title: Impact of heterogeneous soil water distribution on soil and plant water relations
Creators: name: Hayat, Faisal
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: lupin (Lupinus Albus L.)
subject: root architecture
subject: root pressure chamber
subject: transpiration rates
subject: Maize (Zea mays L)
subject: Pressure chamber
subject: Soil drying
subject: Stomatal closure
subject: nodal roots, neutron radiography, hydraulic redistribution, root growth
subject: 500

description: Soil moisture is highly variable in space and time and such variability impacts transpiration, locations of root water uptake and, ultimately, plant growth. Mechanisms by which soil drying impacts plant growth are complex and involve feedbacks between plant hydraulics, stomatal regulation and water distribution. Up to date, our understanding of how plants respond to the heterogeneous soil water contents remain controversial. The aim of this thesis was to understand the effect of soil drying on transpiration, leaf water potential, locations of root water uptake and hydraulic redistribution. The thesis is structured into four chapters where chapter 1 is a brief summary. In chapter 2, a method combining the root pressure chamber technique, which allows measuring the average suction in the leaves of intact transpiring plants exposed to soil drying, with a hydraulic model of root water uptake was introduced. Lupines were grown in PVC pots in a sandy soil which was partitioned into two layers separated by a layer of fine gravel acting as capillary. Three scenarios of soil water contents (wet-wet, dry-wet and dry-dry) were tested. A linear relation between transpiration rate and leaf water potential in all tested scenarios of soil water contents was observed, with a slope decreasing with decreasing water contents. Both a simplified and a complex 3D root architecture model were capable of reproducing this relation and the slopes. The soil-plant conductance in dry-wet and wet-wet scenarios decreased by a factor of 1.65 and 8.26 times compared to the conductance in the wet-wet scenario, respectively. This decrease in conductance indicated the limiting role of the soil conductivity on root water uptake. Furthermore, model simulations showed that at uniform soil water contents (wet-wet and dry-dry scenarios), the relative root water uptake depended uniquely on the root properties and its distribution along the root system did not vary with transpiration rate. In the dry-wet scenarios, root water uptake is shifted to the lower wet layer and both models predicted the occurrence of hydraulic lift in the upper dry soil layer. In chapter 3, the effect of soil drying on the decrease in the soil-plant conductance and stomatal regulation in maize was tested. A simple soil-plant hydraulic model was coupled with measured data from a root pressure chamber and sap flow sensors. Furthermore, transpiration rates for pressurized and not-pressurized plants were measured to test to what extent leaf suction controls stomata closure in drying soils. The results showed a linear relationship between transpiration and leaf water potential in wet soils, while non-linearity was observed at high transpiration rates in dry soil conditions. The soil-plant hydraulic model was capable of reproducing the measured relation. The non-linearity in this relationship corresponds to a decrease in soil-plant conductance, which is interpreted as a loss of hydraulic conductivity around the roots. Transpiration measurements for not-pressurized plants showed that stomata promptly closed when the soil-plant hydraulic conductance decreased. Stomatal regulation reduced transpiration when soil-plant hydraulic conductance dropped, preventing marked non-linearity in the relationship between transpiration and leaf water potential. Besides affecting soil-plant hydraulics and stomatal regulation, soil drying also impacts location of water uptake and hydraulic redistribution through the root system. In chapter 4, neutron radiography was used to visualize and quantify hydraulic redistribution and root growth in maize grown in soil with heterogeneous water distribution. Plants were grown in aluminum containers whose soil water contents were adjusted in both top and bottom layers to the following scenarios: i) dry-wet; and ii) wet-wet. We injected D2O in the bottom soil layers and imaged the root system in the top soil layers overnight. A diffusion-convection model was used to estimate hydraulic redistribution in the roots. During day, D2O was taken up by the roots in the bottom, wet soil layer and transported to the shoot. Overnight, D2O appeared also in nodal and lateral roots in the top compartment. There was a visible efflux of water from lateral roots into the soil (j_r=2.35×10-7 cm s-1). The efflux from nodal roots depended on their length and growth rate and a fraction of the redistributed water flew toward the root tips to sustain their growth. To summarize, I demonstrated the importance of soil drying on the relations between transpiration rate, leaf water potential and soil-plant hydraulic conductance. Stomatal response to soil drying prevented the drop in soil plant-hydraulic conductance by limiting the transpiration rate in drying soils. In natural conditions soils dry heterogeneously, which impact the locations and dynamics of root water uptake, including hydraulic redistribution. In a simplified approach, I have shown the importance of hydraulic redistribution to sustain root growth. These results have been obtained in simplified lab experiments that allowed me for developing the methods. Field measurements in more natural conditions are needed to investigate the ecological and agricultural implications of my findings.