Growth responses of Melilotus siculus accessions to combined salinity and root-zone hypoxia are correlated with differences in tissue ion concentrations and not differences in root aeration

• 15 lines of Melilotus siculus were grown in saline (200 mM NaCl) hypoxic conditions.
• Levels of root porosity were high (18–25%) for all accessions.
• Shoot dry mass was correlated with shoot Na+, Cl− and K+.
• Tolerance to saline-hypoxia was correlated with variation in shoot ion regulation.

Soil salinity and root-zone hypoxia often occur together in saline landscapes. For many plants, this combination of stresses causes greater increases in Na+ and Cl− in shoots, and decreases in K+, than from salinity alone. These changes in ion concentrations from combined salinity and hypoxia can have more adverse consequences for growth than from salinity alone. The herbaceous forage legume Melilotus siculus naturally occurs in saline soils prone to waterlogging; however, accessions differ in their tolerances, although all form high levels of aerenchyma. We hypothesised that tolerance to combined salinity and hypoxia would be associated with either greater aerenchyma formation in roots or the innate ability of the accessions to regulate tissue ion concentrations. Fifteen accessions of M. siculus were grown in nutrient solution with two salinities (0 or 200 mM NaCl) and two aeration treatments (aerated or hypoxic) for 21 days. Dry mass (shoot and root), root porosity and ion concentrations (Cl−, Na+, K+) in shoots and roots were assessed. In the M. siculus accessions variation in the shoot dry mass under saline–hypoxic conditions was negatively correlated with shoot Cl− and Na+, and positively correlated with the shoot K+. Shoot ion concentrations under saline–hypoxic conditions were related to concentrations under saline–aerated conditions, but not to the porosity of the main root, which was relatively high (∼18 to 25%). Differences in the tolerance of M. siculus accessions to combined salinity and root-zone hypoxia were mediated by variation in the plants’ ability to regulate ions, and were not related to variation in root porosity, which was relatively high in all accessions. The interaction between salinity and hypoxia was not detrimental to M. siculus, a waterlogging tolerant species.