Comparisons of annual pasture legumes in growth, ion regulation and root porosity demonstrate that Melilotus siculus has exceptional tolerance to combinations of salinity and waterlogging

Annual pasture legumes with high tolerance of combined salinity and waterlogging are needed for saline areas in many rain-fed agricultural regions. Melilotus siculus is an annual legume from saline marshy areas of the Mediterranean with potential use in saline pastures and, based on descriptions of its native habitat, was hypothesised to tolerate combined salinity and waterlogging. Experiments compared M. siculus to Trifolium michelianum and Medicago polymorpha, other annual pasture legumes with reported salt or waterlogging tolerances, with 0–450 mM NaCl in hydroponics or in sand culture. Tolerance to combined salinity and waterlogging was also evaluated at 150, 400 and 550 mM NaCl in a stagnant deoxygenated nutrient solution. M. siculus was substantially more salt tolerant than the other two species. At 450 mM NaCl, shoot DM of M. siculus was 30% of control, compared with only 15% for the other species. M. siculus was also the most tolerant species to combined salinity and stagnant treatment, and produced new leaves even after 14 d in stagnant nutrient solution with 550 mM NaCl (∼ sea water salinity). In comparison, T. michelianum and M. polymorpha only survived up to 5 d in stagnant solution with 400 mM NaCl. Even at only 150 mM NaCl in stagnant solution, shoot DM was just 30% of control for T. michelianum and M. polymorpha, compared to 60% for M. siculus. Tolerance to combined salinity and waterlogging was associated with higher root porosity and regulation of shoot Na+ and Cl−, with the more tolerant M. siculus having similar shoot Na+ and Cl− concentrations for both aerated-saline and stagnant-saline treatments. In summary, M. siculus is a pasture legume that can grow at high salinity (up to 550 mM) even when in combination with waterlogging, as root porosity and associated O2 transport presumably enables continued regulation by the roots of Na+ and Cl− entry under these dual stress conditions.

► Melilotus siculus is highly salt-tolerant; biomass at 450 mM NaCl was 30% of control.
► M. siculus is tolerant to combined salinity and waterlogging.
► M. siculus was still growing after 14 d in stagnant seawater (550 mM NaCl).
► Trifolium michelianum and Medicago polymorpha died after 5 d in stagnant 400 mM NaCl.
► Tolerance is due to high root porosity and regulation of shoot Na+ and Cl−.