Calcium effects on growth and solute contents of sorghum seedlings under NaCl stress

Increased Ca2+ concentrations in the root environment have been suggested to minimize salinity effects on plant growth. In this work, seedlings of two forage sorghum genotypes (Sorghum bicolor (L.) Moench), one salt tolerant (CSF 20) and the other salt sensitive (CSF 18), were exposed to two Na+ (0 and 75 mmol L-1) and two Ca2+ (0.65 and 2.60 mmol L-1) levels in nutrient solution, in a 2 x 2 x 2 factorial design. After seven days of salt treatment, the shoot and root dry matter as well as Cl-, Na+, K+ Ca2+, and proline concentration were measured. Salt stress reduced shoot and root dry matter yield in both genotypes, especially in the salt sensitive genotype. An increase in Ca2+ concentration in the nutrient solution partially reduced the salt effects, mainly in the shoots of the salt tolerant genotype. The salt stress also increased leaf and root Na+ and Cl- concentration but reduced that of K+ in both genotypes, especially in the sensitive one. Additionally, a reduction in Ca2+ and an increase in proline concentration were observed, though only in the leaves. The increase in the Ca2+ concentration in solution partially alleviated the salt effects by reducing Na+ and Cl- and increasing K+ contents, especially in the leaves of the salt tolerant genotype. This genotype presented a much lower Na+/K+ ratio than the sensitive one, and this difference between genotypes increased along with the increasing Ca2+ concentration in the nutrient solution. Increased Ca2+ concentrations in the nutrient solution also increased leaf and root Ca2+ contents of both genotypes, but salt stressed seedlings were not able to maintain the same tissue concentrations of these ions of unstressed control seedlings. Calcium had no significant effect on proline accumulation in the plants, either in the absence or presence of salt stress.