PHOTOSYNTHESIS, CHLOROPHYLL FLUORESCENCE, LIGHT-HARVESTING SYSTEM ANDPHOTOINHIBITION RESISTANCE OF A ZEAXANTHIN-ACCUMULATING MUTANT OF ARABIDOPSIS-THALIANA

The abscisic-acid-deficient aba-1 mutant of Arabidopsis thaliana is un able to epoxidize zeaxanthin. As a consequence, it contains large amou nts of this carotenoid and lacks epoxy-xanthophylls, HPLC analysis of pigment contents in leaves, isolated thylakoids and preparations of th e major light-harvesting complex of photosystem II (PSII) (LHC-II) ind icated that zeaxanthin replaced neoxanthin, violaxanthin and antheraxa nthin in the light-harvesting system of PSII in aba-1. Non-denaturing electrophoretic fractionation of solubilized thylakoids showed that th e xanthophyll imbalance in aba-1 was associated with a pronounced decr ease in trimeric LHC-II in favour of monomeric complexes, with a subst antial increase in free pigments (mainly zeaxanthin and chlorophyll b) , suggesting a decreased stability of LHC-II. The reduced thermostabil ity of PSII in aba-1 was also deduced from in vivo chlorophyll fluores cence measurements. Wild-type and aba-1 leaves could not be distinguis hed on the basis of their photosynthetic performance: no significant d ifference was observed between the two types of leaves for light-limit ed and light-saturated photosynthetic oxygen evolution, PSII photochem istry and PSII to PSI electron how. When dark-adapted leaves (grown in white light of 80 mu mol m(-2) s(-1)) were suddenly exposed to red li ght of 150 mu mol m(-2) s(-1), there was a strong nonphotochemical que nching of chlorophyll fluorescence, the amplitude of which was virtual ly 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 vio laxanthin in the wild type. A. high concentration of zeaxanthin in aba -1 thylakoids did not, in itself, provide any particular protection ag ainst the photoinhibition of PSII. Taken together, the presented resul ts indicate the following: (1) zeaxanthin can replace epoxy-xanthophyl ls in LHC-II without significantly affecting the photochemical efficie ncy of PSII; (2) zeaxanthin does not play any specific role in direct (thermal) energy dissipation in PSII; (3) the photoprotective action o f the xanthophyll cycle (rapid photoconversion of violaxanthin to zeax anthin) is not based on the mere substitution of violaxanthin by zeaxa nthin in the chlorophyll antennae.