Application of a single root-scale model to improve macroscopic modeling of root water uptake: focus on osmotic stress

Application of a single root-scale model to improve macroscopic modelingof root water uptake: focus on osmotic stressHelena Jorda (1), Adi Perelman (2), Naftali Lazarovitch (2), and Jan Vanderborght (3)(1) Division of Soil and Water Management, KU Leuven, Leuven, Belgium (helena.jordaguerra@kuleuven.be), (2) FrenchAssociates Institute for Agriculture and Biotechnology of Drylands, Jacob Blaustein Institutes for Desert Research,Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel, (3) Institute of Bio- and Geosciences, AgrosphereInstitute, IBG-3, ForschungszentrumJülich GmbH, Jülich, GermanyRoot water uptake is a fundamental process in the hydrological cycle and it largely regulates the water balancein the soil vadose zone. Macroscopic stress functions are currently used to estimate the effect of salinity on rootwater uptake. These functions commonly assume stress to be a function of bulk salinity and of the plant sensitivityto osmotic stress expressed as the salinity at which transpiration is reduced by half or so called tolerance value.However, they fail to integrate additional relevant factors such as atmospheric conditions or root architectural traits.We conducted a comprehensive simulation study on a single root using a 3-D physically-based model thatresolves flow and transport to individual root segments and that couples flow in the soil and root system. Theeffect of salt concentrations on root water uptake was accounted for by including osmotic water potential gradientsbetween the solution at the soil root interface and the root xylem sap in the hydraulic gradient between thesoil and root. A large set of factors were studied, namely, potential transpiration rate and dynamics, root lengthdensity (RLD), irrigation water quality and irrigation frequency, and leaching fraction. Results were fitted to themacroscopic function developed by van Genuchten and Hoffman (1984) and the dependency of osmotic stress andthe fitted macroscopic parameters on the studied factors was evaluated.Osmotic stress was found to be highly dependent on RLD. Low RLDs result in a larger stress to the plantdue to high evaporative demand per root length unit. In addition, osmotic stress was positively correlated topotential transpiration rate, and sinusoidal potential transpiration lead to larger stress than when imposed as aconstant boundary condition.Macroscopic parameters are usually computed as single values for each crop and used for the entire grow-ing season. However, our study shows that both tolerance value and shape parameter p from the van Genuchtenand Hoffman (1984) function were highly dependent on both potential transpiration and RLD. Plant salt tolerancewas lower under high evaporative demand and lower RLD. In addition, the shape of the stress curve, which isdefined by p, was found to be steeper under larger RLD and low transpiration rate. Time-variant macroscopicparameters based on knowledge of current potential transpiration rate per root unit length would be moreconvenient to accurately predict osmotic stress, and hence root water uptake, during a growing season. In a nextstep, simulations considering the whole root systems will be conducted to assess how macroscopic parameters arealso related to root architectural characteristics.van Genuchten, M.T., Hoffman, G., 1984. Analysis of crop production. Soil Salin. Irrig. Springer Berl.258–27