Redox state and energetic equilibrium determine the magnitude of stress in Hydrilla verticillata upon exposure to arsenate.

Arsenic (As) is a potential hazard to plants' health, however the mechanisms of its toxicity are yet to be properly understood. To determine the impact of redox state and energetic in stress imposition, plants of Hydrilla verticillata (L.f.) Royle, which are known to be potential accumulator of As, were exposed to 100 and 500 μM arsenate (AsV) for 4 to 96 h. Plants demonstrated significant As accumulation with the maximum being at 500 μM after 96 h (568 μg g(-1) dry weight, dw). The accumulation of As led to a significant increase in the level of reactive oxygen species, nitric oxide, carbonyl, malondialdehyde, and percentage of DNA degradation. In addition, the activity of pro-oxidant enzymes like NADPH oxidase and ascorbate oxidase also showed significant increases. These parameters collectively indicated oxidative stress, which in turn caused an increase in percentage of cell death. These negative effects were seemingly linked to an altered energetic and redox equilibrium [analyzed in terms of ATP/ADP, NADH/NAD, NADPH/NADP, reduced glutathione/oxidized glutathione, and ascorbate/dehydroascobate ratios]. Although there was significant increase in the levels of phytochelatins, the As chelating ligands, a large amount of As was presumably present as free ion particularly at 500 μM AsV, which supposedly produced toxic responses. In conclusion, the study demonstrated that the magnitude of disturbance to redox and energetic equilibrium of plants upon AsV exposure determines the extent of toxicity to plants.