Differential sensitivity to chloride and sodium ions in seedlings of Glycine max and G. soja under NaCl stress

High Na+ and Cl− concentrations in soil cause hyperionic and hyperosmotic stress effects, the consequence of which can be plant demise. Ion-specific stress effects of Na+ and Cl− on seedlings of cultivated (Glycine max (L.) Merr) and wild soybean (Glycine soja Sieb. & Zucc.) were evaluated and compared in isoosmotic solutions of Cl−, Na+ and NaCl. Results showed that under NaCl stress, Cl− was more toxic than Na+ to seedlings of G. max. Injury of six G. max cultivars, including ‘Jackson’ (salt sensitive) and ‘Lee 68’ (salt tolerant), was positively correlated with the content of Cl− in the leaves, and negatively with that in the roots. In subsequent research, seedlings of two G. max cultivars (salt-tolerant Nannong 1138-2, and salt-sensitive Zhongzihuangdou-yi) and two G. soja populations (BB52 and N23232) were subjected to isoosmotic solutions of 150 mM Na+, Cl− and NaCl, respectively. G. max cv. Nannong 1138-2 and Zhongzihuangdou-yi were damaged much more heavily in the solution of Cl− than in that of Na+. Their leaves were found to be more sensitive to Cl− than to Na+, and salt tolerance of these two G. max cultivars was mainly due to successful withholding of Cl− in the roots and stems to decrease its content in the leaves. The reverse response to isoosmotic stress of 150 mM Na+ and Cl− was shown in G. soja populations of BB52 and N23232; their leaves were not as susceptible to toxicity of Cl− as that of Na+. Salt tolerance of BB52 and N23232 was mainly due to successful withholding of Na+ in the roots and stems to decrease its content in the leaves. These results indicate that G. soja have advantages over G. max in those traits associated with the mechanism of Cl− tolerance, such as its withholding in roots and vacuoles of leaves. It is possible to use G. soja to improve the salt tolerance of G. max.