Non-reciprocal interactions between K⁺ and Na⁺ ions in barley (Hordeum vulgare L.)

The interaction of sodium and potassium ions in the context of the primary entry of Na⁺ into plant cells, and the subsequent development of sodium toxicity, has been the subject of much recent attention. In the present study, the technique of compartmental analysis with the radiotracers ⁴²K⁺ and ²⁴Na⁺ was applied in intact seedlings of barley (Hordeum vulgare L.) to test the hypothesis that elevated levels of K⁺ in the growth medium will reduce both rapid, futile Na⁺ cycling at the plasma membrane, and Na⁺ build-up in the cytosol of root cells, under saline conditions (100 mM NaCl). We reject this hypothesis, showing that, over a wide (400-fold) range of K⁺ supply, K⁺ neither reduces the primary fluxes of Na⁺ at the root plasma membrane nor suppresses Na⁺ accumulation in the cytosol. By contrast, 100 mM NaCl suppressed the cytosolic K⁺ pool by 47-73%, and also substantially decreased low-affinity K⁺ transport across the plasma membrane. We confirm that the cytosolic [K⁺]:[Na⁺] ratio is a poor predictor of growth performance under saline conditions, while a good correlation is seen between growth and the tissue ratios of the two ions. The data provide insight into the mechanisms that mediate the toxic influx of sodium across the root plasma membrane under salinity stress, demonstrating that, in the glycophyte barley, K⁺ and Na⁺ are unlikely to share a common low-affinity pathway for entry into the plant cell.