Effects of foliar application of organic acids on alleviation of aluminum toxicity in alfalfa

Organic acids (OA) may affect plant resistance to aluminum (Al) toxicity in acidic soils. However, limited information is available on the effects of different organic acids on Al resistance in alfalfa. We investigated the effects of foliar application of organic acids (succinic acid, citric acid, malic acid, and oxalic acid) to alfalfa (Medicago sativa L.) under Al stress. Seedlings were grown in pH 4.5 nutrient solution containing Al at 0 or 100 μM, and were sprayed with water or 100 μM of oxalic acid, malic acid, citric acid, or succinic acid every 3 d during a 10 d experiment. Aluminum stress caused significant reduction in alfalfa growth (reflected by above-ground biomass, root weight and root length), root activity, mineral nutrient concentrations (Ca, K, Mg, Mn and Zn), and a significant increase in leaf membrane lipid peroxidation. Foliar application of the four organic acids, especially succinic acid, alleviated Al toxicity, as demonstrated by the increase in plant growth and root activity, as well as reduction in lipid peroxidation. Oxalic acid and malic acid treatments significantly increased oxalate exudation and decreased Al concentration in roots exposed to Al stress. Succinic acid treatment significantly increased accumulation of all four organic acids in roots, accumulation of Ca, K, Mg, Mn and Zn, and up-regulated the gene transcription of malate dehydrogenase (MDH) and phosphoenolpyruvate carboxylase (PEPC) in roots. Our results suggest that the promotion of oxalate exudation from roots through exogenous application of oxalate and malate could contribute to the improvement in Al resistance of alfalfa, and the positive effects of exogenous application of succinate on Al resistance may be associated with the increased endogenous accumulation of all four organic acids in roots, which may constitute an organic-acid detoxification system in alfalfa.