Axial conduit widening, tree height, and height growth rate set the hydraulic transition of sapwood into heartwood

The size-related xylem adjustments required to maintain a constant leaf-specific sapwood conductance (KLEAF) with increasing height (H) are still under discussion. Alternative hypotheses are that: (i) the conduit hydraulic diameter (Dh) at any position in the stem and/or (ii) the number of sapwood rings at stem base (NSWr) increase with H. In addition, (iii) reduced stem elongation (ΔH) increases the tip-to-base conductance through inner xylem rings, thus possibly the NSWr contributing to KLEAF. A detailed stem analysis showed that Dh increased with the distance from the ring apex (DCA) in all rings of a Picea abies and a Fagus sylvatica tree. Net of DCA effect, Dh did not increase with H. Using sapwood traits from a global dataset, NSWr increased with H, decreased with ΔH, and the mean sapwood ring width (SWrw) increased with ΔH. A numerical model based on anatomical patterns predicted the effects of H and ΔH on the conductance of inner xylem rings. Our results suggest that the sapwood/heartwood transition depends on both H and ΔH, and is set when the carbon allocation to maintenance respiration of living cells in inner sapwood rings produces a lower gain in total conductance than investing the same carbon in new vascular conduits.

Exploring within-plant hydraulic trait variation: A test of the vulnerability segmentation hypothesis

Observations show vulnerability segmentation between stems and leaves is highly variable within and between environments. While a number of species exhibit conventional vulnerability segmentation (stem P 50 < ${P}_{50}\lt $ leaf P 50 ${P}_{50}$ ), others exhibit no vulnerability segmentation and others reverse vulnerability segmentation (stem P 50 > ${P}_{50}\gt $ leaf P 50 ${P}_{50}$ ). We developed a hydraulic model to test hypotheses about vulnerability segmentation and how it interacts with other traits to impact plant conductance. We do this using a series of experiments across a broad parameter space and with a case study of two species with contrasting vulnerability segmentation patterns: Quercus douglasii and Populus trichocarpa. We found that while conventional vulnerability segmentation helps to preserve conductance in stem tissues, reverse vulnerability segmentation can better maintain conductance across the combined stem-leaf hydraulic pathway, particularly when plants have more vulnerable P 50 ${P}_{50}$ s and have hydraulic segmentation with greater resistance in the leaves. These findings show that the impacts of vulnerability segmentation are dependent upon other plant traits, notably hydraulic segmentation, a finding that could assist in the interpretation of variable observations of vulnerability segmentation. Further study is needed to examine how vulnerability segmentation impacts transpiration rates and recovery from water stress.

Leaf elongation response to blue light is mediated by stomatal-induced variations in transpiration in Festuca arundinacea

Reduced blue light irradiance is known to enhance leaf elongation rate (LER) in grasses, but the mechanisms involved have not yet been elucidated. We investigated whether leaf elongation response to reduced blue light could be mediated by stomata-induced variations of plant transpiration. Two experiments were carried out on tall fescue in order to monitor LER and transpiration under reduced blue light irradiance. Additionally, LER dynamics were compared with those observed in the response to vapour pressure deficit (VPD)-induced variations of transpiration. Finally, we developed a model of water flow within a tiller to simulate the observed short-term response of LER to various transpiration regimes. LER dramatically increased in response to blue light reduction and then reached new steady states, which remained higher than the control. Reduced blue light triggered a simultaneous stomatal closure which induced an immediate decrease of leaf transpiration. The hydraulic model of leaf elongation accurately predicted the LER response to blue light and VPD, resulting from an increase in the growth-induced water potential gradient in the leaf growth zone. Our results suggest that the blue light signal is sensed by stomata of expanded leaves and transduced to the leaf growth zone through the hydraulic architecture of the tiller.

Assessing and modeling biocatalysis in field denitrification beds reveals key influencing factors for future constructions

Environmental contamination with fertilizers is threatening biodiversity in many ecosystems due to nitrate-based eutrophication. One opportunity for a cost-efficient nitrate elimination are denitrification beds in which a microbial community thrives under anoxic conditions with polymeric plant material as a carbon and an electron source. Incoming nitrate is used as electron acceptor and reduced to molecular nitrogen. Projects realizing denitrification beds in field scale are sparse and robust data on their efficiency throughout the year mostly not available. This study analyzed the nitrate elimination efficiency and microbiology of a 216 m³ denitrification bed over the time course of more than three years. Phylogenetic as well as transcriptomic analysis revealed that the reactor contained a biofilm community growing on the surface of the wood chips and a planktonic community. Both differed in composition but their variance was affected only to a minor extend by seasonal temperature changes. Cellulose degradation was mainly conducted by the biofilm population while denitrification was mostly conducted by the planktonic community. Methanogens were detectable only to a very minor extend. Using online data from the nitrate concentration of in- and outflowing water as well as a hydrological model to predict the water inflow, it was possible to establish a process model that sufficiently describes the denitrification process. This model clearly indicates that the denitrification efficiency is mostly impacted by temperature and hydraulic retention time. It also suggests that the simple design of the denitrification bed most likely leads to different flow paths through the reactor depending on the volumetric flow rate. This study allows for the first time a robust estimation of the necessary reactor size for nitrate removal in a moderate continental climate setting. It also suggests how future denitrification beds could be improved for better performance.

