Chromium (VI)-induced hormesis and genotoxicity are mediated through oxidative stress in root cells of Allium cepa L.

Chromium (VI) genotoxicity was evaluated in Allium bioassay by using different treatment protocols. Treatment of bulbs of Allium cepa L. with Cr(VI) at a range of concentrations for 5 days (120 h) exhibited low dose (12.5 mu M) stimulation and high dose (25-200 mu M) inhibition of root growth apparently indicating hormesis. Inhibition of root growth was correlated with the dose-dependent increase in generation of reactive oxygen species (ROS), cell death, lipid peroxidation, repression of antioxidative enzymes (catalase, superoxide dismutase, ascorbate peroxidase), induction of DNA damage, chromosome aberrations or micronuclei in root cells. The above effects were, however, reversed when the duration of Cr(VI) treatment was limited to 3-24 h followed by recovery in tap water for 4 days that resulted in the dose-dependent stimulation of root growth, mitosis and increased activity of the antioxidative enzymes that obliterated oxidative stress and genotoxicity. The above Cr(VI)-induced stimulation of root growth was effectively countered by pre- or post-treatments of dimethylthiourea, a ROS-scavenger. These findings underscored that Cr(VI), depending on the magnitude of the dose (concentration x time), could either be stimulatory or inhibitory for root growth that underlined the crucial role of ROS having obvious implications in agriculture, post harvest technology and human health.