THE ROLE OF LIGHT-HARVESTING COMPLEX-II IN EXCESS EXCITATION-ENERGY DISSIPATION - AN IN-VIVO FLUORESCENCE STUDY ON THE ORIGIN OF HIGH-ENERGY QUENCHING

A functionally intact light-harvesting complex II (LHC II) was recentl y inferred to be involved in the induction of high-energy-dependent ch lorophyll (Chl) fluorescence quenching (qE) in leaves (Lokstein et al. , J. Photochem. Photobiol. B: Biol, 19 (1993) 217-225). The present st udy was performed to further elucidate this finding and, in particular to analyze the role of the xanthophyll cycle for qE formation. Measur ements of modulated Chl fluorescence using the saturating flash techni que and of xanthophyll cycle activity were performed in barley leaves of wild type (WT) and a Chl-b-less mutant with reduced LHC II (chlorin a 3613) in the absence and presence of the violaxanthin (V) de-epoxida se inhibitor dithiothreitol (DTT). The results can be summarized as fo llows: (a) The content of xanthophyll cycle-constituents in mutant lea ves is lower on a leaf area basis (but significantly higher when relat ed to Chl a content); the mutant has a substantially higher ability to convert V into antheraxanthin (A) and zeaxanthin (Z) when compared wi th the WT. In both genotypes in response to DTT treatment only low lev els of A accumulated and Z formation was almost completely suppressed. (b) The photochemical efficiency of photosystem (PS) II expressed as the ratio of variable to maximum fluorescence, F-V/F-M, in dark-adapte d leaves is virtually the same in WT and the mutant and remains invari ant to treatment with DTT. (c) Nearly complete abolishment of the 'fas t' relaxation phase of non-photochemical quenching (qNf) was observed after DTT treatment. Suppression of qNf (as an approximate measure of qE) accompanied by the disappearance of a 'rapid' phase of F-O quenchi ng relaxation provides strong evidence for qE being an antenna associa ted phenomenon. (d) A 'medium' phase of qN relaxation (qNm) was not in fluenced by DTT treatment at all light intensities investigated. Thus, the results do not indicate a significant contribution of persisting xanthophyll-dependent (A+Z) quenching to qNm. The phenomenon of qNm ca n not be explained by chloroplast movements. The data obtained support the hypothesis that Delta pH-mediated synergistic operation of both, structural rearrangement of LHC II and xanthophyll-cycle activity, is required for efficient qE formation in vivo.