Phytotoxicity and accumulation of mercury in tobacco subjected to different exposure routes

Experiments were conducted to study the absorption, phytotoxicity, and internal distribution of mercury (Hg) in tobacco plants (Nicotiana miersii) exposed to elemental mercury vapor (Hg0) through the shoot or to ionic mercury (Hg2+) through the root. Tobacco plants grown hydroponically were exposed to varying Hg0 treatment concentrations (0 to 100 mg/m3) in a specially designed chamber system, examined for visible symptoms of toxicity, and analyzed for Hg content by cold vapor atomic absorption, 1 and 10 d following treatment. In a separate experiment, the roots of hydroponic tobacco plants were exposed to Hg2+ in the form of a mercury chloride solution (0 to 100 μg/ml) and analyzed for growth changes and Hg content. Accumulation, toxicity response, and Hg distribution differed between the two exposure routes, even when internal Hg concentrations in the treated plants were similar. Plants exposed to Hg0 accumulated Hg in the shoots with no movement to roots. Visible signs of Hg0 stress were apparent at 1.0 mg/m3 exposure levels and greater. Root-exposed plants showed accumulation of Hg in the roots with movement to the shoots by day 10. Inhibition of root and shoot growth occurred at treatment levels of 1.0 μg/ml and greater with very limited tissue damage at higher treatment levels.