Effects of NaCl and silicon on ion distribution in the roots, shoots and leaves of two alfalfa cultivars with different salt tolerance

The effects of exogenous NaCl and silicon on ion distribution were investigated in two alfalfa (Medicago sativa. L.) cultivars: the high salt tolerant Zhongmu No. 1 and the low salt tolerant Defor. The cultivars were grown in a hydroponic system with a control (that had neither NaCl nor Si added), a Si treatment (1 mmol L-1 Si), a NaCl treatment (120 mmol L-1 NaCl), and a Si and NaCl treatment (120 mmol L-1 NaCl + 1 mmol L-1 Si). After 15 days of the NaCl and Si treatments, four plants of the cultivars were removed and divided into root, shoot and leaf parts for Na+, K+, Ca2+, Mg2+, Fe3+, Mn2+, Cu2+ and Zn2+ content measurements. Compared with the NaCl treatment, the added Si significantly decreased Na+ content in the roots, but notably increased K+ contents in the shoots and leaves of the high salt tolerant Zhongmu No.1 cultivar. Applying Si to both cultivars under NaCl stress did not significantly affect the Fe3+, Mg2+ and Zn2+ contents in the roots, shoots and leaves of Defor and the roots and shoots of Zhongmu No.1, but increased the Ca2+ content in the roots of Zhongmu No.1 and the Mn2+ contents in the shoots and leaves of both cultivars, while it decreased the Ca2+ and Cu2+ contents of the shoots and leaves of both cultivars under salt stress. Salt stress decreased the K+, Ca2+, Mg2+ and Cu2+ contents in plants, but significantly increased Zn2+ content in the roots, shoots and leaves and Mn2+ content in the shoots of both cultivars when Si was not applied. Thus, salt affects not only the macronutrient distribution but also the micronutrient distribution in alfalfa plants, while silicon could alter the distributions of Na+ and some trophic ions in the roots, shoots and leaves of plants to improve the salt tolerance.