Salt Stress is Alleviated by Either Proline or Silicon But Not by Their Combination in Alfalfa (Medicago sativa L.) Inoculated with a Salt-Tolerant Ensifer meliloti Strain

Salinity is one of the most deleterious abiotic stresses limiting plant growth and productivity. In the present study, we aimed to assess the interplaying roles of 20 mM proline and 3 mM silicon (Si) in the mitigation of salt stress (200 mM NaCl) in three alfalfa (Medicago sativa L.) varieties: two Moroccan, Ouad Lmaleh (OL) and Demnate 201 (Dm), and one European, NS Mediana ZMS V (NS Med). Salt stress strongly reduced plant biomass, with NS Med being the most affected. A beneficial effect of independently supplied proline or Si in salt-stressed alfalfa plants was observed on dry weight and shoot-to-root ratio, photosynthetic pigments, ion content (phosphorus, potassium) and K+/Na+ ratio. In addition, stress indicators such as Na+, malonyldialdehyde and hydrogen peroxide were significantly reduced. Proline or Si treatment also led to higher superoxide dismutase and catalase activities. However, when applied together, proline and Si beneficial effects were lower than that of single treatment for almost all the investigated parameters. We conclude that application of either proline or Si provides beneficial effects in mitigating the toxic effect of salt in alfalfa and could be a promising way to improve alfalfa growth in salt-affected soils. However, combining Si and proline treatment do not produce any synergistic effects and therefore is not recommended.