Role of Na+, K+, Cl-, proline and sucrose concentrations in determining salinity tolerance and their correlation with the expression of multiple genes in tomato

One of the major abiotic stresses affecting agriculture is soil salinity, which reduces crop yield and, consequently, revenue for farmers. Although tomato is an important agricultural species, elite varieties are poor at withstanding salinity stress. Thus, a feasible way of improving yield under conditions of salinity stress is to breed for improved salt tolerance. In this study, we analysed the physiological and genetic parameters of 23 tomato accessions in order to identify possible traits to be used by plant breeders to develop more tolerant tomato varieties. Although we observed a wide range of Na+ concentrations within the leaves, stems and roots, the maintenance of growth in the presence of 100 mM NaCl did not correlate with the exclusion or accumulation of Na+. Nor could we correlate the growth with accumulation of sugars and proline or with the expression of any gene involved in the homoeostasis of Na+ in the plant. However, several significant correlations between gene expression and Na+ accumulation were observed. For instance, Na+ concentrations both in the leaves and stems were positively correlated with HKT1;2 expression in the roots, and Na+ concentration measured in the roots was positively correlated with HKT1;1 expression also in the roots. Higher and lower Na+ accumulation in the roots and leaves were significantly correlated with higher NHX3 and NHX1 expression in the roots, respectively. These results suggest that, in tomato, for a particular level of tolerance to salinity, a complex relationship between Na+ concentration in the cells and tissue tolerance defines the salinity tolerance of individual tomato accessions. In tomato it is likely that tissue and salinity tolerance work independently, making tolerance to salinity depend on their relative effects rather than on one of these mechanisms alone.