C4 PHOTOSYNTHESIS - THE CO2-CONCENTRATING MECHANISM AND PHOTORESPIRATION

Despite previous reports of no apparent photorespiration in C4 plants based on measurements of gas exchange under 2 versus 21% 02 at varying [CO2], photosynthesis in maize (Zea mays) shows a dual response to varying [O2]. The maximum rate of photosynthesis in maize is dependent on 02 (approximately 10%). This 02 dependence is not related to stomatal conductance, because measurements were made at constant intercellular CO2 concentration (C(i)); it may be linked to respiration or pseudocyclic electron flow. At a given C(i), increasing [02] above 10% inhibits both the rate of photosynthesis, measured under high light, and the maximum quantum yield, measured under limiting light (PHI(CO2)). The dual effect of 02 is masked if measurements are made under only 2 versus 21% 02. The inhibition of both photosynthesis and PHI(CO2), by 02 (measured above 10% O2) with decreasing C(i) increases in a very similar manner, characteristically Of 02 inhibition due to photorespiration. There is a sharp increase in 02 inhibition when the C(i) decreases below 50 mubar of CO2. Also, increasing temperature, which favors photorespiration, causes a decrease in PHI(CO2) under limiting CO2 and 40% O2. By comparing the degree of inhibition of photosynthesis in maize with that in the C3 species wheat (Triticum aestivum) at varying C(i), the effectiveness of C4 photosynthesis in concentrating CO2 in the leaf was evaluated. Under high light, 30-degrees-C, and atmospheric levels of CO2 (340 mubar), where there is little inhibition of photosynthesis in maize by 02, the estimated level of CO2 around ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) in the bundle sheath compartment was 900 mubar, which is about 3 times higher than the value around Rubisco in mesophyll cells of wheat. A high [CO2] is Maintained in the bundle sheath compartment in maize until C(i) decreases below approximately 100 mubar. The results from these gas exchange measurements indicate that photorespiration occurs in maize but that the rate is low unless the intercellular [CO2] is severely limited by stress.