Photosynthetic light utilization and xanthophyll cycle activity in the galactolipid deficient dgd1 mutant of Arabidopsis thaliana

Photochemical and nonphotochemical light utilization was studied in the digalactosyl diacylglycerol-deficient dgd1 mutant of Arabidopsis thaliana. While the contents of the photosynthetic metabolites and carbohydrates 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 electron transport under non-saturating photon flux densities in the dgd1 mutant was fully accounted for by an increase in nonradiative energy dissipation, measured as nonphotochemical fluorescence quenching (qN). Furthermore, the mutant showed a decreased amplitude of the fast relaxing component of qN considered to reflect high-energy state quenching, but displayed an increased slowly relaxing component of qN ascribed to photoinhibition. The slowly relaxing qN component was correlated to persisting amounts of antheraxanthin and zeaxanthin, still present even after prolonged dark exposure of previously illuminated leaves. Violaxanthin deepoxidation was found to be accelerated in the dgd1 mutant, but the proportion of violaxanthin that becomes deepoxidized was similar to that of the wild type. Our data suggest that xanthophyll-cycle operation is involved in the reduction of the photosystem II quantum yield in the dgd1 mutant. The results are discussed in terms of the altered thylakoid membrane organization of the dgd1 mutant.