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

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Abstract
Summary: 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 Quickly Activating Anion Conductance, 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 (Km = 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.