High Activity of the Glycolic Acid Oxidase System in Tobacco Leaves

Glycolic acid has been su,ggested as a major substrate for photorespiration. According to Zelitch (5,6), respiration in the light is several times greater than dark respiration because many plants in the Iiloht synthesize large quantities of glycolic acid which is then oxidized, in 'part at least, to CO2. Yet little glycolic acid is ever found in normal illuminated leaves. If any significant amount of CO2 is released within leaves dhie to glycolic acid meta-bolism, then the glycolic acid oxidase enzymes must have high activity, for this signiificant rate of photorespiration must occur despite very low substrate concentrations. In fact many leaves do, indeed, seem to metabolize glycolic acid very rapidly. Maize, on the other hand, produces little CO2 from gliycolic acid, and its photorespiration is low ( 1, 2, 3, 4, 6). The following experiment demonstrates the rapidity of glycolic acid oxidation to CO, in tobacco and shows the contrastingly slow rate at which maize produces CO, from glycolic acid. Leaves of maize (Zea mays L.) and tobacco (Nicotiana tabacum L.) were excised and recut beneath air-free water and sealed in a double-walled Plexiglas chatber with the cut end or the petiole extending from the chamber into a beaker. At first the beaker was filled with water, and transpiration and CO., evolution were measured. Then the water