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 A(0)(-). It is concluded that weak-light treatment of cucumber leaves at chilling temperature destroys F-X, F-A and F-B and possibly A(1). This gives the molecular basis for the mechanism of selective PSI photodamage that was recently reported [Sonoike and Terashima (1994) Planta 194, 287-293].