INHIBITION OF PHOTOSYSTEM-II OF SPINACH BY LICHEN-DERIVED DEPSIDES

Barbatic acid, a lichen-derived depside, inhibited oxygen evolution in spinach thylakoid membranes. It also affected parameters of Chi fluor escence, (Fm'-F)/F and Fv/Fm. Using specific donors and acceptors of e lectrons, we found two sites of inhibition in the PS II complex. The p rimary site, which is responsible for the inhibition of oxygen evoluti on, is at the reducing side of Q(A), possibly at Q(B). The other site is at the oxidizing side of P680 but not in the oxygen evolving comple x, suggesting Yz as the target. At both sites, irreversible binding of the depside to the targets seems to be responsible for the inhibition s. Among the 8 lichen acids compared, barbatic acid was the most poten t inhibitor for both the reducing site and oxidizing site.