EFFECTS OF INORGANIC-PHOSPHATE ON THE ESR-SPECTRA OF CHLOROPLASTS

The ESR spectra of chloroplasts were studied after illumination of frozen chloroplasts (77 K). The intensity and width of the light-induced ESR signal I depend on the contents of inorganic phosphate (P(i)). Chloroplast preparations having low contents of endogenous P(i) showed an increase in the amplitude and width (H(m) congruent-to 0.82 divided-by 0.87 mT) of the ESR signal I. Additions of P(i), the Photosystem II electron donor diphenylcarbozide and diuron (DCMU) resulted in a decrease of these parameters (DELTA-H(m) = 0.75 mT). The dependence of the ESR signal I on the microwave power in P(i)-deficient chloroplasts also differs from that in the control preparation of chloroplasts. Additions of exogenous P(i) or DCMU reversed this effect. It is suggested that the electron flow from water to P680+ is blocked in the absence of P(i); under these conditions the ESR signal from P680+ (DELTA-H(m) congruent-to 0.87 mT) is observed and superimposed on the ESR signal I from P700+ (DELTA-H(m) congruent-to 0.75 mT). Taking into account the similarities of the ESR difference spectra in chloroplasts and the well-known ESR spectra of phosphate-ion radical we have suggested that phosphate-ion may be the electron donor for P680+.