Loading of nitrate into the xylem:: apoplastic nitrate controls the voltage dependence of X-QUAC, the main anion conductance in xylem-parenchyma cells of barley roots

We report here that NO3 (-) in the xylem exerts positive feedback on its loading into the xylem through a change in the voltage dependence of the Qu ickly Activating A nion C onductance, X-QUAC. Properties of this conductance were investigated on xylem-parenchyma protoplasts prepared from roots of Hordeum vulgare by applying the patch-clamp technique. Chord conductances were minimal around - 40 mV and increased with plasma membrane depolarisation as well as with hyperpolarisation. Two gates with opposite voltage dependences were postulated. When 30 mm Cl- in the bath was replaced by NO3 (-) , a shift in the midpoint potential of the depolarisation-activated gate by about - 60 mV from 43 to - 16 mV occurred (K-m = 3.4 mm). No such effect was seen when chloride was replaced by malate. Addition of 10 mm NO3 (-) to the pipette solution and reduction of [Cl- ] from 124 to 4 mm (to simulate cytoplasmic concentrations) did not interfere with the voltage dependence of X-QUAC activation, nor was it affected by changes in external [K+ ]. If only the NO3 (-) effect on gating was considered, an increase of the NO3 (-) concentration in the xylem sap to 5 mm would result in an enhancement of NO3 (-) efflux by about 30%. Although the driving force for NO3 (-) efflux would be reduced simultaneously, NO3 (-) efflux into the xylem through X-QUAC would be maintained with high NO3 (-) concentrations in the xylem sap; a situation which occurs for instance during the night.