Tracing root-felt sodium concentrations under different transpiration rates and salinity levels

Aims: (1) Monitoring ‘root-felt’ salinity by using rhizoslides as a non-invasive method, (2) Studying how transpiration rate, salinity in irrigation water, and root water uptake affect sodium distribution around single roots, (3) Interpreting experimental results by using simulations with a 3-D root system architecture model coupled with water flow and solute transport models. Methods: Tomato plants were grown on rhizoslides under various salinity levels and two transpiration rates: high and low. Daily root images were processed with GIMP and incorporated into a 3-D numerical model. The experiments were simulated with R-SWMS, a 3-dimensional numerical model that simulates water flow and solute transport in soil, into the root and inside root systems. Results: Both experimental and simulation results displayed higher root-felt sodium concentrations compared with the bulk concentrations, and larger accumulation at higher transpiration rate. The simulations illustrated that the root-felt to bulk concentration ratio changed during the experiment depending both on the irrigation water salinity and transpiration rate. Conclusions: Changes in sodium concentrations with transpiration rates are most likely caused by root water uptake and ion exclusion. Simulation results indicate that root-scale process models are required to link root system architecture, environmental, and soil conditions with root-felt salinities.