The C02-Concentrating Mechanism and Photorespiration

Despite previous reports of no apparent photorespiration in C., plants based on measurements of gas exchange under 2 versus 21% O, at varying [CO,], photosynthesis in maize (Zea mays) shows a dual response to varying [O,]. lhe maximum rate of photosynthesis in maize is dependent on 0, (approximately 10Y0). lhis O2 dependence is not related to stomatal conductance, because measurements were made at constant intercellular CO, concentration (Ci); it may be linked to respiration or pseudocyclic electron flow. At a given Ci, increasing [O,] above 10% inhibits both the rate of photosynthesis, measured under high light, and the maximum quantum yield, measured under limiting light (Qco,). lhe dual effect of O2 is masked if measurements are made under only 2 versus 21% O,. lhe inhibition of both photosynthesis and Qco, by O, (measured above 10% O,) with decreasing Ci increases in a very similar manner, characteristically of O, inhibition due to photorespiration. There is a sharp increase in 0, inhibition when the Ci decreases below 50 wbar of CO,. Also, increasing temperature, which favors photorespiration, causes a decrease in QCO, under limiting CO, and 40% 0,. By comparing the degree of inhibition of photosynthesis in maize with that in the CI species wheat (Triticum aestivum) at varying Ci, the effectiveness of C., photosynthesis in concentrating CO, in the leaf was evaluated. Under high light, 30"C, and atmospheric levels of C02 (340 pbar), where there is little inhibition of photosynthesis in maize by O,, the estimated leve1 of C02 around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the bundle sheath compartment was 900 pbar, which is about 3 times higher than the value around Rubisco in mesophyll cells of wheat. A high [COZI is maintained in the bundle sheath compartment in maize until Ci decreases below approximately 100 Mbar. lhe results from these gas exchange measurements indicate that photorespiration occurs in maize but that the rate is low unless the intercellular [COJ is severely limited by stress.