Non-electrogenic charge recombination in Photosystem II as a source of sub-millisecond luminescence

Abstract

The luminescence emitted by Photosystem II at times between 1 μs and 1 ms after flash illumination was studied. Membrane potentials of up to 1 V, generated in osmotically swollen chloroplasts by externally applied electrical field pulses, had no effect on the decay components of this luminescence. Instead the field induced an additional, much slower luminescence due to a stimulated recombination of the primary charge separation in Photosystem II centers. The field-insensitive luminescence, in contrast to the field-induced luminescence, was independent of the redox state of the primary acceptor Q, and appeared not to originate from the same Photosystem II centers. The decay kinetics consisted mainly of two phases with 10 and 60 μs halftimes. The initial amplitude could be restored at all times by a second flash, indicating that both phases are due to a reversal of the field-insensitive charge separation. In the presence of DCMU and hydroxylamine, no luminescence was observed between 10 μs and 1 ms after the flash. Presumably the reoxidation of the electron acceptor was significantly slowed down, but it was still much faster than that of Q −.