Dynamic changes of rhizosphere properties and antioxidant enzyme responses of wheat plants (Triticum aestivum L.) grown in mercury-contaminated soils

A pot experiment was conducted to investigate the dynamic changes in the rhizosphere properties and antioxidant enzyme responses of wheat plants (Triticum aestivum L.) grown in three levels of Hg-contaminated soils. The concentrations of soluble Hg and dissolved organic carbon (DOC) in the rhizosphere soil solutions of the wheat plants were characterised by the sequence before sowing > trefoil stage > stooling stage, whereas the soil solution pH was found to follow an opposite distribution pattern. The activities of antioxidant enzymes in wheat plants under Hg stress were substantially altered. Greater superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities were observed in the wheat plants grown in a highly polluted soil than in a slightly polluted soil (with increases of 11–27% at the trefoil stage and 26–70% at the stooling stage); however, increasing concentrations of Hg up to seriously polluted level led to reduced enzyme activities. The present results suggest that wheat plants could positively adapt to environmental Hg stress, with rhizosphere acidification, the enhancement of DOC production and greater antioxidant enzyme activities perhaps being three important mechanisms involved in the metal uptake/tolerance in the rhizospheres of wheat plants grown in Hg-contaminated soils.