Ion allocation in two different salt-tolerant Mediterranean Medicago species

The relationship between Na+, major cation concentrations and salt tolerance under long-term saline conditions of Medicago arborea and Medicago citrina was studied. Plants were grown in solution culture in 1, 50, 100, or 200 mmol/L NaCl for 30 days in a climate-controlled greenhouse. Stem and petiole growth was the most affected by salt in both species. Leaf growth was inhibited in M. arborea, with increased salt, while only the 200 mmol/L NaCl-treated M. citrina plants were significantly affected. Both species had the highest Na+ concentrations in the shoots, however, the allocation pattern was different; M. arborea showed the highest concentrations in the leaf blades, whereas M. citrina distributed the salt into the petioles. K+/Na+ ratio decreased with salt in both species; however, leaf K+ use efficiency (g leaf DW mg-1 leaf K+) was higher in M. citrina. The difference in Na+ allocation and cation concentrations found in these medic species and their importance is discussed in relation to their response to NaCl salinity.