Changes in xanthophyll-cycle components and in fluorescence yield in leaves of a crassulacean-acid-metabolism plant, Clusia rosea Jacq., throughout a 12-hour photoperiod of constant irradiance.

The content of the xanthophylls violaxanthin, antheraxanthin and zeaxanthin, which are interconvertible in the so-called xanthophyll cycle in the chloroplasts, was determined in leaves of the crassulacean-acid-metabolism plant, Clusia rosea, at the end of a 12-h dark period and at three times during a 12-h photoperiod of constant illumination (200 μmol photons·m(-2)·s(-1)). Whereas the level of violaxanthin was high at the end of the dark period and that of zeaxanthin low, zeaxanthin increased and violaxanthin decreased during the early light period, when atmospheric CO2 was taken up rapidly by the leaves. In the middle of the light period, when net uptake of atmospheric CO2 was not observed, yet when decarboxylation of malic acid allowed for high rates of CO2 reduction in the chloroplasts, zeaxanthin was low and violaxanthin was high. At the end of the light period, when uptake of atmospheric CO2 re-occurred, zeaxanthin increased again and violaxanthin decreased. Measurements of chlorophyll-a fluorescence with a modulated fluorometer showed that the increases in the level of zeaxanthin during the early and late light period were paralleled by decreased rates of electron transport and by increased rates of nonradiative dissipation of excitation energy.