Increased rate of net photosynthetic carbon dioxide uptake caused by the inhibition of glycolate oxidase.
There is considerable variation among species in their rate of photorespiration, and photorespiration increases greatly at higher temperatures. The addition of an inhibitor of glycolate oxidase, alpha-hydroxy-2-pyridinemethanesulfonic acid, to tobacco leaf disks at 35 degrees stimulated photosynthetic (14)CO(2) uptake at least 3-fold, but (14)CO(2) uptake was not changed by the inhibitor at 25 degrees . The inhibitor did not increase photosynthesis in maize leaf disks at either temperature.The evolution of CO(2) from glycolate was greatly enhanced in tobacco at 35 degrees compared with 25 degrees . Labeling of the glycolate of tobacco with glycolate-1-(14)C and -2-(14)C showed that the increased CO(2) evolved in the light (photorespiration) arose specifically from the carboxyl-carbon atom of glycolate. Maize, a species known to have a negligible photorespiration, produced (14)CO(2) poorly from glycolate-1-(14)C in comparison to tobacco.Acetate-1-(14)C, a substrate metabolized by dark respiration, produced similar amounts of (14)CO(2) in the light in both tobacco and maize. This respiration was changed little relative to photosynthesis by increasing temperature.Most plants, such as tobacco, have a high photorespiration. The loss of fixed carbon causes an increase in the internal concentration of CO(2) especially at higher temperatures, and results in a lower CO(2) concentration gradient and therefore a lower net photosynthetic CO(2) uptake. Some species, like maize, have a negligible photorespiration and are thus more efficient photosynthetically. The use of an inhibitor of the oxidation of glycolate, the substrate for photorespiration, changed tobacco so that it behaved photosynthetically like maize. Thus high rates of photorespiration may limit the net CO(2) uptake in many plant species.
Zelitch
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Department of Biochemistry, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504.