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 (Zea mays L) roots using a range of microelectrode and imaging techniques. Addition of 100mm 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 (A_K+) within ~1min after stress onset. At this time, no detectable amounts of Na⁺ were released into the xylem vessels. The observed drop in A_K+ 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 5min of root exposure to NaCl), stelar cells become more depolarized, and a gradual recovery in A_K+ occurs.