THE RELATIONSHIP BETWEEN ZEAXANTHIN, ENERGY-DEPENDENT QUENCHING OF CHLOROPHYLL FLUORESCENCE, AND TRANS-THYLAKOID PH GRADIENT IN ISOLATED-CHLOROPLASTS

The quantitative relationship between energy-dependent quenching of chlorophyll fluorescence (q(E)) and trans-thylakoid pH difference DELTA-pH, estimated with 9-aminoacridine) was compared in chloroplasts from leaves preilluminated at low oxygen in the absence of CO2 with chloroplasts from leaves darkened under the same conditions. The extent of both phenomena was varied by changes in actinic light intensity. Chloroplasts from preilluminated leaves contained high levels of zeaxanthin (up to 15% of total carotenoid) and were capable of forming q(E) at lower DELTA-pH values than chloroplasts from dark-adapted leaves, which lacked zeaxanthin. Infusion of dithiothreitol into leaves prior to preillumination prevented the light-induced formation of zeaxanthin; chloroplasts prepared from these leaves showed the same relationship between q(E) and DELTA-pH observed in chloroplasts from dark-adapted leaves. The rate of appearance and disappearance of the change in relationship between q(E) and DELTA-pH upon leaf preillumination and darkening was closely matched by the kinetics of synthesis and degradation of zeaxanthin. The quantitative relationship between q(E) and quenching of the dark-level of fluorescence (F0) was very similar in chloroplasts from preilluminated or dark-adapted leaves, containing disparate amounts of zeaxanthin. In both sets of chloroplasts, q(E) was inhibited by antimycin A. These latter observations suggest that the formation of q(E) involves mechanistically similar features in chloroplasts containing or lacking zeaxanthin. Overall, the data are discussed in terms of action of zeaxanthin as a 'quenching amplifier', functioning physiologically to allow q(E) formation in the absence of high DELTA-pH potentially prohibitive to high rates of CO2 fixation.