Determination of the different components of cadmium short-term uptake by roots

As the various components of the cadmium (Cd) root sink have not been clearly described, there is a need to precisely measure the respective contributions of apoplast and symplast to short-term root Cd uptake and to explain the linear component of the absorption isotherms. A new method of fractionating Cd in roots was applied to two plant species with contrasting abilities to accumulate Cd: maize (Zea mays) and a Cd-hyperaccumulating ecotype of alpine pennycress (Noccaea caerulescens). Their roots were exposed for 1 h to increasing concentrations of labeled Cd. Series of desorption baths were used to obtain the root apoplastic Cd in combination with a brief freezing step in liquid nitrogen to separate the intracellular metal from the apoplastic one. The apoplastic uptake accounted for 15% to 82% and for 48% to 96% of the total Cd uptake of maize and of alpine pennycress roots, respectively. In the case of maize, the concentration-dependent symplastic net flux fitted a biphasic Michaelis-Menten function, while in the case of alpine pennycress, a Michaelis-Menten-plus-linear function proved a better fit. The second component of the symplastic net flux may reflect absorption through a low-affinity transport system. Short-term Cd uptake by roots is dominated by the high-affinity transport system for exposure concentrations below 1 mu M for maize and 0.2 mu M for alpine pennycress, while cell-wall binding prevailed for higher exposure concentrations.