Abstract:    The effects of neutral salt and alkali on the ion distribution were investigated in two alfalfa (Medicago sativa L.) cultivars, including Zhongmu 1, a high salt-tolerant cultivar, and Algonquin, a low salt-tolerant cultivar. The alkali stress expressed more serious growth inhibition than the neutral salt stress at the same Na+ concentration. Compared with Algonquin, Zhongmu 1 did not exhibit a higher alkali tolerance under the Na₂CO₃-NaHCO₃ treatment with the low Na+ concentration (50 mmol L^–1). The alkali increased the accumulation of Na⁺, Ca²⁺, and Mg²⁺ in the root and changed the Ca²⁺ and Mg²⁺ balance in the entire alfalfa plant. The salt and alkali stresses decreased the K+ and Fe³⁺ contents of the roots and leaves, the root Mn²⁺ content, and the shoot Zn²⁺ content, but they increased the Fe³⁺ accumulation of the shoots, the shoot and leaf Cu²⁺ contents, and the leaf Zn²⁺ content in both alfalfa cultivars. Based on the results obtained under the conditions of this experiment, we found that the salt and alkali stresses reduced the plant growth in both alfalfa cultivars, while the alkali caused a stronger stress than the neutral salt in alfalfa. Thus, we conclude that under hydroponic conditions, the deleterious effects of the alkali on plants are due to the distribution change of some trophic ion balance in the roots, shoots, and leaves of the plants by causing of Na+, CO₃^2–, and/or HCO₃^– stresses.

Key words: alfalfa ,  ion distribution ,  neutral salts ,  alkali ,  stress