CONICET | Buscador de Institutos y Recursos Humanos

A combination of flooding and salinity stress is detrimental to most plants. Roots in waterlogged soils can suffer oxygen deprivation and the resulting energy deficits compromise ion transport processes essential for salinity resistance. We studied tolerance of combined flooding and salinity stress in Melilotus siculus, an annual legume with potential to be used as a forage pasture on waterlogged and/or moderately saline soils. Tolerance of 200 mM NaCl in aerated or stagnant agar nutrient solution was assessed for fifteen accessions. No adverse interaction of the two stresses combined was evident on shoot dry mass. Shoot dry mass in saline conditions, both aerated and stagnant, was positively correlated with shoot K+ concentration and K+/Na+ ratio, and it was negatively correlated with Cl and Na+ concentrations. Root porosity was high (>22%) in stagnant conditions. Roots contained primary aerenchyma and also developed aerenchymatous phellem (secondary aerenchyma). Regulation of ion uptake into the shoot was maintained even in stagnant solution, presumably owing to the adequate internal aeration afforded by the high root porosity. Tolerance of complete submergence was assessed for one accession, which survived complete submergence of one week at low salinity (25 mM NaCl), but did not recover when submerged in 100 mM NaCl. Leaves of submerged plants retained a surface gas film for the first few days this feature enhanced underwater net photosynthesis and also delayed entry of Na+ and Cl- during submergence in saline floodwater. In summary, M. siculus can tolerate the combined effects of moderately high salinity (200 mM NaCl) and root-zone oxygen deficiency, and even complete submergence at low salinity (up to 50 mM), reinforcing field observations of growth of this species in flood-prone and moderately saline soils.