AN IMPROVED METHOD, USING SATURATING LIGHT-PULSES, FOR THE DETERMINATION OF PHOTOSYSTEM-I QUANTUM YIELD VIA P700+-ABSORBENCY CHANGES AT 830 NM

An improved method is introduced for the determination of the quantum yield of photosystem I. The new method employs saturating light pulses with steep rise characteristics to distinguish, in a given physiological state, centers with an open acceptor side from centers with a reduced acceptor side. The latter do not contribute to PSI quantum yield (PHI(I)). Oxidation of P700 is measured by a rapid modulation technique using the absorbance change around 830 nm. The quantum yield PHI(I), is calculated from the amplitude of the rapid phase of absorbance change (DELTAA; 830 nm) upon application of a saturation pulse in a given state, divided by the maximal DELTAA (830 nm) which is induced by a saturation pulse with far-red background illumination. Using this technique, PHI(I), can be determined even under conditions of acceptor-side limitation, as for example in the course of a dark-light induction period or after elimination of CO2 from the gas stream. Thus determined PHI(I) values display a close-to-linear relationship with those for the quantum yield of PSII (PHI(II)) calculated from chlorophyll fluorescence parameters. It is concluded that the proposed method may provide new information on the activity of the PSI acceptor side and thus help to separate the effects of acceptor-side limitation from those of cyclic PSI, whenever a nonlinear relationship,between PHI(II), and the P700-reduction level is observed.