Checking the relevance of Michaelis-Menten function for modelling of root cadmium uptake

摘要

Thlaspi caerulescens (J. & C. Presl) hyperaccumulates several trace elements and is of first interest for phytoextraction, while maize does not. To determine the root absorption function that should be used for cadmium (Cd) uptake modelling, Cd uptake by different root compartments was studied and compared between maize and two contrasting ecotypes of T. caerulescens (Tc) having high (Tc-H) and low (Tc-L) Cd accumulation in shoots. In one hour concentration- dependent kinetics, we observed that exchangeable adsorbed Cd increased linearly with the external solution concentration, with a bigger slope for Tc-H ecotype, whereas non-exchangeable apoplastic Cd remained approximately constant. At external concentrations below 1μM, symplastic Cd was about 50% of the total root Cd, while at higher external Cd activity, exchangeable apoplastic Cd reached 90%. Over the 1 to 50μM concentration range, symplastic Cd uptake by Tc was characterized by a Michaelis-Menten type kinetics, Tc-L ecotype showing a lower Imax and a higher Km than Tc-H. Maize roots showed a six-fold lower Imax than that of Tc-H ecotype, together with an additive linear component. At Cd concentrations below 1μM, Cd uptake isotherms of all species were formalized best using a Michaelis-Menten function, while at higher concentrations, a linear function could be used as the limiting condition for Cd transport modelling in soil.