Evaluation of the light/dark C assay of photorespiration: tobacco leaf disk studies with glycidate and glyoxylate.

Preincubation of illuminated tobacco (Nicotiana tabacum L.) leaf disks in glycidate (2,3-epoxypropionate) or glyoxylate inhibited photorespiration by about 40% as determined by the ratio of (14)CO(2) evolved into CO(2)-free air in light and in darkness. However, under identical preincubation conditions used for the light/dark (14)C assays, the compounds failed to reduce photorespiration or stimulate net photosynthesis in tobacco leaf disks based on other CO(2) exchange parameters, including the CO(2) compensation concentration in 21% O(2), the inhibitory effect of 21% O(2) on net photosynthesis in 360 microliters per liter of CO(2) and the rate of net photosynthetic (14)CO(2) uptake in air.The effects of both glycidate and glyoxylate on the (14)C assay are inconsistent with other measures of photorespiratory CO(2) exchange in tobacco leaf disks, and thus these data question the validity of the light to dark ratio of (14)CO(2) efflux as an assay for relative rates of photorespiration (Zelitch 1968, Plant Physiol 43: 1829-1837). The results of this study specifically indicate that neither glycidate nor glyoxylate reduces photorespiration or stimulates net photosynthesis by tobacco leaf disks under physiological conditions of pO(2) and pCO(2), contrary to previous reports.