XANTHOPHYLL CYCLE AND THERMAL-ENERGY DISSIPATION IN PHOTOSYSTEM-II - RELATIONSHIP BETWEEN ZEAXANTHIN FORMATION, ENERGY-DEPENDENT FLUORESCENCE QUENCHING AND PHOTOINHIBITION

The influence of zeaxanthin formation on energy-dependent (qE) and pho toinhibitory fluorescence quenching (decrease in Fv/Fm ratio) was stud ied with isolated thylakoids of spinach (Spinacia oleracea L.) suspend ed in a medium of pH 7.6. It was found that in the absence of any qE ( due to addition of uncoupler) the degree of photoinhibition and maximu m fluorescence yield were unaffected by the presence of zeaxanthin. Th ylakoid samples with and without added methylviologen both exhibited t hese effects. In the absence of any artificial electron acceptor, subs tantial qE occurred only in the presence of zeaxanthin. When methylvio logen was added, which increased electron flow and pH gradient, substa ntial zeaxanthin-independent qE was exhibited. This qE was significant ly stimulated (i.e. nearly doubled) by formation of zeaxanthin. The mi nimum luminal pH required for occurrence of zeaxanthin-independent qE was estimated as about 4.6, which was attained in the presence of meth ylviologen only. Antheraxanthin formation did not correlate with zeaxa nthin-independent qE. Dithiothreitol, an inhibitor of violaxanthin de- epoxidase, stimulated photoinhibition even in the absence of zeaxanthi n, indicating a secondary effect on photoinhibition that is not relate d to its influence on zeaxanthin formation. Photoinhibition was decrea sed when qE had been formed, showing the protective function of the qE process. Under all applied conditions photoinhibition was increased b y the presence of methylviologen compared with its absence. The hypoth esis is advanced that zeaxanthin acts as photoprotector only in the en ergized (coupled) thylakoid system via stimulation of the qE process a nd is not a quencher of excessive energy per re.