Differential effects of turgor on sucrose and potassium transport at the tonoplast and plasma membrane of sugar beet storage root tissue

When turgor was increased, by decreasing the concentration of mannitol bathing discs of sugar beet storage root tissue, the rates of sucrose and potassium uptake into the vacuole were decreased, At all external mannitol concentrations the rate of sucrose and potassium uptake across the plasma membrane was an order of magnitude greater than the rate of quasi-steady uptake into the vacuole, implying a very large efflux. Efflux of both sucrose and potassium was increased at high turgor, However, while increasing turgor decreased the rate of K+ uptake, the rate of sucrose uptake at the plasma membrane increased with time, Compartmental analysis of tracer exchange kinetics was used to determine unidirectional Kf fluxes, From these results, it was estimated that the increase in Kf efflux accompanying a 1.5 MPa increase in turgor could lead to a net increase of 140 mol(-3) h(-1) in the external potassium concentration, It is suggested that the turgor-imposed increase in solute efflux is a means of regulating intracellular osmotic pressure and/or turgor in sugar beet storage roots, but that sucrose is preferentially retrieved from the apoplast, even under conditions of excessively high turgor, However, much of this sucrose is probably lost from the cell, implying a 'futile' sucrose transport cycle at the plasma membrane, The turgor-stimulated leak of potassium could play a major role in the regulation of turgor pressure in sugar beet storage root tissue.