Joint action of natural and synthetic photosystem II inhibitors

The inhibitory action on photosystem II of four sorgoleone analogues, isola ted from Sorghum bicolor (L) Moench, and two synthetic inhibitors, diuron a nd bentazone, was tested by measuring oxygen evolution of thylakoid membran es. The inhibition of oxygen evolution for mixtures of inhibitors was compa red with the Additive Dose Model (ADM). ADM assumes that, at a defined resp onse level, the effect of a mixture of inhibitors can be unambiguously expr essed by the potency of either of the inhibitors applied separately. The sl ope of the logistic dose-response curves differed between the inhibitors; s orgoleone analogues had the steepest and bentazone the shallowest slope. Th e difference in slopes makes the interpretation of the isoboles less genera l and may reflect the differences in the interaction between the natural an d the synthetic inhibitors with the binding site. The results suggest that there may be some limitation to ADM, namely that compounds with the same si te of action might have different response curves if their mechanism of bin ding is different. The joint action of inhibitors follows ADM at I-50. Ther efore, the inhibitors can replace each other in any mixture ratio, based on the relative potencies of the pure inhibitors, without changing each other 's effect on oxygen evolution. The joint action at I-20 and I-80 sometimes diverged from ADM. (C) 1999 Society of Chemical Industry.