Evaluation of Na+ Concentration and K+/Na+ Ratio as A Criterion for Salinity Tolerance in Wheat and Barley

Most researches on wheat and barley breeding for salt tolerance have focused mainly on excluding Na+ from different tissues but the results of some experiments suggest that contribution of Na+ exclusion to salt tolerance is overshadowed by other physiological responses. Three bread wheat cultivars differing in salt tolerance (Arg, Tajan and Baharan) and one barley cultivar (Nik) were employed to assess tissues Na+ concentration and K+/Na+ ratios as a criterion for salt tolerance using a factorial experiment based on a randomized complete block design with three replications. Three levels of NaCl (0, 75 and 150 mM NaCl) were imposed as the salinity treatment when the leaf 4 was fully expanded. Salinity decreased root dry weight, root K+, shoot K+ and shoot K+/Na+ ratio and increased root and shoot Na+. Shoot Na+ concentrations of barley and salt tolerant cultivar of wheat (Arg) were greater than Tajan and Baharan under salinity stress.  There was no meaningful relationship between Na+ exclusion and salt tolerance in the examined wheat cultivars and Nik barley cultivar. Shoot K+/Na+ ratio was found to be the most responsive trait to salinity and no significant differences were observed between the wheat cultivars and Nik barley cultivar under salinity conditions in this aspect. Given the negative effects of salinity on root growth, it seemed that the major factor in root dry weight losses of the present wheat cultivars and Nik barley cultivar was due, mainly, to the osmotic effect of salt. The results of this experiment suggest that Na+ exclusion does not necessarily confer salt tolerance. It, hence, seems breeding for salt tolerance needs to select for traits related to both Na+ exclusion and other physiological responses, most likely those associated with tissue tolerance.