Introduction

Soil salinity, a major abiotic stress under the excess of sodium chloride (Na+Cl−), affects ∼20% of the world’s 230 million irrigated agricultural lands (see Munns and Tester, 2008). High Na+ concentrations (>40 mM) have detrimental effects on crop growth mainly due to hyperosmotic stress (water deficit under strongly negative water potential), excessive absorption and ion imbalance (see Munns and Tester, 2008), particularly in semi-arid and arid regions where Na+ is up to 50–100 mM in soil. Under salt stress, potassium ion (K+) and sodium ion (Na+) are the major ions contributing to osmotic pressure and ionic strength (Serrano and Rodriguez-Navarro, 2001). As K+ and Na+ have similar ionic radius (0.331 versus 0.358 nm), they may enter cytoplasm through the same ion channels, especially these are non-selective cation channels (Schaschtman and Liu, 1999). Salt-tolerant plants generally maintain intracellular K+ and Na+ homeostasis, which is important for the maintenance of plant growth and water status (Zhu, 2003).Therefore, the selectivity of K+ over Na+ is essential to understand the tolerance of plants to salt stress.