Symptoms of Mn toxicity and the effect of salt on Mn toxicity and accumulation in the halophyte Lepidium latifolium

Abstract

Manganese (Mn) is an essential micronutrient for higher plants but becomes toxic at excess concentrations in soil. Halophytes are salt-tolerant plants and exhibit a salt-mediated increase in tolerance to other stresses, including heavy metals toxicity. This study investigated Lepidium latifolium (Brassicaceae), a facultative halophyte with invasive behavior and rapid spread under submerged conditions, for Mn accumulation and tolerance in the absence or presence of salt (100 mM NaCl). Under excess Mn concentration (1 mM), toxicity symptoms were observed as black spots on the young leaves but as brown, dark-purple speckles on the older leaves, with colocation of phenolics and Mn. The H2O2 concentration and peroxidase activity in the leaf apoplast were both significantly reduced by excess Mn. This indicated a decrease in the oxidation of MnII and phenolics in the leaf apoplast, likely a mechanism for this species’ high Mn toxicity tolerance. Salt treatment improved the biomass and leaf pigments of Mn-stressed plants, enhanced the activity of antioxidant enzymes, decreased Mn accumulation, and restored the leaf Fe concentration. Our data suggest that L. latifolium is a species with high Mn tolerance and accumulation capacity (∼10 mg g−1 DW) and is suitable for studying salt-induced heavy metal tolerance in Brassicaceae.