Using non-equilibrium thermodynamics to model cadmium accumulation by maize

• Published in: Current plant biology 2024-09, Vol.39, p.100369, Article 100369
• Main Authors: Moyne, Christian, Leglize, Pierre, Sterckeman, Thibault
• Format: Article
• Language: English
• Subjects: Chemical potential; Diffusion; Environmental Sciences; Hydroponics; Life Sciences; Root cortex; Translocation; Xylem sap
• ISSN: 2214-6628; 2214-6628
• DOI: 10.1016/j.cpb.2024.100369

Many people around the world are overexposed to cadmium through their consumption of plant products. A model predicting Cd content in crops would improve risk assessment and cultural practices. As no such model exists, we evaluated different methods to simulate the root uptake of Cd and its translocation to the aerial parts of maize. Using non-equilibrium thermodynamics, the Cd flux (JA,B) from one compartment (A) to another (B) was considered to be proportional to the difference in electrochemical potential between the compartments and given by an equation of the type JA,B=βA,Bln(KBCA/KACB), where βA,B and KB are constants and CA and CB the actual Cd concentrations in compartments A and B. The compartments considered were rhizosphere solution (Rh), root cortex (Co), xylem sap (X) and aerial tissues. The model was evaluated against the experimental uptake of Cd by maize exposed for 8 h to a constant Cd concentration in the rhizosphere solution. The formalism made it possible to describe the flow of Cd from the rhizosphere to the root cortex, with βRh,Co = 8.7E-11 mol m−2 s−1 and KCo = 73. This questions the common use of Michaelis-Menten kinetics to model root absorption over the long term (throughout the cultivation period). In this case, the apparent validity of the Michaelis-Menten uptake kinetics is probably more closely linked to the root growth than to the Cd internalization mechanisms. To take into account the resistance to the ion transport linked to crossing the root cortex, thermodynamic and diffusion formalisms had to be associated, which enabled the prediction of the Cd flux towards xylem, with KX = 12.48 and a diffusion coefficient DCo = 3.44E-11 m2 s−1. The Cd flux from xylem to aerial tissues was better predicted by modelling the sap flow due to plant transpiration. This work opens perspectives towards a relatively simple modelling of plant Cd accumulation. •Cd absorption flux is proportional to the difference in electrochemical potential.•Michealis-Menten kinetics for root absorption are questioned by thermodynamics.•Diffusion in root cortex delaying xylem loading of Cd was simulated.•Cd flux to aerial tissues was predicted by xylem sap flow due to transpiration.•Thermodynamic modelling confirms the key role of root cortex in Cd accumulation.