Control of salt transport from roots to shoots of wheat in saline soil

Wheat genotypes with 5-fold difference in shoot Na⁺ concentrations were studied over a salinity range of 1-150 mM NaCl and CaCl ₂ of 0.5-10 mM to assess their performance in saline and sodic soils. All genotypes had a maximum shoot Na⁺ concentration at 50 mM external NaCl when the supplemental Ca²⁺ provided an activity of 1 mM or more. Shoot Na⁺ concentrations either stayed constant from 50 to 150 mM external NaCl, or decreased in some genotypes at the higher salinity. Calculated rates of root uptake, and root: shoot transport, were at a maximum at 50 mM NaCl in all genotypes, and decreased at higher NaCl in some genotypes, indicating feedback regulation. K⁺ showed a pattern inverse to that of Na⁺. Cl^- uptake and transport rates increased linearly with increasing salinity, and differed little between genotypes. Increasing external Ca²⁺ concentration reduced the accumulation of Na ⁺ in the shoot, the effects being greater in the low Na⁺ genotypes, and greater as the salinity increased, indicating that the plateau in shoot Na⁺ concentration relied on the maintenance of a minimal Ca²⁺ activity of 1 mM. Increasing external Ca²⁺ concentration did not reduce the root Na⁺ concentration, however, suggesting that Ca²⁺ influenced the loading of Na⁺ in the xylem.