Evaluation of the Light/Dark 14C Assay of Photorespiration

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Abstract
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 14CO2 evolved into CO2-free air in light and in darkness. However, under identical preincubation conditions used for the light/dark 14C assays, the compounds failed to reduce photorespiration or stimulate net photosynthesis in tobacco leaf disks based on other CO2 exchange parameters, including the CO2 compensation concentration in 21% O2, the inhibitory effect of 21% O2 on net photosynthesis in 360 ml/l of CO2 and the rate of net photosynthetic 14CO2 uptake in air. The effects of both glycidate and glyoxylate on the 14C assay are inconsistent with other measures of photorespiratory CO2 exchange in tobacco leaf disks, and thus these data question the validity of the light to dark ratio of 14CO2 efflux as an assay for relative rates of photorespiration. 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 pO2 [partial pressure of O2] and pCO2, contrary to previous reports.