Biostimulatory effects of ascorbic acid in improving plant growth, photosynthesis-related parameters and mitigating oxidative damage in alfalfa (Medicago sativa L.) under salt stress condition

Biostimulants such as ascorbic acid, known as vitamin C, have been reported to have numerous positive roles in plant tolerance to abiotic stresses. However, little is known about the biostimulant effects of ascorbic acid on alfalfa (Medicago sativa). Accordingly, a pot experiment was conducted to investigate the effects of 1 mM ascorbic acid, applied as foliar spray, on the salt tolerance of a Moroccan alfalfa population Demnate 201. One month-old M. sativa seedlings were exposed to 200 mM NaCl for four weeks with or without 1 mM of exogenous ascorbic acid treatment. The results showed that salinity stress significantly (p < 0.001) reduced plant biomass, disturbed photosynthesis-related parameters and induced oxidative stress. However, ascorbic acid foliar spray counteracted the observed negative effects of salinity. It significantly (p < 0.001) improved plant growth and photosynthetic parameters. Besides, stress indicators, including Na⁺ in shoot and root, hydrogen peroxide and electrolyte leakage, were significantly reduced by 42%, 29%, 12% and 34%, respectively, in treated and salt-stressed alfalfa plants. Interestingly, the decrease in oxidative stress markers was positively correlated to the ability of ascorbic acid to induce the accumulation of flavonoids and to increase the antioxidant activity of guaiacol peroxidase. Furthermore, compatible solutes, such as proline and soluble sugars, were found higher especially in salt-stressed alfalfa plants treated with 1 mM ascorbic acid. Our findings showed that ascorbic acid supply could be an eco-friendly and sustainable technique to mitigate the toxic effect of salt and could improve alfalfa forage production when grown in salt-affected soils.