Variations in ion transportation and accumulation in alfalfa plants under NaCl and Na2HCO3 stresses with different pH levels

For revealing the effects of NaCl and NaHCO₃ stress at different pH levels on the absorption and accumulation of main ions in roots, stems and leaves, alfalfa (Medicago sativa L.) plants were cultivated in a nutrient solution system. After NaCl and Na₂HCO₃ imposition, the contents of main ions in roots, stems, leaves and xylem bleeding sap were determined. Under NaCl and Na₂HCO₃ stresses with the same Na⁺ concentration, Na₂HCO₃ stress showed severer inhibitory effects on alfalfa growth in contrast to NaCl stress at different pH levels. Under NaCl stresses at different pH levels, alfalfa plants showed much lower Na⁺ accumulation and Na⁺ delivery rate in root bleeding sap than that under NaHCO₃ stress. The plants of NaHCO₃ stress showed a severer inhibition in K⁺ accumulation, lower K⁺ delivery rate, but higher Na⁺/K⁺ ratios than the plants of NaCl stress. NaHCO₃ stress increased Ca²⁺ and Mg²⁺ accumulation in alfalfa roots, decreased Ca²⁺ and Mg²⁺ delivery rates and increased leaf Na⁺/Ca²⁺ ratio and Na⁺/Mg²⁺ ratio. Both NaCl and NaHCO₃ stresses decreased the contents of Fe³⁺ and Mn²⁺ in roots, but increased the contents of Cu²⁺ in roots, stems and leaves, the content of Mn²⁺ in stems and leaf Zn²⁺ content in alfalfa plants. However, the contents of Fe³⁺ and Mn²⁺ in roots and stem Mn²⁺ content in the NaHCO₃ treatments were much lower than those in the NaCl treatments. In conclusion, NaHCO₃ stress produced severer injury and induced severer Na⁺ toxicity to alfalfa plants than NaCl stress. The effects of NaHCO₃ stress and NaCl stress on the absorption and accumulation of trophic ions were different. Under NaCl stress, the increased pH did not alter Na⁺ absorption and accumulation in plants, but altered the distribution of some nutrient ions in roots, stems and leaves.