LAUSR

BACKGROUND AND AIMS Upland rice is often grown where water and phosphorus (P) are limited. To better understand the interaction between water and P availability, functional-structural models that mechanistically represent small-scale nutrient gradients and water dynamics in the rhizosphere are needed. METHODS Rice was grown in large columns using a P-deficient soil at three P supplies in the topsoil (deficient, suboptimal, non-limiting) in combination with two water regimes (field capacity versus drying periods). Root system characteristics, such as nodal root number, lateral types, interbranch distance, root diameters, and the distribution of biomass with depth, as well as water and P uptake were measured. Based on the observed root data 3D root systems were reconstructed by calibrating the structural architecure model CRootBox for each scenario. Water flow and P transport in the soil to each of the individual root segments of the generated 3D root architectures were simulated using a multiscale flow an transport model. Total water and P uptake were then computed by adding up the uptake by all the root segments. KEY RESULTS Measurements showed that root architecture was significantly affected by the treatments. The moist, high P scenario had 2.8 times the root mass, double the numer of nodal roots, and more S-type laterals than the dry, low P scenario. Likewise, measured plant P uptake increased over threefold by increasing P and water supply. However, drying periods reduced P uptake at high but not at low P supply. Simulation results adequately predicted P uptake in all scenarios when the Michaelis Menten constant (Km) was corrected for diffusion limitation. They showed that the key drivers for P uptake are the different types of laterals (i.e. S- and L-type) and growing root tips . The L-type laterals become more important for overall water and P uptake than the S-type laterals in the dry scenarios. This is true across all the P treatments, but the effect is more pronounced as the P availability decreases. CONCLUSIONS This functional-structural model can predict the function of specific rice roots in terms of P and water uptake under different P and water supplies, when the structure of the root system is known. A future challenge is to predict how root systems' structure responds to nutrient and water availability.