CHANGES IN THYLAKOID BEHAVIOR IN CHLORIDE-FREE MEDIA

The photosynthetic activity of spinach thylakoids has been monitored under light-limiting conditions with a modulated oxygen electrode. At pH 7.9, replacing chloride with sulphate in the bathing medium causes a larger inhibition of oxygen evolution if the solution change is performed in the dark, but replacing the chloride with nitrate causes an inhibition which is unaffected by the illumination conditions. At pH 6.5, replacing chloride with sulphate does not inhibit oxygen evolution but replacing chloride with nitrate does. These findings show that sulphate but not nitrate conforms to the chloride replacement mechanism of Theg and Homann [1]. Returning chloride to thylakoids which have been inhibited with nitrate restores the original phase angle and a large part of the lost oxygen evolution. Thus the electron transport chains which respond to the chloride restoration do not appear to have been structurally damaged. Thylakoids which have been inhibited with a sulphate medium do not respond to the restoration of a chloride medium. At pH 7.9, the variable fluorescence observed after dialysis against sulphate or nitrate media and in the presence of DCMU rises slowly compared to a chloride control and there is a large increase in the complementary area but no changes in F(o) or F(max). The fluorescence changes can be abolished under conditions which permit the inhibition of oxygen evolution and thus may represent a separate chloride-sensitive process.