Experiments were conducted during the growing seasons of 1993-1995 to determine whether exposure to ozone (O(3)) affected the synthesis of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in hybrid poplar, Populus maximowizii A. Henry x trichocarpa Torr. & A. Gray, Clone 245. As the canopy aged, the concentration of Rubisco decreased at a more rapid rate in lower leaves of hyrid poplar ramets subjected to chronic O(3) exposure in open-top chambers than in comparable foliage of plants grown in charcoal-filtered air. There was no difference in rate of synthesis of Rubisco between treatments, suggesting that loss of this protein in O(3)-treated leaves was caused by an accelerated rate of proteolysis. In foliage higher in the canopy, both concentration and rate of synthesis of Rubisco were stimulated by O(3) for a brief period when the leaves were young. Quantification of mRNA for the small (rbcS) and large (rbcL) transcripts of Rubisco did not reveal changes that were likely to reflect altered synthesis of Rubisco as a prime response to O(3). Analyses of Rubisco concentration and rate of Rubisco synthesis in foliage connected by vascular traces within the canopy indicated that loss of Rubisco in older leaves was associated with an increase in this protein in younger leaves higher in the canopy. These data support the notion that accelerated senescence may provide some compensatory benefit to the plant. In 1995, the rate of synthesis of Rubisco was almost always higher in O(3)-treated foliage than in nontreated foliage, even when the concentration of Rubisco was adversely affected by the O(3) treatment. Because accelerated foliar abscission in response to O(3) was minimal in 1995 compared to other years, we speculate that, when abscission is delayed, Rubisco synthesis and concentration become uncoupled.

Ozone (O 3 )-induced accelerated senescence of leaves was measured in four tree species: black cherry (Prunus serotina), hybrid poplar (Populus maximowizii x trichocarpa, clone 245), northern red oak (Quercus rubra) and sugar maple (Acer saccharum). Seedlings or ramets of the four species were subjected to chronic O 3 exposures and designated leaves harvested periodically from emergence to senescence. Gas exchange was analysed, and concentrations of total soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase were measured as indicators of leaf senescence. Total antioxidant potential and ascorbate peroxidase and glutathione reductase activities also were determined. Black cherry and hybrid poplar exhibited O 3 -induced accelerated leaf senescence, whereas sugar maple and northern red oak did not. When the O 3 effects were related to cumulative uptake of the gas, black cherry was the most sensitive of the four species. Although hybrid poplar exhibited similar symptoms of O 3 -induced accelerated senescence after the same exposure period as did black cherry, this species took up much greater quantities of O 3 to achieve the same response. The O 3 -induced increase in glutathione reductase activity in hybrid poplar was consistent with the capacity of this species to take up high concentrations of the gas. Relative tolerance of northern red oak and sugar maple could be explained only in part by lower cumulative O 3 uptake and lower rate of uptake. Sugar maple had the highest antioxidant potential of all four species, which may have contributed to O 3 tolerance of this species. Ascorbate peroxidase activity, when expressed on a fresh weight basis, could not account for differential sensitivity among the four species.