Effect of butyl 2-hydroxy-3-butynoate on sunflower leaf photosynthesis and photorespiration.

Detached leaves and whole plants of sunflower were supplied with butyl 2-hydroxy-3-butynoate (BHB), a competitive inactivator of glycolate oxidase, to evaluate the possibility of inhibiting photorespiration and increasing photosynthetic efficiency. In all treatments in vivo and in vitro, BHB inhibited glycolate oxidase. With partially purified glycolate oxidase from spinach leaves, the apparent K(i) for BHB was 13.2 micromolar.Low concentrations of BHB neither decreased photorespiration nor increased net photosynthesis. At higher concentrations, either a proportional decrease in photosynthesis and photorespiration or an inhibition of net photosynthesis greater than photorespiration was observed. CO(2) evolution in BHB-treated leaves was O(2)-sensitive and was derived from recent photosynthate. BHB inhibited photosynthesis in 2, 21, or 50% O(2) but the ratio of the rates of photosynthesis in these O(2) concentrations was the same as in control leaves. BHB treatment resulted in a stimulation of dark respiration.As photosynthesis, photorespiration, and dark respiration were all affected by BHB, the action of BHB on whole leaf metabolism appears to be complex. Substantial inhibition of photorespiration was accompanied by inhibition of photosynthesis and increases in photosynthesis were not observed.