Abstract

In green plants and algae light energy is absorbed by a large array of light-harvesting chlorophyll (Chl) a/b proteins and by Chl a proteins associated with the reaction centers of either PS I or II. Still a great deal of uncertainty exists on the detailed organization and function of the photosynthetic apparatus and the processes relevant for the first steps in photosynthesis after absorption of a photon. During the last few years agreement has been reached between different laboratories as to the general features of the decay kinetics in chloroplasts and green algae (1-3). At least three decay components have been required whose amplitudes and also lifetimes respond differently to the redox-state of PS II. A fast decay component of 80 - 150 ps lifetime has been ascribed to open PS II centers (1-3). An intermediate component of several hundred ps was attributed mainly to LHC II (1,3,4) or alternatively to PS II in a different redox state (2). A long-lived decay component of 1 - 2 ns was recognized as being related to the amount of closed PS II centers. Its amplitude is zero at fully open centers (3) (F_o-state) and maximal when all PS II centers are closed either by light (1, 4) or inhibitors of PS II (3). It was concluded that this component almost exclusively responsible for the variable fluorescence F_var observed upon closing PS II centers (1,3,4). These basic features are now fairly well established. Many questions remain to be solved, however. Measuring the time-resolved emission and excitation spectra of the various decay components should provide a powerful method to elucidate the characteristics of of the emitting pigments and the excitation or absorption spectra of the connected antennae.

© 1984 Optical Society of America