A large-scale glasshouse trial, including nearly 70 barley cultivars (5300 plants in total), was conducted over 2 consecutive years to investigate plant physiological responses to salinity. In a parallel set of experiments, plant salt tolerance was assessed by non-invasive microelectrode measurements of net K+ flux from roots of 3-day-old seedlings of each cultivar after 1 h treatment in 80 mm NaCl as described in our previous publication (Chen et al. 2005). K+ flux from the root in response to NaCl treatment was highly (P < 0.001) inversely correlated with relative grain yield, shoot biomass, plant height, net CO2 assimilation, survival rate and thousand-seed weight measured in glasshouse experiments after 4-5 months of salinity treatment. No significant correlation with relative germination rate or tillering was found. In general, 62 out of 69 cultivars followed an inverse relationship between K+ efflux and salt tolerance. In a few cultivars, however, high salt tolerance (measured as grain yield at harvest) was observed for plants showing only modest ability to retain K+ in the root cells. Tissue elemental analysis showed that these plants had a much better ability to prevent Na+ accumulation in plant leaves and, thus, to maintain a higher K+/Na+ ratio. Taken together, our results show that a plant's ability to maintain high K+/Na+ ratio (either retention of K+ or preventing Na+ from accumulating in leaves) is a key feature for salt tolerance in barley.