Kinetics of cadmium and zinc absorption by rice seedling roots

The present experiment in which intact roots of rice seedlings were used was conducted primarily to determine the reason for the high accumulation of Cd in roots compared with the absorption isotherms reflecting Cd and Zn uptake in roots. The amounts of the metals accumulated in the roots increased linearly with time within 1 h. The absorption isotherms of Cd and Zn in the roots showed that Cd was more rapidly absorbed than Zn at low medium concentrations up to about 0.6 µm, while the reverse pattern was observed at concentrations higher than 0.6 µm. From the Lineweaver-Burk equation, a dual pattern of transport mechanism was clearly observed for both Cd and Zn at the concentration range of 0.089 to 8.9 µm. The phenomenon of more rapid absorption of Cd than Zn at low concentrations was ascribed to a lower Km, namely higher affinity of Cd, while the more rapid absorption of Zn than Cd at higher concentrations was ascribed to the considerably higher Vmax of Zn.