Glycine as a substrate for photorespiration1

Substrates for photorespiration were examined by feeding 14C labeled compounds to tobacco and corn leaf segments and by measuring 14CO2 evolution in light and darkness. CO2 release in the dark was rapid, but in light CO2 release was slow due to refixation by photosynthesis. Carboxyl labeled glycine was more rapidly decarboxylated than were glyoxylate, glycolate or serine. Hydroxypyridinemethane sulfonate, an inhibitor of glycolate oxidase, blocked CO2 release from glycolate but not from glycine. Isonicotynyl hydrazide blocked CO2 release from both glycine and glycolate. DCMU blocked photosynthetic refixation of the released CO2, consequently the rates of CO2 release in light and dark were about equal. It was concluded that CO2 release during photo-respiration came from the conversion of 2 molecules of glycine to one serine and one CO2. 14CO2 release from glycine-l-14C in the dark or with DCMU in light can be used as an assay for photorespiration ability. CO2 release from glycine and glycolate by corn leaf segments in the dark proceeded at ⅔ the rate of that in normal tobacco leaf. This result, together with other work on O2 exchange and enzymatic analysis, indicates that corn and other plants do carry on photorespiration, but it is not manifested by CO2 release in light. A yellow tobacco mutant, Consolation 402, had high rates of photorespiration by the 14CO2 assay, nearly half (or more) as many peroxisomes as chloroplasts, and high rates of CO2 release from glycine-l-14C or glycolate-l-14C. A common tobacco, Bright Yellow, had lower rates of photorespiration, fewer visible peroxisomes, and slower decarboxylation of glycine and glycolate. The amount of 14CO2 release from glycine-l-14C or glycolate-l-14C increased only slightly when the temperature was raised from 25 to 35°C.