Osmotic adjustment in roots and leaves of two sorghum genotypes under NaCl stress

Abstract

Seedlings of two sorghum genotypes [Sorghum bicolor (L.) Moench], one salt tolerant (CSF 20) and the other salt sensitive (CSF 18) were grown in nutrient solution containing 0, 50 and 100 mmol.L-1 NaCl for seven days and the osmotic potential (Ψs) and the contribution of organic and inorganic solutes to the Ψs were determined in the leaves and roots. Salinity reduced the Ψs of the cellular sap of leaves and roots in both genotypes, mainly in the salt sensitive one. The higher decrease in the Ψs in the salt sensitive genotype was mostly due to higher accumulation of Na+ and Cl- that probably exceeded the amount needed for the osmotic adjustment. Among the inorganic solutes, K+ contributed the most to the Ψs in control unstressed seedlings, but its contribution decreased as salt stress increased, especially in the salt sensitive genotype. Soluble carbohydrates and amino acids were the organic solutes that contributed the most to the leaf and root Ψs, respectively. No statistically significant difference in these organic solute contributions to the leaf Ψs between genotypes was observed. Their contributions to the root Ψs, however, were higher in the salt tolerant genotype, especially at higher NaCl concentration. Proline contribution to leaf and root Ψs was quite small in both genotypes and its accumulation was not related to salt tolerance. Our results suggest that the salt tolerant genotype was able to maintain a more adequate osmotic pool in the leaves and roots under salt stress than the salt sensitive genotype.