PHOTOACOUSTIC STUDY OF CHANGES IN THE ENERGY-STORAGE OF PHOTOSYSTEM-I AND PHOTOSYSTEM-II DURING STATE-1-STATE-2 TRANSITIONS

Using photoacoustic spectroscopy, state 1-state 2 transitions were demonstrated in vivo in intact sugar maple leaves (Acer saccharum Marsh.) by following the changes in energy storage of photosystems (PS) I and II. Energy storage measured with 650 nm modulated light (light 2) in the presence of background white light indicated the total energy stored by both photosystems (ES(T)), and in the presence of background far-red light showed the energy stored by PSI (ES(PSII). The difference between ES(T) and ES(PSI) gave the energy stored by PSII (ES(PSII). While ES(T) remained nearly constant during state transitions, both ES(PSI) and ES(PSII) changed considerably. The ratio of ES(PSII) to ES(PSI), an indicator of the energy distribution between the two photosystems, decreased or increased during transition to state 2 or state 1, respectively. State transitions were completed in about 20 min and were fully reversible. During transition from state 1 to state 2, the fraction of excitation energy gained by PSI was nearly equal to that lost by PSII. This fraction of excitation energy transferred from PSII to PSI accounted for about 5% of the absorbed light (fluorescence is not considered), 19% of ES(T), 34% of ES(PSII), and 43% of ES(PSI) in state 2. NaF treatment inhibited the transition to state 1. Data in the present study confirm the concept of changes in absorption cross-section of photosystems during state transitions.