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-investigate d in a 33-d study. Gas exchange fell gradually with time, but under O- 3 exposure declines were accelerated. Photosynthetic capacity, stomata l conductance, and carboxylation efficiency were highly correlated, su ggesting a coordinated reduction in the physiological activity of stre ssed leaves. Intercellular CO2 was not significantly different among t reatments, indicating that stomatal opening was not the primary factor in limiting photosynthesis. Analysis of A:C-i response curves reveale d 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 l eaves to leaves produced, and total leaf area declined with O-3 exposu re. Whole-plant response curves were less linear than curves for the o ldest subset of leaves because new leaves were produced continuously d uring the study, resulting in within-plant variation in O-3 exposure. Declines in photosynthesis were greater than leaf area losses when dat a were expressed as a percentage of the control treatment. These resul ts indicate that exposure to O-3 for a relatively short time severely limits plant carbon gain in a sensitive free-growing species like aspe n.