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 photoinhibitory fluorescence quenching (decrease in Fv/Fm ratio) was studied with isolated thylakoids of spinach (Spinacia oleracea L.) suspended 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 maximum fluorescence yield were unaffected by the presence of zeaxanthin. Thylakoid samples with and without added methylviologen both exhibited these effects. In the absence of any artificial electron acceptor, substantial qE occurred only in the presence of zeaxanthin. When methylviologen was added, which increased electron flow and pH gradient, substantial zeaxanthin-independent qE was exhibited. This qE was significantly stimulated (i.e. nearly doubled) by formation of zeaxanthin. The minimum luminal pH required for occurrence of zeaxanthin-independent qE was estimated as about 4.6, which was attained in the presence of methylviologen only. Antheraxanthin formation did not correlate with zeaxanthin-independent qE. Dithiothreitol, an inhibitor of violaxanthin de-epoxidase, stimulated photoinhibition even in the absence of zeaxanthin, indicating a secondary effect on photoinhibition that is not related to its influence on zeaxanthin formation. Photoinhibition was decreased when qE had been formed, showing the protective function of the qE process. Under all applied conditions photoinhibition was increased by the presence of methylviologen compared with its absence. The hypothesis is advanced that zeaxanthin acts as photoprotector only in the energized (coupled) thylakoid system via stimulation of the qE process and is not a quencher of excessive energy per re.