Functional-structural root system model validation using soil MRI and tracer experiment

Functional-structural root system models simulate the relations between root system architectural and hydraulic properties, and the spatio-temporal distributions of water and solute in the root zone. Such models might help identify optimal plant properties for breeding and contribute to increased water use efficiency. However, it must be first demonstrated that they accurately reproduce the processes they intend to describe. This is challenging because the flow and transport processes towards individual roots are hard to observe. We demonstrate how this deadlock could be broken by combining co-registered root and tracer distributions obtained from magnetic resonance imaging with a root system model in an inverse modeling scheme. The main features in the tracer distributions were well reproduced by the model using realistic root hydraulic parameters. By combining functional-structural root system model with 4D tracer observations, we were able to quantify the water uptake distribution of a growing root system. We showed that 76% of the transpiration was extracted through 3 rd order roots. The simulations also demonstrated that accurate water uptake distribution cannot be directly derived neither from observations of tracer accumulation nor from water depletion. However, detailed tracer experiments combined with process-based models help decipher mechanisms underlying root water uptake.