Photodamage of the photosynthetic apparatus and its dependence on the leafdevelopmental stage in the npq1 Arabidopsis mutant deficient in the xanthophyll cycle enzyme violaxanthin de-epoxidase

The npq1 Arabidopsis mutant is deficient in the violaxanthin de-epoxidase e nzyme that converts violaxanthin to zeaxanthin in excess light (xanthophyll cycle). We have compared the behavior of mature leaves (ML) and developing leaves of the mutant and the wild type in various light environments. Ther moluminescence measurements indicated that high photon flux densities (>500 mu mol m(-2) s(-1)) promoted oxidative stress in the chloroplasts of npq1 ML, which was associated with a loss of chlorophyll and an inhibition of th e photochemical activity. Illuminating leaf discs in the presence of eosin, a generator of singlet oxygen, brought about pronounced lipid peroxidation in npq1 ML but not in wild-type leaves. No such effects were seen in young leaves (YL) of npq1, which were quite tolerant to strong light and eosin-i nduced singlet oxygen. Non-photochemical energy quenching was strongly inhi bited in npq1 YL and ML and was not improved with high-light acclimation. O ur results confirm that the xanthophyll cycle protects chloroplasts from ph otooxidation by a mechanism distinct from non-photochemical energy quenchin g and they reveal that the absence of xanthophyll cycle can be compensated by other protective mechanisms. npq1 YL were observed to accumulate conside rable amounts of vitamin E during photoacclimation, suggesting that this li pophilic antioxidant could be involved in the high phototolerance of those leaves.