The Delivery of Salts to the Xylem. Three Types of Anion Conductance in the Plasmalemma of the Xylem Parenchyma of Roots of Barley

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Abstract
To explore possible pathways for anions to enter the xylem in the root during the transport of salts to the shoot, we used the patch-clamp method on protoplasts prepared from the xylem parenchyma of barley (Hordeum vulgare L.) plants. K+ currents were suppressed by tetraethylammonium orN-methylglucamine in the solutions in the pipette and the bath, and the permeating anions were Cl− or NO3 −. We recorded the activities of three distinct anion conductances: (a) an inwardly rectifying anion channel (X-IRAC), characterized by activation at hyperpolarization and open times of up to several seconds; (b) a quickly activating anion conductance (X-QUAC), important for anion efflux at voltages between −50 mV and the equilibrium potential of the prevailing anion; and (c) a slowly activating anion conductance (X-SLAC), activating above −100 mV. Both X-IRAC and X-QUAC were permeable for Cl− and NO3 −; X-QUAC was also permeable for malate. The occurrence of X-IRAC became more frequent with an increase in cytoplasmic Ca2+, while the occurrence of X-QUAC decreased. Anion currents through X-SLAC, and particularly through X-QUAC, were estimated to be large enough to account for reported rates of xylem loading, which is in accordance with the notion that xylem loading is a passive process.