Characterizing root-water-uptake of wheat under elevated CO₂ concentration

Delineating root-water-uptake (RWU) under conditions with augmented CO₂ concentrations is very important for scheduling irrigation to contend with climate change. Responses of plant growth to elevated CO₂ concentration (e[CO₂]) have been widely reported, while the effects of e[CO₂] on RWU has hardly been studied. A hydroponic experiment of wheat (Triticum aestivum L.) with five NO₃⁻-N concentrations (Exp. 1) was conducted to investigate and quantify the effects of e[CO₂] on RWU activity. Another experiment growing wheat in soil columns with four combinations of water and N supply levels (Exp. 2) was conducted to validate the results obtained in Exp. 1, establishing a macroscopic RWU model to simulate soil water dynamics under e[CO₂]. Although CO₂ acclimation was observed in both experiments, plant canopy and root growth were generally stimulated under e[CO₂], while transpiration consumption was not synchronously enhanced due to decreased stomatal conductance, indicating an increase in water use efficiency while a decrease in RWU activity. Potential transpiration was found more linearly related to root nitrogen mass (RNM) than root length under various CO₂ concentrations, regardless of wheat growth stage, water and N supply level. Consequently, RNM density was used to drive the RWU model. The results from Exp. 1 indicated that the effects of e[CO₂] on water uptake coefficient per RNM could be quantified by a recently proposed nonlinear stomatal conductance response model (R² = 0.84, RMSE = 0.55 cm³ mg⁻¹ d⁻¹). The RWU model reliably simulated the dynamics of soil water transport and wheat transpiration under e[CO₂] in Exp. 2 with the RMSE and relative errors mostly less than 0.03 cm³ cm⁻³ and 10 %, respectively. Practical application of the established RWU model for any other specific conditions is expected to benefit from optimization of parameters following choice of most appropriate stomatal conductance response model.