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.Abbreviations A_sat net CO₂ uptake rate per unit leaf area at light saturation - A net CO₂ uptake rate per unit leaf area - A_max net CO₂ uptake rate per unit leaf area at CO₂ and light saturation - c_i mole fraction of CO₂ in the intercellular air space - g_s stomatal conductance to CO₂ - F_m maximum chlorophyll fluorescence - F_v variable chlorophyll fluorescence - _c quantum yield of CO₂ uptake for absorbed light - ₀ quantum yield of oxygen evolution for incident light - PPFD photosynthetically active radiation - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate - Vc_max maximum rate of carboxylation