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 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 Ca 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 Ca 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 Ca activity of 1 mm. Increasing external Caconcentration did not reduce the root Na concentration, however, suggesting that Ca influenced the loading of Na in the xylem.