Uptake of cadmium from hydroponic solutions by willows (Salix spp.) seedlings
Willow (Salix sp.) has large biomass production and high resistance to environmental stress. As an important multipurpose tree species in afforestation, it has been proved to be effective in the uptake and accumulation of metals from contaminated waters and soils. Suliu172 (Salix × jiangsuensis 'J172'), Hanliu (Salix matsudana), Weishanhu (Salix integra 'Weishanhu') and Yizhibi (S. integra 'Yizhibi') were chosen as model plants to evaluate their potential for uptake of cadmium from hydroponic culture and relative uptake mechanism. Cadmium uptake showed a linear increase in the short time course, and a nonlinear and slow increase in the long time course. After one week cultivation, cadmium accumulation in different parts of willows generally followed the order of root > stem > leaf. Cadmium influx in willow roots increased with the increase of cadmium concentration in hydroponic solutions. A modified Michaelis-Menten equation was employed to describe the concentration-dependent kinetics of cadmium uptake through the roots. Cadmium influx could be resolved into linear and saturable components under concentration–dependent kinetics. The saturable component followed Michaelis–Menten kinetics, which indicated that cadmium uptake across the plasma membrane was transporter–mediated. The uptake capacity (Vmax/Km) jointly decided by the Vmax and Km followed descending order of Hanliu > Weishanhu ≈ Yizhibi > Suliu172, indicating that their inherent potential of cadmium uptake reduced in turn. Low temperature and metabolic inhibitor inhibited the apparent uptake of cadmium in willow. Both active absorption and passive absorption occurred in the cadmium uptake by willow roots.
Key words: Cadmium, willow, uptake kinetics.