OZONE-INDUCED CHANGES IN PRIMARY CARBON METABOLISM ENZYMES OF LOBLOLLY-PINE NEEDLES

Current ambient ozone concentrations in the southeastern U.S.A. have the potential to affect carbon fixation rates and growth of forest tree species. In this study, loblolly pine saplings were exposed to ozone fumigation in open-top chambers during four growing seasons in the Piedmont of North Carolina, U.S.A. to study biochemical responses in pathways of carbon fixation and metabolism. The trees exhibited a significant decrease in needle photosynthetic rates proportional to the cumulative ozone exposure, decreasing by 50% after 350ppm.h (12-h summation). This decrease was correlated with a parallel decline in Rubisco activity. In contrast, increases in the activities of several enzymes linked to the degradative pathways of sugars were observed. The largest increase was observed in the activity of glucose-6-phosphate dehydrogenase (65% for 300ppm.h). However, this marked increase and the slighter increases in phosphofructokinase and fumarase activities (about 25%) showed a tendency to a further decline when the trees were exposed to higher cumulative ozone doses. Thus, current ambient ozone concentrations could lead progressively to biochemical disequilibrium within leaf cells with reduced production of assimilates and transient increased respiration.