Nonphotochemical quenching of chlorophyll fluorescence in higher plant leaves studied by delayed fluorescence decay measurements

Delayed chlorophyll fluorescence decay measurements on the second time scale were applied to investigate the state of photosystem II under different photosynthetic conditions. Leaves adapted to high and low light intensities were used to study the effects of nonphotochemical quenching (energy quenching) on the photosynthetic state. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated leaves were used to characterize the photosynthetic state in the absence of a transthylakoid Delta pH, dithiothreitol (DTT)-treated leaves in the absence of the xanthophyll zeaxanthin. The fast decay components were the most affected by energy quenching as indicated by increased decay times. The slowest decay component was hardly affected, neither in amplitude nor in decay time. The measurements indicate a relaxation of energy quenching on the second time scale and the absence of damages in the electron transfer chain of PS II. The constant decay times of the DTT-treated leaf, comparable to those of the DCMU-treated leaf, indicate the obligatory role of zeaxanthin for most of the detected energy quenching.