Role of transpiration and metabolism in translocation and accumulation of cadmium in tobacco plants (Nicotiana tabacum L.).

Tobacco plants grown in pots and in hydroponic culture accumulated cadmium (Cd) particularly: the Cd content of tobacco leaves exceeded 100 mg/kg and the enrichment factor (the ratio of Cd in leaves to that in soil) was more than 4. These high levels of accumulation identify tobacco as a hyperaccumulator of Cd. Two transpiration inhibitors (paraffin or CaCl2) and shade decreased the Cd content of tobacco leaves, and the decrease showed a linear relationship with the leaf transpiration rate. A metabolism inhibitor, namely 2,4-dinitrophenol (DNP), and low temperature (4 °C) also lowered the Cd content of tobacco leaves, but the inhibitory effect of low temperature was greater. In the half number of leaves that were shaded, the Cd content decreased to 26.5% of that in leaves that were not shaded in the same tobacco plants. These results suggests that translocation of Cd from the medium to the leaves is driven by the symplastic and the apoplastic pathways. Probably, of the two crucial steps in the translocation of Cd in tobacco plants, one, namely uptake from the medium to the xylem, is energy-dependent whereas the other, namely the transfer from the xylem to the leaves, is driven mainly by transpiration.