Reassessment of tissue Na⁺ concentration as a criterion for salinity tolerance in bread wheat

Wheat is the most important crop grown on many of world's saline and sodic soils, and breeding for improved salinity tolerance (ST) is the only feasible way of improving yield and yield stability under these conditions. There are a number of possible mechanisms by which cereals can tolerate high levels of salinity, but these can be considered in terms of Na⁺ exclusion and tissue tolerance. Na⁺ exclusion has been the focus of much of the recent work in wheat, but with relatively little progress to date in developing high-yielding, salt-tolerant genotypes. Using a diverse collection of bread wheat germplasm, the present study was conducted to assess the value of tissue Na⁺ concentration as a criterion for ST, and to determine whether ST differs with growth stage. Two experiments were conducted, the first with 38 genotypes and the second with 21 genotypes. A wide range of Na⁺ concentrations within the roots and shoots as well as in ST were observed in both experiments. However, maintenance of growth and yield when grown with 100 mm NaCl was not correlated with the ability of a genotype to exclude Na ⁺ either from an individual leaf blade or from the whole shoot. The K⁺ : Na⁺ ratio also showed a wide range among the genotypes, but it did not explain the variation in ST among the genotypes. The results suggested that Na⁺ exclusion and tissue tolerance varied independently, and there was no significant relationship between Na⁺ exclusion and ST in bread wheat. Consequently, similar levels of ST may be achieved through different combinations of exclusion and tissue tolerance. Breeding for improved ST in bread wheat needs to select for traits related to both exclusion and tissue tolerance.