Destruction of photosystem I iron-sulfur centers in leaves of Cucumis sativus L. by weak illumination at chilling temperatures

The activity of photosystem (PS) I in cucumber leaves was selectively inhibited by weak illumination at chilling temperatures with almost no loss of P-700 content and PSII activity. The sites of inactivation in the reducing side of PSI were determined by EPR and flash photolysis. Measurement by EPR showed the destruction of iron-sulfur centers, F_X, F_A and F_B,in parallel with the loss of quantum yield of electron transfer from diaminodurene to NADP⁺. Flash photolysis showed the increases in the triplet states of P-700 and antenna pigments, along with the decrease in the electron transfer from P-700 to F_A/F_B. This indicates the increase in the charge recombination between P-700⁺ and A₀^-. It is concluded that weak-light treatment of cucumber leaves at chilling temperature destroys F_X, F_A and F_B and possibly A₁. This gives the molecular basis for the mechanism of selective PSI photodamage that was recently reported [Sonoike and Terashima (1994) Planta 194, 287-293].