Low Temperature Secondary Pathway Donation in Photosystem II of Spinach

Abstract

At low temperature (5 K) light-induced charge separation in photosystem II (PS II) is followed by charge stabilization due to reduction of P680⁺ by secondary donors such as carotenoid (Car) or chlorophyll (Chl) that compete with P680⁺ Q_A ⁻ charge recombination.

In order to study charge stabilization in photosystem II at low temperature we utilized a newly developed CCD-based absorption spectrometer providing minimal actinic fluence and reasonable time (100 ms) and spectral (0.5 nm) resolution while covering a large spectral window from 470 to 1,100 nm. This enabled us to follow the electron transfer reactions in photosystem II by monitoring the specific signals for Q_A ⁻, Chl⁺ and Car⁺ simultaneously in the wavelength and time domain. We show that, as a result of inducing multiple turnovers in photosystem II core complexes from spinach, during the course of illumination with continuous green light the relative amounts of the donors Car⁺ and Chl⁺ depend on the number of photons absorbed. In a prolonged illumination Car⁺ is largely replaced by more stable donors.