Timing of ozone stress and resulting status of ribulose bisphosphate carboxylase/oxygenase and associated net photosynthesis

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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 designated leaf. Saplings of Populus maximowizii×trichocarpa NE 388 received 5-d exposures to O3 in growth chambers during and at the termination of presumed synthesis of Rubisco in a designated leaf. Ozone had no detectable impact on Rubisco activity or quantity when the exposure occurred during the time of increasing concentration of the protein in the leaf. When the concentration of Rubisco was near its peak, O3 induced a reduction in quantity and activity of Rubisco, but after cessation of the O3 stress, levels converged with those of the untreated tissue. When O3exposure occurred after full leaf expansion, minimal effects of the gas could be detected. When plants of hybrid popular or Raphanus sativus L. cv. Cherry Belle received chronic O3 treatment throughout the lifespan of the leaf, Rubisco activity and quantity declined more rapidly and never converged with values of untreated tissue.

Studies of gas exchange revealed that changes in Rubisco were associated with 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 leaf age in O3-treated than in untreated poplars. There was no effect of O3 on stomatal limitation to CO2, assimilation except for a slight increase during the last 2 wk of the 9-wk experiment. This supported the hypothesis that O3 effects on A were associated with CO2-fixing capability of the leaf.

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