Utilization of O-2 in the metabolic optimization of C-4 photosynthesis

The combined effects of O-2 on net rates of photosynthesis, photosystem II activity, steady-state pool size of key metabolites of photosynthetic metabolism in the C-4 pathway, C-3 pathway and C-2 photorespiratory cycle and on growth were evaluated in the C-4 species Amaranthus edulis and the C-3 species Flaveria pringlei. Increasing O-2 reduced net CO2 assimilation in F. pringlei due to an increased flux of C through the photorespiratory pathway. However, in A. edulis increasing O-2 up to 5-10% stimulated photosynthesis. Analysis of the pool size of key metabolites in A. edulis suggests that while there is some O-2 dependent photorespiration, O-2 is required for maximizing C-4 cycle activity to concentrate CO2 in bundle sheath cells. Therefore, the response of net photosynthesis to O-2 in C-4 plants may result from the balance of these two opposing effects. Under 21 versus 5% O-2, growth of A. edulis was stimulated about 30% whereas that of F. pringlei was inhibited about 40%.