title: Root and soil hydraulic traits controlling plant water status under drought
Creators: name: Abdalla, Mohanned
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: eco-physiology
subject: leaf water potential
subject: plant hydraulics
subject: stomatal regulation
subject: transpiration rate
subject: 500
subject: 570
subject: 580

description: Plants are expected to experience intense and extended periods of drought in many regions worldwide. Stomatal regulation, which governs water loss and hence plant water use, has been proposed as a key feature facilitating plant adaptation to water-limited environments. Nevertheless, the fundamental question as to what triggers stomatal closure during soil drying remains disputed. Recent soil–plant models proposed that the loss in hydraulic conductivities is the driver of stomatal closure, stomata responding to non-linearity in the relationship between transpiration rate (E) and leaf water potential (ψleaf). However, the primary hydraulic constraint along the soil–plant system, being in the soil or an element of the plants, remains without consensus. The overall objective of my thesis was to investigate how below-ground biophysical processes impact stomatal regulation and plant water status, especially in drying soils. This thesis comprises six chapters: chapter one introduces the state-of-the-art, summarizes the main findings, and provides an outlook. I asked the following questions: do stomata close when the E(ψleaf)-relation becomes non-linear? And what is the primary hydraulic limitation along the soil–plant system (chapter two)? Would differences in the root system impact the relationships between stomatal conductance, E, and ψleaf during soil drying (chapter three)? What is the role of abscisic acid (ABA) in stomatal response to non-linearity (chapter four)? What is the impact of salt accumulation at the root surface on plant response to water stress (chapter five)? Do arbuscular mycorrhiza fungi (AMF) impact the gradients in water potential around roots and hence the soil–plant hydraulic conductance during soil drying (Chapter six)? To answer these questions, I combined novel experiments and a soil–plant hydraulic model to shed light on the role of below-ground processes on plant response to edaphic and atmospheric drought. The main hypotheses were: 1) stomata close at the onset of hydraulic non-linearity; 2) the loss in below-ground hydraulic conductances (namely soil, root and/or their interface) represent the primary driver of stomatal closure; 3) ABA plays a central role in plant response hydraulic non-linearity; 4) salt accumulation at the root surface limits transpiration by creating an osmotic gradient, especially as the soil dries; 5) AMF attenuate the drop in matric potential across the rhizosphere and hence enhance soil–plant hydraulic conductance under water stress. In chapter two, I combined a novel root pressure chamber system and a soil–plant hydraulic model to investigate the primary driver of stomatal closure in tomato during soil drying. The method accurately measures the E(ψleaf)-relation in intact plants. Additionally, I have measured E and ψleaf without plant pressurization. The E(ψleaf)-relation was linear in wet soil and non-linear as the soil dried. The comparison of pressurized and unpressurized plants revealed no decline in xylem hydraulic conductance, and the primary hydraulic limitation to E occurred within root-soil continuum, which proves the hypothesis that stomata close to obviate the non-linear drop in ψleaf. Thus, I conclude that, in tomatoes, a decline in root-soil hydraulic conductance was the primary driver of stomatal closure in drying soil. To investigate the impacts of below-ground hydraulics, namely root system, on stomatal regulation, I grafted tomato plants, having identical shoots and two contrasting roots, a short and a long one. The E(ψleaf)-relation varied between the two root systems, i.e., short-rooted plant exhibited a marked non-linearity. Stomatal closure matched the onset of hydraulic non-linearity and was significantly different between root systems. This proves there is no unique relationship between E and ψleaf and below-ground hydraulics play a pivotal role in plant response to soil drying. The role of ABA in stomatal response to hydraulic non-linearity was explored in chapter four. I used two tomato genotypes: ABA-deficient mutant and its parental line. The latter showed linear E(ψleaf)-relation, while the mutant exhibited non-linear relation already in wet soil. This initial finding proposes ABA as a potential mechanism allowing stomata to obviate hydraulic non-linearity. Salinity has an adverse effect on water availability. Such effects are highly dynamic, as they depend on soil moisture gradients, diffusion and convection, and are difficult to predict. Thus, in chapter five, I have demonstrated how salt accumulation at the root surface induces an osmotic stress which dictates a more negative leaf water potential in the predawn and successively throughout the daytime. AMF symbiosis plays a positive role in plant water status under drought. In chapter six, I have elucidated that AMF enhance plant water status by increasing soil-root hydraulic conductance during soil drying. These findings together suggest root-soil interface as a crucial biological hotspot that plays a major role in controlling plant water status. These findings disclose the tied link between stomatal regulation and below-ground hydraulics, and call for reconciling soil and root hydraulics and rhizosphere biophysical processes to fully understand and optimally predict plant behavior and adaptation to future climate change.