Reductions in gas exchange of Populus tremuloides caused by leaf aging and ozone exposure

Declines in gas exchange of mature leaves of Populus tremuloides Michx. (quaking aspen) seedlings under four O-3 treatments were-investigated in a 33-d study. Gas exchange fell gradually with time, but under O-3 exposure declines were accelerated. Photosynthetic capacity, stomatal conductance, and carboxylation efficiency were highly correlated, suggesting a coordinated reduction in the physiological activity of stressed leaves. Intercellular CO2 was not significantly different among treatments, indicating that stomatal opening was not the primary factor in limiting photosynthesis. Analysis of A:C-i response curves revealed that stomatal limitation increased only slightly, while biochemical limitation more than doubled, reaching a maximum of 47.1% by the third week of fumigations. Number of surviving leaves, ratio of surviving leaves to leaves produced, and total leaf area declined with O-3 exposure. Whole-plant response curves were less linear than curves for the oldest subset of leaves because new leaves were produced continuously during the study, resulting in within-plant variation in O-3 exposure. Declines in photosynthesis were greater than leaf area losses when data were expressed as a percentage of the control treatment. These results indicate that exposure to O-3 for a relatively short time severely limits plant carbon gain in a sensitive free-growing species like aspen.