Effects of monovalent cations on light energy distribution between two pigment systems of photosynthesis in isolated spinach chloroplasts

Abstract

The effects of monovalent cations on the light energy distribution between two pigment systems of photosynthesis were studied in isolated spinach chloroplasts by measuring chlorophyll a fluorescence and photochemical reactions. The addition of NaCl to the chloroplast suspension produced a 40–80% increase in fluorescence yield measured at 684 nm at room temperature. The fluorescence increase was completed about 5 min after the addition. The effect saturated at 100 mM NaCl. Low-temperature fluorescence spectra showed that NaCl increased the yields of two fluorescence bands of pigment system II at 684 and 695 nm but decreased that of pigment system I at 735 nm. Similar effects on chlorophyll a fluorescence at room and at low temperatures were obtained with NaBr, NaNO 3, Na 2SO 4, LiCl, KCl, RbCl, CsCl, NH 4Cl and CH 3NH 3Cl. NaCl suppressed the quantum efficiency of NADP + reduction supported by the ascorbate-2,6-dichlorophenolindophenol (DCIP) couple as an electron donor system in the presence of 3-(3′,4′-chlorophenyl)-1,1-dimethylurea (DCMU). On the other hand, NaCl only slightly enhanced the quantum yield of photoreaction II measured by the Hill reaction with DCIP. It is concluded that the monovalent cations tested suppressed the excitation transfer from pigment system II to pigment system I; the effects were the same as those of alkaline earth metals and Mn 2+ (refs. 1, 2).