TIMING OF OZONE STRESS AND RESULTING STATUS OF RIBULOSE BISPHOSPHATE CARBOXYLASE OXYGENASE AND ASSOCIATED NET PHOTOSYNTHESIS

Experiments were conducted to determine how the timing of an exposure to ozone influenced the impact of the gas on ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) throughout the life span of a designa ted leaf. Saplings of Populus maximowiczii x trichocarpa NE 388 receiv ed 5-d exposures to O3 in growth chambers during and at the terminatio n of presumed synthesis of Rubisco in a designated leaf. Ozone had no detectable impact on Rubisco activity or quantity when the exposure oc curred during the time of increasing concentration of the protein in t he leaf. When the concentration of Rubisco was near its peak, O3 induc ed a reduction in quantity and activity of Rubisco, but after cessatio n of the O3 stress, levels converged with those of the untreated tissu e. When O3 exposure occurred after full leaf expansion, minimal effect s of the gas could be detected. When plants of hybrid popular or Rapha nus sativus L. cv. Cherry Belle received chronic O3 treatment througho ut the lifespan of the leaf, Rubisco activity and quantity declined mo re rapidly and never converged with values of untreated tissue. Studie s of gas exchange revealed that changes in Rubisco were associated wit h a decline in net photosynthesis (A) and that these effects preceded the observed reduction in foliar conductance. CO2 response curves were measured periodically, and the initial slope (linear portion) of the curve, reflecting carboxylation capacity, declined more rapidly with l eaf age in O3-treated than in untreated poplars. There was no effect o f O3 on stomatal limitation to CO2 assimilation except for a slight in crease during the last 2 wk of the 9-wk experiment. This supported the hypothesis that O3 effects on A were associated with CO2-fixing capab ility of the leaf.