A hydraulic model of cardiovascular physiology and pathophysiology embedded into a computer-based teaching system for student training in laboratory courses

Functional understanding of the different parts of the cardiovascular system is essential for an insight into pathomechanisms of numerous diseases. During training cardiovascular physiology, students and early-stage medical personnel should understand the role of different functional parameters for systolic and diastolic blood pressure, as well as for blood flow. The impact of isolated parameters can only be studied in models. Here physical hydraulic models are an advantage in which the students have a direct contact to the mechanical properties of the circulatory system. But these models are often difficult to handle. The aim of the present study was to develop a comprehensive model of the cardiovascular system, including a mechanical heart with valves, an elastic aorta, a more rigid peripheral artery system, a total peripheral resistance, and a venous reservoir representing the variable cardiac preload. This model allows one to vary systematically several functional parameters and to continuously record their impact on pressure and flow. This model is embedded into a computer-based teaching system (LabTutor) in which the students are guided through the handling of the model (as well as the systematic variation of parameters), and the measured data can be analyzed. This hybrid teaching system, which is routinely integrated in physiology laboratory courses of medical students, allows students to work with a complex hydraulic model of the cardiovascular system and to analyze systematically the impact of influencing variables (e.g., increased peripheral resistance or changed cardiac preload) as well as pathophysiological dysfunctions (e.g., reduced aortic compliance).

Stomatal Sensitivity to Vapor Pressure Deficit and the Loss of Hydraulic Conductivity Are Coordinated in Populus euphratica, a Desert Phreatophyte Species

There are considerable variations in the percentage loss of hydraulic conductivity (PLC) at mid-day minimum water potential among and within species, but the underpinning mechanism(s) are poorly understood. This study tested the hypothesis that plants can regulate leaf specific hydraulic conductance (K _l) via precise control over PLC under variable ΔΨ (water potential differential between soil and leaf) conditions to maintain the -m/b constant (-m: the sensitivity of stomatal conductance to VPD; b: reference stomatal conductance at 1.0 kPa VPD), where VPD is vapor pressure deficit. We used Populus euphratica, a phreatophyte species distributed in the desert of Northwestern China, to test the hypothesis. Field measurements of VPD, stomatal conductance (g _s), g _s responses to VPD, mid-day minimum leaf water potential (Ψ _lmin), and branch hydraulic architecture were taken in late June at four sites along the downstream of Tarim River at the north edge of the Taklamakan desert. We have found that: 1) the -m/b ratio was almost constant (=0.6) across all the sites; 2) the average Ψ ₅₀ (the xylem water potential with 50% loss of hydraulic conductivity) was -1.63 MPa, and mid-day PLC ranged from 62 to 83%; 3) there were tight correlations between Ψ ₅₀ and wood density/leaf specific hydraulic conductivity (k _l) and between specific hydraulic conductance sensitivity to water potential [d(k _s)/dln(-Ψ)] and specific hydraulic conductivity (k _s). A modified hydraulic model was applied to investigate the relationship between g _s and VPD under variable ΔΨ and K _l conditions. It was concluded that P. euphratica was able to control PLC in order to maintain a relatively constant -m/b under different site conditions. This study demonstrated that branchlet hydraulic architecture and stomatal response to VPD were well coordinated in order to maintain relatively water homeostasis of P. euphratica in the desert. Model simulations could explain the wide variations of PLC across and within woody species that are often observed in the field.

A systematic mixed-integer differential evolution approach for water network operational optimization

The operational management of potable water distribution networks presents a great challenge to water utilities, as reflected by the complex interplay of a wide range of multidimensional and nonlinear factors across the water value chain including the network physical structure and characteristics, operational requirements, water consumption profiles and the structure of energy tariffs. Nevertheless, both continuous and discrete actuation variables can be involved in governing the water network, which makes optimizing such networks a mixed-integer and highly constrained decision-making problem. As such, there is a need to situate the problem holistically, factoring in multidimensional considerations, with a goal of minimizing water operational costs. This paper, therefore, proposes a systematic optimization methodology for (near) real-time operation of water networks, where the operational strategy can be dynamically updated using a model-based predictive control scheme with little human intervention. The hydraulic model of the network of interest is thereby integrated and successively simulated with different trial strategies as part of the optimization process. A novel adapted mixed-integer differential evolution (DE) algorithm is particularly designed to deal with the discrete-continuous actuation variables involved in the network. Simulation results on a pilot water network confirm the effectiveness of the proposed methodology and the superiority of the proposed mixed-integer DE in comparison with genetic algorithms. It also suggests that 23.69% cost savings can be achieved compared with the water utility's current operational strategy, if adaptive pricing is adopted for all the pumping stations.