Model-based insights on the relationship between planar root length density observed in minirhizotron images and volumetric root length density in the field

Minirhizotrons (MR) enable non-destructive investigation of plant root systems in the field. However, MR images only provide information about root systems in the 2D plane, whose relationship to 3D root system measures remains unclear. This study uses model simulation to investigate the relationship between planar root length density (pRLD) as determined from MR images and volumetric root length density (vRLD) in the field. We set up a virtual MR facility resembling the field MR facilities in Selhausen. Root systems of maize and winter wheat were grown around horizontally laid rhizotubes in a virtual field setup using the root architecture model CPlantBox. We calculated pRLD from virtual MR images, as well as vRLD from virtual soil layers. Our simulations confirmed experimental observations of weak correlations between pRLD and vRLD in topsoil, and of strong correlations in subsoil. The ratio of vRLD to pRLD remained relatively constant across the entire subsoil depth. The greater the heterogeneity in the distribution of root length density or in the anisotropy of root growth across depth, the higher the ratio of vRLD to pRLD. Different numbers of MR images led to similar mean pRLD values if the MR images were distributed evenly along the rhizotube length. Larger rhizotube diameters resulted in lower vRLD-to-pRLD ratios, while different plant densities had no effect. Model simulation provides valuable insights into the factors influencing the relationship between pRLD and vRLD. It also draws attention to the potential and limitations of using minirhizotron image data, making it a useful complement to experimental studies. • The root architecture model CPlantBox was used to set up a virtual minirhizotron facility, and to derive planar root length density from virtual minirhizotron images, as well as volumetric root length density from virtual soil layers • Weak correlations between planar and volumetric root length density in the topsoil could be attributed to the inherent architecture of root systems • The ratio of volumetric to planar root length density remained relatively constant throughout the subsoil, indicating that planar root length density is an appropriate measure of relative root length density in the field • Even distribution of minirhizotron images along the length of the rhizotube proved important for obtaining accurate planar root length densities