A quantitative analysis of the chlorophyll fluorescence induction in terms of electron transfer rates at donor and acceptor sides of photosystem II

Abstract

The light-induced chlorophyll fluorescence increase in dark-adapted leaves and chloroplasts upon a 1s light pulse shows the following characteristics: i) the half time of the initial fluorescence increase is not inversely related to the light intensity at excitation rates above 5*103 s-1; ii) the sigmoidal fluorescence rise at excitation rates of the order of 103 s-1 converts into an exponential rise upon an increase in the range above 104-105 s-1, and iii) the rate constant of the fluorescence rise becomes invariable at high excitation rates and modulates with the redox state of the oxygen evolving complex (OEC). Numerical solutions of differential equations for the double hit requiring closure of reaction centers (RC) are derived. Experimental induction curves can be resolved in terms of the rate constants of the oxidation of the four OEC redox states, the rate constants of QA- oxidation by QB and QB-, the excitation rate and the S0/S1 heterogeneity constant of the system. Analysis of the induction curve offers a supreme tool for localizing and quantifying the effect of stress conditions (UV, drought, heat, and inhibitors) on photosynthetic performance. Current classification of RC¿s differing in fluorescence properties will require renewed attention and analysis