An in vivo analysis of photosynthesis during short-term O3 exposure in three contrasting species.

The depressions of photosynthetic CO2 uptake following O3 exposures of 200 and 400 nmol mol(-1) for between 4 and 16 h were compared between Pisum sativum, Quercus robur and Triticum aestivum, and the potential causes of change identified in vivo. Photosynthetic change was examined by analysis of CO2, O2, O3 and water vapour exchanges together with chlorophyll fluorescence in controlled environments. Under identical fumigation conditions, each species showed very similar rates of O3 consumption. The light-saturated rate of CO2 uptake showed a statistically significant decrease in each species with increasing O3 dose. Although stomatal conductance declined in parallel with CO2 uptake this did not account for the observed decrease in photosynthesis. The decrease in mesophyll conductance resulted primarily from a decrease in the apparent carboxylation capacity, implying in decreased activity of ribulose 1,5-bisphosphate carboxylase/oxygenase. The maximum capacity of carboxylation was consequently reduced by over 30% and 50% after 16 h fumigation with 200 and 400 nmol mol(-1) O3 respectively. Additionally, in Q. robur, a statistically significant inhibition of the CO2 saturated rate of photosynthesis occurred after 16 h with 400 nmol mol(-1) O3, suggesting that the ability to regenerate ribulose 1,5-bisphosphate was also impaired. None of the species showed any significant decrease in the efficiency of light-limited photosynthesis following fumigation at 200 nmol mol(-1) O3, but effects were apparent at 400 nmol mol(-1) O3. The common feature in all three species was a decline in carboxylation capacity which preceded any other change in the photosynthetic apparatus.