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

Abstract

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.