Increased Rate of Net Photosynthetic Carbon Dioxide Uptake Caused by the Inhibition of Glycolate Oxidase

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Abstract
There is considerable variation among species in their rate of photorespiration which increases greatly at higher temperatures. The addition of an inhibitor ol glycolate oxidase [alpha]-hydroxy-2-pyridinemethanesulfonic acid, to tobacco leaf disks at 35[degree] but not at 25[degree] C stimulated photosynthetic C14O2 uptake at least 3-fold. The inhibitor did not increase photosynthesis in maize leaf disks at either temperature. The evolution of CO2 from glycolate was greatly enhanced in tobacco at 35[degree] compared with 25[degree]. Labeling of the glycolate of tobacco with glycolate-l-C14 and -2-C14 revealed that the increased CO2 evolved in the light (photorespiration) arose specifically from the carboxyl-carbon atom of glycolate. Maize produced 14CO2 poorly from glycolate-1-14C in comparison to tobacco. Acetate-1-14C, a substrate metabolized by dark respiration, produced similar amounts of 14CO2 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 C causes an increase in the internal concentration of CO2 especially at higher temperatures, and results in a lower CO2 concentration gradient and therefore a lower net photosynthetic CO2 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. High rates of photorespiration may limit the net CO2 uptake in many plant species.