IS ZEAXANTHIN CAPABLE OF ENERGY-TRANSFER TO CHLOROPHYLL-A IN PARTIALLY GREENED LEAVES - A STUDY OF FLUORESCENCE EXCITATION-SPECTRA DURING VIOLAXANTHIN DEEPOXIDATION

Absorbance spectra and excitation spectra of chlorophyll a fluorescence were recorded during the light-induced deepoxidation of violaxanthin to zeaxanthin in bean leaves (Phaseolus coccineus) greened under intermittent light. Light minus dark fluorescence excitation difference spectra showed distinct minima at 440, 465, and 500 nm corresponding to maxima in the absorbance difference spectra. Both difference spectra were prevented by the deepoxidase inhibitor dithiothreitol and were inverted when zeaxanthin was epoxidized. The fluorescence excitation difference spectra were successfully modeled by considering the absorbance differences between violaxanthin and zeaxanthin, assuming no energy transfer from the two pigments to chlorophyll a, and accounting for light-induced scattering changes. The pigment stoichiometry and the scattering changes of the simulation were in accordance with experimental data. The results indicate that, in the early stage of leaf development, light absorbed by the cycle pigments violaxanthin and zeaxanthin is not transferred to chlorophyll.