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 recently inferred to be involved in the induction of high-energy-dependent chlorophyll (Chl) fluorescence quenching (qE) in leaves (Lokstein et al., J. Photochem. Photobiol. B: Biol, 19 (1993) 217-225). The present study was performed to further elucidate this finding and, in particular to analyze the role of the xanthophyll cycle for qE formation. Measurements of modulated Chl fluorescence using the saturating flash technique and of xanthophyll cycle activity were performed in barley leaves of wild type (WT) and a Chl-b-less mutant with reduced LHC II (chlorina 3613) in the absence and presence of the violaxanthin (V) de-epoxidase inhibitor dithiothreitol (DTT). The results can be summarized as follows: (a) The content of xanthophyll cycle-constituents in mutant leaves is lower on a leaf area basis (but significantly higher when related to Chl a content); the mutant has a substantially higher ability to convert V into antheraxanthin (A) and zeaxanthin (Z) when compared with the WT. In both genotypes in response to DTT treatment only low levels 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-adapted leaves is virtually the same in WT and the mutant and remains invariant to treatment with DTT. (c) Nearly complete abolishment of the 'fast' 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 quenching relaxation provides strong evidence for qE being an antenna associated phenomenon. (d) A 'medium' phase of qN relaxation (qNm) was not influenced 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 can 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.