Willow (Salix sp.) has large biomass production and high resistance to environmental stress as an important multipurpose tree species in afforestation, hence has been proved to be effective in the uptake and accumulation of metals from contaminated waters and soils. Salix jiangsuensis CL J-172 and Salix babylonica Linn were chosen as model plants to investigate their potential for uptake of Cu²⁺ from hydroponics culture solution.
The uptake kinetic characteristics of Salix jiangsuensis CL J-172 and Salix babylonica Linn to Cu²⁺ and the effect of organic acids including acetic and malic acids to the Cu²⁺ accumulation by both willows species were investigated under hydroponics. The results suggested that the uptake influx of Cu²⁺ by roots of Salix jiangsuensis CL J-172 and Salix babylonica Linn increased with Cu²⁺ concentration varied from 0 to 200 μmol/L and the time prolonged from 0 to 48 hours. Concentration-dependent Cu²⁺ influx kinetics in Salix jiangsuensis CL J-172 and Salix babylonica Linn were characterized by smooth, non-saturating curves. The analysis of parameters derived from Michaelis-Menten equation showed that the absorption ability of Cu²⁺ by the root of Salix babylonica Linn was better than that of Salix jiangsuensis CL J-172. The addition of acetic acid enhanced the uptake of Cu²⁺ in roots and reached the highest values when the ratios of acetic acid to Cu²⁺ is 10:1, which increased the uptake of Cu²⁺ by 43.1% and 22.9% for Salix jiangsuensis CL J-172 and Salix babylonica Linn, respectively. The addition of malic acid enhanced the uptake of Cu²⁺ in roots and reached the highest values when the ratios of acetic acid to Cu²⁺ is 5:1, which increased the uptake of Cu²⁺ by 61% and 9.2% for Salix jiangsuensis CL J-172 and Salix babylonica Linn, respectively. The content of Cu²⁺ in root cap was significantly higher than that in root manure zone in Salix jiangsuensis CL J-172 which suggest that root cap was more active in uptake of Cu²⁺ from solution. The addition of acetic acid increased the concentration of Cu²⁺ by 16.5% in root tip and by 23.7% in root mature zone, while malic acid increased by 33.7% in root tips and 43% in root mature zone at the ratio of organic acids to Cu²⁺ at 10:1 for Salix jiangsuensis CL J-172, while the ratio of Cu²⁺ content in root cap to root mature zone changed slightly compared the value of 1.40 and 1.39 to the value of 1.49 without the addition of organic acid. With the ratio of acetic acid to Cu²⁺ at 10:1, Cu²⁺ uptake by root of Salix babylonica Linn were firstly enhanced when Cu²⁺ concentration were lower than 20 μmol/L and then decreased as Cu²⁺ concentration were higher than 20 μmol/L.The addition of acetic acid always decreased the content of Cu²⁺ in aboveground part of Salix babylonica Linn which implied that acetic acid inhibited the translocation of Cu²⁺ form root to aboveground part of Salix babylonica Linn and the inhibiting effect would be more obvious as the concentration of acetic acid increased.