ESTIMATION OF ENERGY DISTRIBUTION AND REDISTRIBUTION AMONG TWO PHOTOSYSTEMS USING PARALLEL MEASUREMENTS OF FLUORESCENCE LIFETIMES AND TRANSIENTS AT 77 K

Abstract— A single-sample method for estimating energy distribution and redistribution among the two photosystems using fluorescence lifetimes and transients at 77 K is presented. In this method,α(the fraction of photons absorbed by photosystem I, PSI) is F_1(α)/(F_1(α)+ (τ_{F 1(M)}/τ_{F 2(M)}).F_2(M)) where, F_1(α) is the fluorescence intensity from PSI excited by photons initially absorbed by the latter, τ_{F 1(M)} and τ_{F 2(M)} are the maximum lifetimes of fluorescence from chlorophyll- a in PSI (1) and II (2), and, F_2(M) is the maximum fluorescence intensity from PSII (P level). Analysis of the intensities and lifetimes of wavelength resolved fluorescence of thylakoids (pH 7.0), with and without cations, leads to the following conclusions: The addition of 10 m M Na⁺ to cation-depleted thylakoids (pH 7.0) increases α by ˜ 10%, while the subsequent addition of 10 m M Mg²⁺ leads to three principal concomitant changes (in the order of importance): a 50% decrease in PSII to PSI energy transfer, a 20% increase in other radiation-less losses, and a 10% decrease in α.