Calcium mediates root K+/Na+ homeostasis in poplar species differing in salt tolerance

Using the non-invasively ion-selective microelectrode technique, flux profiles of K+, Na+ and H+ in mature roots and apical regions, and the effects of Ca2+ on ion fluxes were investigated in salt-tolerant poplar species, Populus euphratica Oliver and salt-sensitive Populus simonii x (P. pyramidalis + Salix matsudana) (Populus popularis 35-44, P. popularis). Compared to P. popularis, P. euphratica roots exhibited a greater capacity to retain K+ after exposure to a salt shock (SS, 100 mM NaCl) and a long-term (LT) salinity (50 mM NaCl, 3 weeks). Salt shock-induced K+ efflux in the two species was markedly restricted by K+ channel blocker, tetraethylammonium chloride, but enhanced by sodium orthovanadate, the inhibitor of plasma membrane (PM) H+ -ATPase, suggesting that the K+ efflux is mediated by depolarization-activated (DA) channels, e. g., KORCs (outward rectifying K+ channels) and NSCCs (nonselective cation channels). Populus euphratica roots were more effective to exclude Na+ than P. popularis in an LT experiment, resulting from the Na+/H+ antiport across the PM. Moreover, pharmacological evidence implies that the greater ability to control K+/Na+ homeostasis in salinized P. euphratica roots is associated with the higher H+ -pumping activity, which provides an electrochemical H+ gradient for Na+/H+ exchange and simultaneously decreases the NaCl-induced depolarization of PM, thus reducing Na+ influx via NSCCs and K+ efflux through DA-KORCs and DA-NSCCs. Ca2+ application markedly limited salt-induced K+ efflux but enhanced the apparent Na+ efflux, thus enabling the two species, especially the salt-sensitive poplar, to retain K+/Na+ homeostasis in roots exposed to prolonged NaCl treatment.