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

Abstract

Declines in gas exchange of mature leaves of Populustremuloides Michx. (quaking aspen) seedlings under four O3 treatments were investigated in a 33-d study. Gas exchange fell gradually with time, but under O3 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:Ci 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 O3 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 O3 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 O3 for a relatively short time severely limits plant carbon gain in a sensitive free-growing species like aspen.