Effects of Certain Inhibitors on Photorespiration by Wheat Leaf Segments

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Abstract
The effect on the carbon metabolism of wheat leaf segments of certain inhibitors of photorespiration was studied. Sodium 2-hydroxy-3-butynoate supplied for 40 min resulted in accumulation of 14C in glycolic acid with only a 7% inhibition of photosynthesis; when supplied for 90 min, photosynthesis was inhibited by 47%. When 14CO2 was replaced by 1000 vpm 12CO2, radioactivity in glycine decreased but increased more rapidly in sucrose with less release of 14CO2. Isonicotinyl hydrazide (INH) inhibited photosynthesis from 14CO2 by 50% and glycine replaced sucrose as the main product. When, after 15 min, 14CO2 was replaced by 150 vpm 12CO2, in the presence of INH less 14CO2 was released, 14CO in glycine decreased more slowly, and less [14CO]sucrose accumulated. Glycidate (potassium 2,3-epoxypropionate) at 2 mM had no effect on photosynthetic rate and little effect on carbon metabolism; 20 mM glycidate inhibited photosynthesis by 64% and resulted in less radioactivity in glycine, more in phosphate esters, and less 14CO2 released. When photosynthesis was measured in 1000 vpm CO2 the inhibitors gave smaller effects on metabolism than during photosynthesis from 150 vpm 14CO2 but 20 mM glycidate still resulted in a 42% inhibition of photosynthesis. When U- [14CO]glycerate was applied to leaf segments in air with 320 vpm 14CO2 the total uptake of glycerate was not changed by the inhibitors. INH and glycidate both decreased the amount of glycerate metabolised. More 14CO accumulated in glycine in the presence of INH and in phosphate esters and serine in the presence of glycidate. Hydroxybutynoate increased the production of glycolate from glycerate but did not affect the total amount of glycerate metabolised. Although all three inhibitors affected photorespiratory metabolism none stimulated photosynthesis. The results are consistent with the main release of CO2 in photorespiration arising from the conversion of glycine to serine.

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