Photorespiratory phenomena in maize: oxygen uptake, isotope discrimination, and carbon dioxide efflux

Concurrent O(2) evolution, O(2) uptake, and CO(2) uptake by illuminated maize (Zea mays) leaves were measured using (13)CO(2) and (18)O(2). Considerable O(2) uptake occurred during active photosynthesis. At CO(2) compensation, O(2) uptake increased. Associated with this increase was a decrease in O(2) release such that a stoichiometric exchange of O(2) occurred. The rate of O(2) exchange at CO(2) compensation was directly related to O(2) concentration in the atmosphere at least up to 8% (v/v).When illuminated maize leaves were exposed to saturating CO(2) concentrations containing approximately equal amounts of (12)CO(2) and (13)CO(2), the latter was taken up more rapidly, thus depressing the atom% (13)C in the atmosphere. Moreover, upon exposure to CO(2) containing 96 atom% (13)C, there occurred a directly measurable efflux of (12)CO(2) from the leaves for at least 15 minutes. During this period an equimolar evolution of (16)O(2) and uptake of (13)CO(2) was observed. Thereafter, although the rate of (16)O(2) evolution remained unchanged, the rate of (13)CO(2) uptake declined markedly, suggesting continual (13)C enrichment of the photorespiratory substrate.It is concluded that a finite photorespiratory process occurs in maize and that the CO(2) generated thereby is efficiently recycled. Recycling maintains the internal CO(2) concentration at a level difficult to detect by most photorespiratory assays.