Effects of sodium and magnesium cations on the “dark-” and light-induced chlorophyll a fluorescence yields in sucrose-washed spinach chloroplasts

Abstract

The effects of Na + and Mg 2+ on the “dark” level (O level) and light-induced (P level) fluorescence in sucrose-washed spinach chloroplasts were studied. Low concentrations of NaCl (2–10 mM) cause a significant decrease in both the O and P levels in the chlorophyll fluorescence transient. The effect on the O level may reflect changes in the bulk chlorophyll a. At 77 °K NaCl increases the F735 F685 emission peak ratio in dark-adapted and preilluminated chloroplasts, but has no significant effect on this ratio in sucrose-washed Photosystem II particles. This evidence is consistent with a sodium-induced excitation-energy distribution in favor of Photosystem I. In the presence of MgCl 2, with or without NaCl, there is a slight decrease in the O and P level fluorescence as compared with the salt-free control, but an increase as compared with the NaCl-treated sample. Magnesium appears to override the sodium-induced changes. At low temperatures in chloroplasts and Photosystem II particles, MgCl 2 has different effects on the F735 F685 ratio apparently depending on the state of the membrane. Magnesium, however, always induces an increase in the F695 F685 ratio. These results suggest that magnesium may influence Photosystem II reaction centers as well as energy distribution between the two photosystems.