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

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Abstract
After a preliminary period in light, leaf disks floated on 10 mm α-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 14CO2, 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-2-14C and especially pyruvate-3-14C is incorporated predominantly into the C-2 of glycolate in both species, but the specific radioactivity is much greater in maize. Glyoxylate-2-14C is readily converted to glycolate-2-14C in both species. The addition of phosphoenolpyruvate stimulated glycolate formation in maize and inhibited its synthesis in tobacco, and in the presence of 14CO2 the specific radioactivity in glycolate-14C was decreased greatly by the added phosphoenolpyruvate only in maize.