Kinetics of Delta psi-dependent sucrose transport in plasma membrane vesicles from higher plant cells

The inside-out vesicles of plasma membranes were isolated from pumpkin stem cells, and the kinetics of sucrose efflux induced by the K+-diffusion pote ntial (Delta psi (D)) was studied by measuring light transmission. Two phas es differing in their rates and duration were identified in Delta psi (D)-d ependent changes of light transmission. The increase in Delta psi (D) eleva ted the rate and magnitude of the fast phase in light transmission changes but did not markedly affect the rate of the slow phase. These two phases al so differed in their sensitivity to inhibitors and to changes in sucrose co ncentration in the external medium. Measurements of Delta psi (D) during su crose transport by means of the fluorescence probe dis-C-3-(5) revealed dif ferences in the magnitude of Delta psi (D) and its stability in vesicles lo aded with sucrose and mannitol, as well as under the action of inhibitors. The two-phase dependence of sucrose efflux from vesicles on the applied dif fusion potential is discussed in the context of modern concepts on the func tioning of sucrose carriers in the membranes.