In case of low soil water availability the main driver of transpiration reduction by plants is via stomatal closure. We tried to model different stomatal behavior and examined the influence on root water uptake. We compared isohydric behavior, where leaf water potential is kept constant if a certain threshold is reached, with a behavior where stomatal conductance is as a function of leaf water potential and a hormone concentration. Hormone production happens at the root tips and is increasing with increasing root water potential. Thus the drier the soil the more hormones are produced.
We used a 3D soil and root model with fully coupled soil and root water flow to conduct a virtual experiment for a vertical split-root system. This setup is often used to observe the influences of artificially created soil heterogeneities on plants, where roots of a single plant are forced to grow in two independent soil compartments. The two compartments were hydraulically independent and could be irrigated independently from each other. We compared uniform and non-uniform irrigation. The total amount of irrigation remained constant for both cases and was slightly higher than potential transpiration. To simulate hormone production a particle tracking algorithm was implemented. The particles that were produced at the root tips as a function of root water potential were transported via advection towards the root collar. The root collar had to be used as a proxy for the above ground part of the plant as this is not modeled explicitly. Actual transpiration was computed as a function of either collar water potential or as a combined function of collar water potential and hormone concentration [Tardieu and Davies, 1993].
We observed different transpiration reduction for the two different mechanisms and could see a large influence of the transport velocity in the combined approach. As soon as water uptake for the drying part ceased, the transport of particles was stopped and the transpiration began to increase again. Furthermore we could show that compensation of root water uptake is not affected by adjustment of transpiration. Comparison of virtual experiments with data from literature [Dodd et al., 2008] showed qualitative correlations for leaf hormone concentration over time.