Sequential depolarization of root cortical and stelar cells induced by an acute salt shock - implications for Na+ and K+ transport into xylem vessels

Early events in NaCl-induced root ion and water transport were investigated in maize (Zeamays L) roots using a range of microelectrode and imaging techniques. Addition of 100 mM NaCl to the bath resulted in an exponential drop in root xylem pressure, rapid depolarization of trans-root potential and a transient drop in xylem K+ activity (AK+) within ~1 min after stress onset. At this time, no detectable amounts of Na+ were released into the xylem vessels. The observed drop in AK+ was unexpected, given the fact that application of the physiologically relevant concentrations of Na+ to isolated stele has caused rapid plasma membrane depolarization and a subsequent K+ efflux from the stelar tissues. This controversy was explained by the difference in kinetics of NaCl-induced depolarization between cortical and stelar cells. As root cortical cells are first to be depolarized and lose K+ to the environment, this is associated with some K+ shift from the stelar symplast to the cortex, resulting in K+ being transiently removed from the xylem. Once Na+ is loaded into the xylem (between 1 and 5 min of root exposure to NaCl), stelar cells become more depolarized, and a gradual recovery in AK+ occurs.