Xylem ionic relations and salinity tolerance in barley

Control of ion loading into the xylem has been repeatedly named as a crucial factor determining plant salt tolerance. In this study we further investigate this issue by applying a range of biophysical [the microelectrode ion flux measurement (MIFE) technique for non-invasive ion flux measurements, the patch clamp technique, membrane potential measurements] and physiological (xylem sap and tissue nutrient analysis, photosynthetic characteristics, stomatal conductance) techniques to barley varieties contrasting in their salt tolerance. We report that restricting Na⁺ loading into the xylem is not essential for conferring salinity tolerance in barley, with tolerant varieties showing xylem Na⁺ concentrations at least as high as those of sensitive ones. At the same time, tolerant genotypes are capable of maintaining higher xylem K⁺/Na⁺ ratios and efficiently sequester the accumulated Na⁺ in leaves. The former is achieved by more efficient loading of K⁺ into the xylem. We argue that the observed increases in xylem K⁺ and Na⁺ concentrations in tolerant genotypes are required for efficient osmotic adjustment, needed to support leaf expansion growth. We also provide evidence that K ⁺-permeable voltage-sensitive channels are involved in xylem loading and operate in a feedback manner to maintain a constant K/Na⁺ ratio in the xylem sap.