The abscisic-acid-deficient aba-1 mutant of Arabidopsis thaliana is unable to epoxidize zeaxanthin. As a consequence, it contains large amounts of this carotenoid and lacks epoxy-xanthophylls. HPLC analysis of pigment contents in leaves, isolated thylakoids and preparations of the major light-harvesting complex of photosystem II (PSII) (LHC-II) indicated that zeaxanthin replaced neoxanthin, violaxanthin and antheraxanthin in the light-harvesting system of PSII in aba-1. Non-denaturing electrophoretic fractionation of solubilized thylakoids showed that the xanthophyll imbalance in aba-1 was associated with a pronounced decrease in trimeric LHC-II in favour of monomeric complexes, with a substantial increase in free pigments (mainly zeaxanthin and chlorophyll b), suggesting a decreased stability of LHC-II. The reduced thermostability of PSII in aba-1 was also deduced from in vivo chlorophyll fluorescence measurements. Wild-type and aba-1 leaves could not be distinguished on the basis of their photosynthetic performance: no significant difference was observed between the two types of leaves for light-limited and light-saturated photosynthetic oxygen evolution, PSII photochemistry and PSII to PSI electron flow. When dark-adapted leaves (grown in white light of 80 mumol m-2s-1) were suddenly exposed to red light of 150 mumol m-2s-1, there was a strong nonphotochemical quenching of chlorophyll fluorescence, the amplitude of which was virtually identical (at steady state) in aba-1 and wild-type leaves, despite the fact that the xanthophyll cycle pigment pool was completely in the form of zeaxanthin in aba-1 and almost exclusively in the form of violaxanthin in the wild type. A high concentration of zeaxanthin in aba-1 thylakoids did not, in itself, provide any particular protection against the photoinhibition of PSII. Taken together, the presented results indicate the following: (1) zeaxanthin can replace epoxy-xanthophylls in LHC-II without significantly affecting the photochemical efficiency of PSII; (2) zeaxanthin does not play any specific role in direct (thermal) energy dissipation in PSII; (3) the photoprotective action of the xanthophyll cycle (rapid photoconversion of violaxanthin to zeaxanthin) is not based on the mere substitution of violaxanthin by zeaxanthin in the chlorophyll antennae.