Effect of variable sulfur supply on arsenic tolerance and antioxidant responses in Hydrilla verticillata (L.f.) Royle.

In the present study, Hydrilla verticillata plants were exposed to arsenate (AsV; 50 microM) and arsenite (AsIII; 5 microM) under variable S supply: deficient (2 microM S, -S), normal (1 mM S, +S) and excess (2 mM S, +HS). Arsenic accumulation (microg g(-1) dw) in +HS plants was about 2-fold higher upon exposure to both AsV (30) and AsIII (50) than that observed in +S (12 & 24) and -S (14 & 26) plants. Despite lower As accumulation, -S plants experienced the maximum oxidative stress owing to an inadequate response of enzymatic and molecular antioxidants and significant decline in total thiols and the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG). By contrast +HS plants had significant increase in total thiols and an improved redox status, did not demonstrate any negative impact to antioxidants except catalase and hence experienced the least increase in oxidative stress parameters. In conclusion, an increase in S supply to plants may improve their accumulation capacity for As through enhanced tolerance caused by a positive effect on thiol metabolism and antioxidant status of the plants.