Stimulation of Delayed Light Emission by Salt Gradients and Estimation of the Relative Ionic Permeabilities of the Thylakoid Membranes

Abstract

The sudden addition of various salts to pre-illuminated ohloroplasts enhances the rate of delayed light-emission. It is argued that this stimulation of luminescence is, with some salts, due entirely to the electrical potential developed as the ions diffuse across the thylakoid membranes. By varying the magnitude of the concentration gradients and applying the Goldman voltage equation it has been possible to obtain quantitative estimates for the relative ionic permeabilities of the thylakoids. It seems that chloride permeates as much as 10 times slower than K⁺. Predictions of the effects of using mixed salt solutions and of preincubation of the chloroplasts with salt at various levels have been made and tested experimentally. From this it is apparent that the action of salts of weak acids, such as benzoic acid, although particularly effective in stimulating luminescence, cannot be satisfactorily explained on the diffusion potential hypothesis. Over all the results seem to give quantitative information on the relative ionic permeabilities of the thylakoids and supports the model that delayed light may originate from a charge transfer complex specifically orientated in this membrane system.