Screening plants for salt tolerance by measuring K+ flux:: a case study for barley

Development of salt-tolerant genotypes is central both to remediation of salinity-affected land and to meet increasing global food demand, which has been driving expansion of cropping into marginal areas. The bottleneck of any breeding programme is the lack of a reliable screening technique. This study tested the hypothesis that the ability of plants to retain K+ under saline conditions is central to their salt tolerance. Using seven barley cultivars contrasting in salt tolerance (CM72, Numar, ZUG293, ZUG95, Franklin, Gairdner, ZUG403), a comprehensive study was undertaken of whole-plant (growth rate, biomass, net CO2 assimilation, chlorophyll fluorescence, root and leaf elemental and water content) and cellular (net fluxes of H+, K+, Na+ and Ca2+) responses to various concentrations of NaCl (20-320 m M). Na+ selective microelectrodes were found to be unsuitable for screening purposes because of non-ideal selectivity of the commercially available Na+ LIX. At the same time, our results show very strong negative correlation between the magnitude of K+ efflux from the root and salt tolerance of a particular cultivar. K+ efflux from the mature root zone of intact 3-day-old seedlings following 40 min pretreatment with 80 mM NaCl was found to be a reliable screening indicator for salinity tolerance in barley. As a faster and more cost-effective alternative to microelectrode measurements, a procedure was developed enabling rapid screening of large numbers of seedlings, based on amount of K+ leaked from plant roots after exposure to NaCl.