MECHANISM OF NONPHOTOCHEMICAL CHLOROPHYLL FLUORESCENCE QUENCHING .2. RESOLUTION OF RAPIDLY REVERSIBLE ABSORBENCY CHANGES AT 530 NM AND FLUORESCENCE QUENCHING BY THE EFFECTS OF ANTIMYCIN, DIBUCAINE AND CATION-EXCHANGER, A23187

The putative relationship between the light-induced absorbance increas e at 530 nm (Delta A(530)), the so-called light-scattering change, and non-photochemical chlorophyll fluorescence quenching (NPQ) was examin ed by the effect of inhibitors. Antimycin at a low concentration (350 nM) completely inhibited fluorescence quenching while only partially i nhibiting A(530). This effect was independent of the mode of thylakoid energisation and preinduction of violaxanthin de-epoxidation. Dibucai ne at 20 mu M abolished NPQ but had little effect on Delta A(530). Mor eover, the light-induced Delta A(530) Signal was present even in the a bsence of de-epoxidised xanthophylls. The cation exchanger A23187 bloc ked the development of NPQ as well as relaxed fluorescence quenching a t steady state without involving a major portion of Delta A(530). Thus , the relationship between energy-dependent A(530) changes and fluores cence quenching was non-linear under all conditions tested. The light- induced absorbance increase at 530 nm, therefore, is insufficient for NPQ. The differential effects of inhibitors are explained schematicall y, depicting three phases for NPQ: (a) formation of zeaxanthin and ant heraxanthin by the xanthophyll cycle; (b) formation of a state reflect ed by A(530) that is induced by the transthylakoid Delta pH, possibly involving aggregation of LHCII; and (c) fluorescence quenching by the combined effect of both steps and by the H+-cation exchange properties of thylakoid membranes.