SUMMARYControl 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.

Changes in the bioelectric activity of maize leaves caused by a single light pulse (6 s; 70 mol m-2 s-1) were used to compare the effects of NaCl treatment (20–200 mM) on plant growth, Na+ accumulation in leaves, chlorophyll fluorescence and pigment composition. Bioelectric responses seemed to be the most sensitive indicator of NaCl effects. Even the weakest salt treatment (20 mM) caused a statistically significant decrease (about 40%) in the amplitude of the bioelectric response. The higher the NaCl concentration, the smaller was the amplitude. Over the full concentration range, the characteristic time of response increased from about 30 to 60 sec, indicating that the rate of bioelectric changes was slowed by increasing salinity. Other reliable characteristics were found to be the fluorescence yield and quenching coefficients. The Fv/Fm ratio was not significantly affected by NaCl treatment. Changes in growth rate, biomass or pigment composition were either insensitive, or showed a plateau over a wide range of NaCl concentrations, and were inappropriate for screening. A possible link between bioelectric and fluorescence characteristics is discussed. We conclude that leaf bioelectric activity can be used together with, or instead of, chlorophyll fluorescence measurements, to screen genotypes for salt tolerance.