An evaluation of the recycling in measurements of photorespiration.

All measurements of photorespiration and gross photosynthesis in leaves, whether using isotopes or not, are underestimated because of the recycling of O(2) or CO(2). On the basis of a simple diffusion model, we propose a method for the calculation of the recycling and the corresponding underestimation of the measurements. This procedure can be applied when the stomatal resistance is known, and allows for a correction of certain results in the literature. It is found that measurements of the photorespiratory CO(2) release are usually underestimated by 20 to 100%, which sets the estimated rate of CO(2) photorespired at 30 to 50% of the net photosynthesis in C3 plants under normal conditions. In water stress studies, the correction of the photorespiration is still more important (1.5-3.3) because the stomata are closed more. Analysis of the diffusion of O(2) shows that its recycling is low and that the underestimation of photorespiration with (18)O(2) is negligible.