Linking micro-X-ray fluorescence spectroscopy and X-ray computed tomography with model simulation explains differences in nutrient gradients around roots of different types and ages.

Plant roots create chemical gradients within the rhizosphere, but little information exists on the effect of root properties on the distribution of chemical gradients. The research aim was to analyse and model the effects of root type and age, radial root geometry and root hairs on nutrient gradients in the rhizosphere. Using micro-X-ray fluorescence spectroscopy (μ-XRF) combined with X-ray computed tomography (X-ray CT), we analysed nutrient gradients around root segments with different diameters and ages of two Zea mays genotypes (wild-type and root hair defective mutant) growing in two substrates (loam and sand). Gradients of key nutrients were compared with gradients obtained by a process-based, radially symmetric 1D rhizosphere model. Results show that root hairs matter for nutrient uptake during supply limitation (phosphorus (P)), but not when it is limited by uptake kinetics (calcium (Ca), sulphur (S)). Higher Ca and S accumulation was observed at the surface of older and thicker root segments than at younger and thinner root segments. Micro-XRF proved suitable for the detection of nutrient gradients of Ca and S, but not of P. While continuum modelling was well suited to explain observed nutrient gradients, it was less effective in representing pore-related phenomena, such as precipitation reactions, which calls for new homogenization approaches.