On the mechanism of the energy-dependent quenching of atebrin fluorescence in isolated chloroplasts.

Recently, a very interesting observation relating energy generation to the quenching of the fluorescence of the uncoupler atebrin in isolated chloroplasts has been described by Kraayenhof [ 11. It was shown that the required energy could be provided by electron transport, ATP hydrolysis or a pH gradient, and it was therefore suggested that the extent of quenching may be used to measure the ‘energy state’ of the chloroplast [I, 21 . Since several pools of energy storage can be in equilibrium with the ‘energized state’ (defined here as the state necessary for ATP formation) it is not clear whether the fluorescence quenching is measuring one of these pools or the state itself. One candidate may be the H’ gradient across the membrane [3] . It was previously shown that in subchloroplast particles H’ uptake can be abolished without affecting the rate of ATP formation [4] . We, therefore, checked in this system the relationship between fluorescence quenching and H+ gradients. As will be shown atebrin was found to be distributed between the inside of the chloroplast and the solution according to the ratio of proton concentration. In subchloroplast particles the quenching of atebrin fluorescence bore no relation to the capacity of the subchloroplast particles to form ATP. The quenching of the fluorescence observed seems due to several factors, among which a screening effect of the chlorophyll molecules on the exciting light is a major factor. 2. Materials and methods