In the present study, Hydrilla verticillata plants were exposed to arsenate (AsV; 50 mM) and arsenite
(AsIII; 5 mM) under variable S supply: deficient (2 mM S, –S), normal (1 mM S, +S) and excess (2 mM S,
+HS). Arsenic accumulation (mg g1 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.