Differential tolerance to Mn toxicity in perennial ryegrass genotypes: involvement of antioxidative enzymes and root exudation of carboxylates

Mechanisms underlying differential tolerance to Manganese (Mn) toxicity in perennial ryegrass (Lolium perenne L.) cultivars are poorly understood. We evaluated activity of antioxidative enzymes and root exudation of carboxylates in four ryegrass cultivars subjected to increasing Mn supply under nutrient solution conditions. A growth reduction caused by Mn toxicity was smaller in Jumbo and Kingston than Nui and Aries cultivars. Shoot Mn accumulation varied in the order Nui > Aries > Kingston > Jumbo. Ascorbate peroxidase and guaiacol peroxidase activities increased with Mn excess. Mn-tolerant Jumbo and Kingston had high activity of these enzymes and relatively low lipid peroxidation. Kingston was most tolerant to high tissue Mn concentrations and had the highest superoxide dismutase activity. Increased activity of antioxidative enzymes in Mn-tolerant cultivars could protect their tissues against oxidative stress triggered by Mn excess. Mn toxicity induced root exudation of carboxylates; oxalate and citrate may decrease Mn availability in the rhizosphere, thus enhancing Mn tolerance in ryegrass.