Photosynthetic regulation by cations in spinach chloroplasts.

(1) In the presence of > or =3 mM divalent cations or > or =100 mM monovalent cations, weak 650-nm light, presumably containing an excess of Photosystem II quanta, caused substantial reduction of the electron carrier pool between the two photoacts. In low-salt media, most electron carriers were oxidized during red light. (2) In the presence of low concentrations of carbonylcyanide m-chlorophenylhydrazone (CCCP) (0.3 microM) or antimycin A (40 microM) the pool remained oxidized even in the presence of adequate cation concentrations. (3) The non-linear dependence of the O2-evolution rate upon the concentration of active Photosystem II trapping centers, which implies energy transfer between Photosystem II units, was observed only in the presence of sufficient cation. (4) Enhancement of the rate of methyl viologen reduction in far-red light by red light was found to be a weak function of Mg2+ concentration. (5) With ferredoxin-NADP+ as acceptor, enhancement of the rate of O2 evolution in 650-nm light by far-red light was obtained only with added Mg2+. Furthermore, Mg2+ addition markedly stimulated the quantum yields of NADP+ reduction, independent of wavelength, with either water or ascorbate-reduced 2,6-dichlorophenolindophenol (DCIP) as the electron donor. A dual effect of cations in the ferredoxin-NADP+ system is suggested. (6) With methyl viologen or potassium ferricyanide as acceptor and water as donor, Mg2+ did not affect the quantum yield in 650-nm light and slightly increased the yield at longer wavelengths. The quantum yield of the ascorbate-reduced DCIP-methyl viologen (Photosystem I) reaction was slightly decreased in 650-nm light and unaffected at long wavelengths. Our interpretation is that cation concentration affects the pigment distribution between Photosystems I and II.