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After a preliminary period in light, leaf disks floated on 10 mm alpha-hydroxy-2-pyridinemethanesulfonic acid to inhibit glycolate oxidase accumulate glycolate at average initial rates of 67 micromoles in tobacco and 8 micromoles per gram fresh weight per hour in maize under optimal conditions in air. In the presence of (14)CO(2), the glycolate synthesized has a high specific radioactivity in illuminated tobacco and a low one in maize. Isonicotinic acid hydrazide also inhibits glycolate oxidation and causes a slow accumulation of glycolate in maize but not in tobacco, while it inhibits glycolate synthesis in tobacco but not in maize. Radioactive carbon in acetate-2-(14)C and especially pyruvate-3-(14)C is incorporated predominantly into the C-2 of glycolate in both species, but the specific radioactivity is much greater in maize. Glyoxylate-2-(14)C is readily converted to glycolate-2-(14)C in both species. The addition of phosphoenolpyruvate stimulated glycolate formation in maize and inhibited its synthesis in tobacco, and in the presence of (14)CO(2) the specific radioactivity in glycolate-(14)C was decreased greatly by the added phosphoenolpyruvate only in maize.Thus, unsymmetrically labeled glycolate is mainly synthesized from pyruvate-3-(14)C by a slow pathway in maize. Tobacco possesses an additional rapid pathway that produces equally labeled glycolate more directly from fixed CO(2) during photosynthesis. Glycolate is believed to be the primary substrate of photorespiration, and sufficiently rapid rates of glycolate synthesis have been observed in tobacco to account for this function. Hence the high rates of photorespiration observed in tobacco leaves compared with maize result partly from differences between these species in the pathway of glycolate synthesis.

Alternate Pathways of Glycolate Synthesis in Tobacco and Maize Leaves in Relation to Rates of Photorespiration