C-4 PHOTOSYNTHESIS - THE EFFECTS OF LEAF DEVELOPMENT ON THE CO2-CONCENTRATING MECHANISM AND PHOTORESPIRATION IN MAIZE

The effect of O-2 on photosynthesis was determined in maize (Zea mays) leaves at different developmental stages. The optimum level of O-2 for maximum photosynthetic rates was lower in young and senescing tissues (2-5 kPa) than in mature tissue (9 kPa). Inhibition of photosynthesis by suboptimal levels of O-2 may be due to a requirement for functional mitochondria or to cyclic/pseudocyclic photophosphorylation in chloroplasts; inhibition by supraoptimal levels of O-2 is considered to be due to photorespiration. Analysis of a range of developmental stages (along the leaf blade and at different leaf ages and positions) showed that the degree of inhibition of photosynthesis by supraoptimal levels of O-2 increased rapidly once the ribulose-1,5-bisphosphate carboxylase/oxygenase and chlorophyll contents were below a critical level and was similar to that of C-3 plants. Tissue having a high sensitivity of photosynthesis to O-2 may be less effective in concentrating CO2 in the bundle sheath cells due either to limited function of the C-4 cycle or to higher bundle sheath conductance to CO2. An analysis based on the kinetic properties of ribulose-1,5-bisphosphate carboxylase/oxygenase was used to predict the maximum CO2 level concentrated in bundle sheath cells at a given degree of inhibition of photosynthesis by supraoptimal levels of O-2.