Sodium and Chloride Sensitivity in Alfalfa (Medicago sativa L.): Growth, Photosynthesis, and Tissue Ion Regulation in Contrasting Genotypes

Alfalfa (Medicago sativa L.) is a moderately salt-tolerant plant. This study was conducted to evaluate responses of two contrasting alfalfa genotypes (OMA-84-salt sensitive and OMA-285-salt-tolerant) to components (Na⁺, and/or Cl⁻) of salt stress. Alfalfa genotypes were exposed to Na⁺ − salts (without chloride), Cl⁻ − salts (without sodium), and NaCl (sodium chloride) stresses with two concentrations (30 and 150 mM). The treatments, involving macronutrients, with the same osmotic potentials, were taken as control. Salt stress, irrespective of type and intensity, caused a significant reduction in plant biomass, physiological (net photosynthetic rate, photosystem II efficiency, chlorophyll fluorescence, water use efficiency, maximum yield of primary photochemistry, and electron transport rate), and shoot mineral (calcium, magnesium, phosphorus, and potassium) contents compared to control; however, this reduction was in the order of NaCl (150 mM) > Na⁺ (150 mM) > Cl⁻ (150 mM). The alfalfa genotype OMA-285 sustained growth under both types of salt stresses than the genotype OMA-84 due to less accumulation of Na⁺ and Cl⁻ ions, maintenance of higher K⁺/Na⁺ ratio, and better photosynthetic activities. In conclusion, salt stress caused a significant reduction in alfalfa growth, this reduction was more under NaCl stress and the effect was mainly additive. The alfalfa genotype OMA-285 sustained growth under salt stresses than the genotype OMA-84 due to ionic homeostasis. However, the tested genotypes were more sensitive to Na⁺ toxicity than the Cl⁻ toxicity, and the contrasting genotypes differed in tissue tolerance of high Na⁺ and Cl⁻. Further research is needed to evaluate tissue tolerance in a diverse and large group of alfalfa genotypes to elucidate the general salt tolerance mechanism in alfalfa.