The Significance of a Two-Phase Growth Response to Salinity in Wheat and Barley

Wheat and barley genotypes that differed in salt tolerance were used to test a hypothesis that the growth response to salinity has two phases. In the first phase there would be a large decrease in growth rate caused by the salt outside the roots, i.e. an 'osmotic' response. In the second phase there would be an additional decline in growth caused by salt having built up to toxic levels within plants, i.e. a 'salt-specific' response. If this two-phase model is correct, genotypes that differ in their ability to exclude salt or tolerate high internal salt concentrations would not differ in growth rate for some period of time in saline conditions. This hypothesis was tested using many genotypes that had previously been found to differ greatly in salt tolerance, as defined by differences in percent reduction in yield or biomass after prolonged exposure to NaCl. Leaf extension of 15 wheat and barley genotypes was measured daily while the NaCl in the nutrient solution was increased over 10 days to 250 mM. All 15 genotypes showed a similar percentage reduction in leaf extension rate. Dry matter production of four selected wheat genotypes that again differed greatly in salt tolerance was measured for up to 6 weeks in 150 mM NaCl. All genotypes showed the same growth reduction for 4 weeks. After this initial period the more salt- sensitive genotype showed a greater decline in growth. This occurred after 60% of the leaves were dead. These data strongly support the hypothesis that the growth response to salinity has two phases, and indicate that most changes in metabolism or gene expression leading to growth reductions during the first phase relate to the osmotic effect of salinity, not to any salt-specific effect. They also indicate that the salt within the plant reduces growth by causing premature senescence of old leaves and hence a reduced supply of assimilates to the growing regions.