Effect of increased alkalinity on Na+ and K+ contents, lipid peroxidation and antioxidative enzymes in two populations of Populus cathayana

We compared two populations of Populus cathayana Rehder, originating from altitudes 2 840 m and 1 450 m, to determine whether trees from different altitudes exhibit different tolerance to alkalinity. The tree cuttings were exposed to nutrient solutions with pH 7.9, 8.8, 9.8 and 10.4 and the salt concentration 200 mM. Na⁺ and K⁺ contents, and Na⁺/K⁺ ratios in leaves and roots were greatly affected by pH values. At pH 10.4, the Na⁺/K⁺ ratios in both leaves and roots sharply dropped in the higher altitude population but were maintained at higher levels in the lower altitude population. The patterns of pH-induced changes in contents of malondialdehyde and free proline, and antioxidative enzyme activities indicated that the higher altitude population exhibits greater tolerance to alkalinity stress than does the lower altitude population.