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

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 or N-methylglucamine in the solutions in the pipette and the bath, and the permeating anions were Cl(−) or NO(3)(−). 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 NO(3)(−); X-QUAC was also permeable for malate. The occurrence of X-IRAC became more frequent with an increase in cytoplasmic Ca(2+), 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.