RESUMEN
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.