PHOTOSYNTHETIC LIGHT UTILIZATION AND XANTHOPHYLL CYCLE ACTIVITY IN THE GALACTOLIPID DEFICIENT DGD1 MUTANT OF ARABIDOPSIS-THALIANA

Photochemical and nonphotochemical light utilization was studied in th e digalactosyl diacylglycerol-deficient dgd1 mutant of Arcrbidopsis th aliana. While the contents of the photosynthetic metabolites and carbo hydrates analyzed were found to be unchanged, chlorophyll fluorescence quenching and xanthophyll-cycle activity were distinctly different in the mutant. A decrease in the quantum yield of photosystem II electro n transport under non-saturating photon flux densities in the dgd1 mut ant was fully accounted for by an increase in nonradiative energy diss ipation, measured as nonphotochemical fluorescence quenching (qN). Fur thermore, the mutant showed a decreased amplitude of the fast relaxing component of qN considered to reflect high-energy state quenching, bu t displayed an increased slowly relaxing component of qN ascribed to p hotoinhibition. The slowly relaxing qN component was correlated to per sisting amounts of antheraxanthin and zeaxanthin, still present even a fter prolonged dark exposure of previously illuminated leaves. Violaxa nthin deepoxidation was found to be accelerated in the dgd1 mutant, bu t the proportion of violaxanthin that becomes deepoxidized was similar to that of the wild type. Our data suggest that xanthophyll-cycle ope ration is involved in the reduction of the photosystem II quantum yiel d in the dgd1 mutant. The results are discussed in terms of the altere d thylakoid membrane organization of the dgd1 mutant. (C) Elsevier, Pa ris.