The phototolerance of three chlP transgenic tobacco (Nicotiana tabacum) lines, affected in geranylgeranyl reductase and, hence, deficient in tocopherols (vitamin E), was estimated by in vivo luminescence and fluorescence measurements and was compared with that of the wild type (WT). Exposure of leaf discs to high light (1 mmol photon m(-2) s(-1)) and low temperature (10 degrees C) led to a rapid inhibition of photosystem II (PSII) photochemistry that showed little dependence on the tocopherol level. PSII photo-inhibition was followed by lipid peroxidation with a time delay of about 4 h, and this phenomenon was exacerbated in the tocopherol-deficient leaves. A linear correlation was observed in these short-term experiments between resistance to photooxidation and tocopherol content. When whole plants were exposed to the same treatment, PSII was severely photo-inhibited in mature leaves of all genotypes. Lipid peroxidation was also observed in all plants, but it occurred much more rapidly in tocopherol-deficient transgenic plants relative to WT plants. The time at which extensive lipid peroxidation occurred was correlated with the tocopherol content of the leaves. The present results show that tocopherols protect thylakoid membranes against photodestruction through lipid peroxidation. However, tocopherol deficiency was compensated in young, developing leaves that were able to photo-acclimate in the long term and did not suffer from photooxidative damage. Soluble antioxidants (glutathione and ascorbate) did not accumulate in photo-acclimated chlP transgenic leaves relative to WT leaves. In contrast, a selective accumulation of xanthophyll cycle pigments was observed in young transgenic leaves, and this could represent a compensatory mechanism for tocopherol deficiency.