaf age in UL. These data indicate N reallocation within the leaves from carbon fixation components to light harvesting components and a dynami c regulation of photochemical processes of PSII in response to increased se lf-shading of UL. Despite the difference in light environment with leaf age between UL and LL, maximum photosynthetic rates and nitrogen-use efficienc y decreased with leaf aging in both UL and LL. Constancy in the chl/N ratio with leaf age in LL indicated that the decrease in photosynthetic capacity was caused by effects other than shading, such as leaf aging. We conclude that N reallocation and acclimation of PSII to self-shading occurred even i n mature leaves, whereas the change in photosynthetic capacity with leaf ag e was more conservative.