Dynamic adjustments in the functional absorption cross section of PSII in higher plants

Abstract

The functional absorption cross section of PSII (sPSII) describes the efficiency of light utilization in PSII, and allows one to calculate the rate of electron transport in PSII: JPSII = E sPSII qP (where E is the background spectral irradiance and qP is photochemical quenching). Using a high frequency pulsed fluorescence technique, we followed the kinetic behavior sPSII for higher plants in real time. We observed three components: (1) The fast component in ms-ms time scale originates from the PSII. This sPSII changes has an oscillatory pattern with a minimum for S2-(QAQB)- states and a maximum under S3-(QAQB)2- states. The observed oscillations in sPSII are triphasic, resulting from the superimposition of a two electron gate (with a binary phase function) on a period four oscillation. The maximum changes in sPSII for dark adapted leaves from this fast component are about 25% of the total. (2) An intermediate component in ms-s time scale arising from up-regulation at near infrared illumination related to the processes of oxidation PSII electron acceptors by PSI and the generation of D f and/or DpH gradients. This energy-dependent quenching, qE, accounts for about 14-20% of the total change in the sPSII. (3) A slow component (s-min time scale) was induced under high actinic irradiance and manifested by up to a 50% decrease in sPSII (down-regulation). The slow component is a due to the xanthophyll cycle and is correlated with a reflectance at 485 nm (corresponding to zeaxanthin accumulation). Vacuum infiltration of DTT (inhibitor for de-epoxidase) or 2,4-Dinitrophenol (DpH decoupler) into leaves completely blocked the slow component. These results are the first to reveal how dynamic changes in PSII cross sections faciliate variations in linear electron transport in higher plants.