Silicon improves physiological, biochemical, and morphological adaptations of alfalfa (Medicago sativa L.) during salinity stress

Silicon (Si) application to crops is a promising way for the deployment of sustainable agriculture. Here, the effects of Si on salt stress tolerance were investigated in alfalfa (Medicago sativa L.)-rhizobia symbioses. Two Moroccan, Ouad-Lmaleh (OL) and Demnate-201 (Dm), and one European, NS-Mediana-ZMS-V (NS-Med), alfalfa varieties were associated to Ensifer meliloti Rm41 rhizobial strain. One-month-old alfalfa plants were exposed to 120 mM NaCl for five weeks with or without 3 mM of Si. The plants subjected to salt stress showed reduced biomass, chlorophyll (Chl) contents and relative water content (RWC) in comparison to the controls. The alfalfa-rhizobia symbiosis was also impaired under stress as reflected by less root nodulation and lower nitrogen (N) content and nitrogen content index (NCI). Added Si significantly increased plant biomass, nodules number, N content, NCI, Chl contents and RWC under salt stress. Results showed that salt-stressed alfalfa increased malonyldialdehyde (MDA), hydrogen peroxide (H2O2) and electrolyte leakage (EL). However, Si incorporation in the cultured media reduced oxidative damages under salt-stress particularly in NS-Med variety by 26%, 70% and 70% for MDA, H2O2 and EL respectively. The lower amount of MDA, H2O2 and EL in the Si-treated plants seems to be related to its capacity to modulate superoxide dismutase and polyphenol oxidase activities and increase total polyphenol, flavonoid and carotenoid contents. Besides, compatible osmolytes, such as proline, glycine betaine and soluble sugars were found increased particularly in Si-treated OL plants by 46%, 33% and 26% respectively in comparison to Si-untreated plants. Alfalfa varieties reacted differently to Si treatment. Sodium concentration in alfalfa plants increased under salinity and reduced by Si treatment with an increase in the potassium content. Our findings showed that exogenous Si application could be a promising way to mitigate the toxic effect of salt and could improve alfalfa growth and its rhizobial symbiosis when grown in salt-affected soils.