Water Uptake Dynamics Research Articles

Water/Methanol Transport in Polyethylene Based Block Copolymers as Anion Conductors for Applications in Fuel Cells

Anion exchange membranes have been a central point of focus for the low temperature fuel cell application. The primary goal of AEMs in fuel cells is to transport hydroxide (OH-) ions from cathode to anode and to act as a separator between the electrodes. In this work we have studied a block copolymer of polyethylene and polyvinyl benzyl trimethyl ammonium bromide (PE-b-PVBTMA[Br], Fig. 1) for various transport properties. By using the dynamic vapor sorption apparatus we were able to study water and methanol uptake dynamics for this AEM. Fickian diffusion coefficients for both water and methanol were calculated during this study giving idea about the solvent uptake kinetics.We also measured the conductivity of the AEM under 95% relative humidity (Figure 2). It was found that the AEM followed VFT theory behavior for transporting ions through it as opposed to the classical Arrhenius behavior. VFT behavior suggests that ion transport in the membrane is facilitated by the polymer chain motion as opposed to ion transport in water which is governed mainly by the Arrhenius type of behavior.We also report methanol/water ion transport using the pulse gradient stimulated spin echo (PGSTE) NMR technique to determine the tortuosity of the membrane along with self diffusion of each species. Acknowledgements: We would like to thank the Army Research Office for funding this Multidisciplinary University Research Initiative under contract #W911NF-10-1-0520 and the Colorado School of Mines NMR facility, funded by National Science Foundation under the grant CHE-0923537. We would like to thank Yifan Li, D M Knauss for providing with the polymer for this study.

Modeling potato root growth and water uptake under water stress conditions

Potato (Solanum tuberosum L.) is considered a drought sensitive crop. Accurate simulation of root growth is critical for estimating water uptake dynamics. However, data required to build and test advanced potato root simulation approaches is lacking. Previously unpublished data from our soil–plant–atmosphere-research (SPAR) chambers was used to evaluate a new two-dimensional diffusive root growth module linked to the existing potato model SPUDSIM. The root module consisted of diffusive parameters controlling the direction of root growth in horizontal and vertical directions, and an additional convective term for vertical growth within the soil. This modified SPUDSIM was tested against observed SPAR data which consisted of root distribution in the soil profile and organ dry weights (DW) at harvest, plus daily water uptake patterns for each of six different irrigation treatments. The difference between simulated and observed DW data was within two standard errors for most plant organs—root DW was over-predicted for well-watered plants. Spatial and temporal patterns of root distribution and water contents were reproduced well. However, the model tended to over-estimate water uptake from soil layers closer to the surface. Differences in simulated root growth patterns among irrigation treatments were the result of fluctuations in soil water status, bulk density, and root density which, in turn, affected the amount of carbon allocated to the roots in different soil layers and the value of the convective term. These results suggest the new module will provide more reliable predictions of potato water uptake for improved agricultural decision support tools.

Effect of Irrigation Pattern and Timing on Root Density of Young Citrus Trees Infected with Huanglongbing Disease

Citrus (Citrus sp.) root length density (RLD) can help in understanding and predicting nutrient and water uptake dynamics. A study was conducted at two sites in Florida to investigate root and water distribution patterns among different irrigation and fertigation systems. The results over the 2 years showed that RLD was highest in the 0- to 15-cm soil depth and decreased with depth for all treatments at both sites. About 64% to 82% of the fibrous roots (<1 mm diameter) were concentrated in the irrigated zones of drip- and microsprinkler-irrigated trees and 18% to 36% were found in the nonirrigated zones at the Spodosol site (SS). At the Entisol site (ES), the RLD (<0.5 mm diameter) in the 0- to 15-cm depth soil for intensive microsprinkler or drip irrigation was 3- to 4-fold (nonirrigated zone) and 4- to 7-fold (irrigated zone) greater at the 0- to 15-cm soil depth than that for conventional irrigation system. The trees at SS were symptomatic for Huanglongbing (HLB; Candidatus Liberibacter asiaticus) in the second year, while those at ES were asymptomatic throughout the study. This might have limited the density and extent of root distribution at SS. The water contents remained either close to or slightly above the field capacity. The results showed higher RLD for intensive irrigation and fertigation practices in irrigated zones compared with conventional grower applications suggesting greater water and nutrient uptake potential for the former.

Simulation of the evolution of root water foraging strategies in dry and shallow soils.

The dynamic structural development of plants can be seen as a strategy for exploiting the limited resources available within their environment, and we would expect that evolution would lead to efficient strategies that reduce costs while maximizing resource acquisition. In particular, perennial species endemic to habitats with shallow soils in seasonally dry environments have been shown to have a specialized root system morphology that may enhance access to water resources in the underlying rock. This study aimed to explore these hypotheses by applying evolutionary algorithms to a functional-structural root growth model. A simulation model of a plant's root system was developed, which represents the dynamics of water uptake and structural growth. The model is simple enough for evolutionary optimization to be computationally feasible, yet flexible enough to allow a range of structural development strategies to be explored. The model was combined with an evolutionary algorithm in order to investigate a case study habitat with a highly heterogeneous distribution of resources, both spatially and temporally--the situation of perennial plants occurring on shallow soils in seasonally dry environments. Evolution was simulated under two contrasting fitness criteria: (1) the ability to find wet cracks in underlying rock, and (2) maximizing above-ground biomass. The novel approach successfully resulted in the evolution of more efficient structural development strategies for both fitness criteria. Different rooting strategies evolved when different criteria were applied, and each evolved strategy made ecological sense in terms of the corresponding fitness criterion. Evolution selected for root system morphologies which matched those of real species from corresponding habitats. Specialized root morphology with deeper rather than shallower lateral branching enhances access to water resources in underlying rock. More generally, the approach provides insights into both evolutionary processes and ecological costs and benefits of different plant growth strategies.

A modeling approach to determine the importance of dynamic regulation of plant hydraulic conductivities on the water uptake dynamics in the soil-plant-atmosphere system

We present here a new model, PlaNet-Maize, with the purpose of investigating the effect of environmental and endogenous factors on the growth and water relations of the maize plant. This functional–structural plant model (FSPM) encompasses the entire soil-plant-atmosphere continuum with a sub-organ resolution. The model simulates the growth and development of an individual maize plant and the flux of water through the plant structure, from the rhizosphere to the leaf boundary layer. Leaf stomatal conductance and root radial and axial conductivities are considered as functions of local water potential. Finally, a simple carbon allocation rule is included in the model to allow the feedback effect of water deficit on plant growth. The model was successfully used to reproduce experimental plant hydraulic behavior in response to water deficit. The quantitative contribution of leaf conductance and root conductivities were assessed individually and in combination.Our results highlight the importance of regulating hydraulic properties in FSPM as these can strongly modify the water uptake dynamics and lead to emerging water uptake behaviors. The modeling results also indicate that plant hydraulic properties can theoretically be tailored to improve plant water use in challenging environments.

Dynamics of root water uptake and water use efficiency under alternate partial root-zone irrigation

The distribution of root length density of maize in ridge culture under alternate partial root-zone furrow irrigation (APFI) and conventional furrow irrigation (CFI) was investigated, and two-dimensional models of root length density and root uptake were developed to investigate the water use efficiencies under different furrow irrigation modes. Results indicated that APFI increased root depth and horizontal extended distance of maize, as well as water use efficiency (WUE). Values of the mean absolute error (MAE), the root mean square error (RMSE), and the index of agreement (di) between measured and simulated values of root length density of maize under APFI ranged from 0.26 to 0.68, 0.03 to 1.51, and 0.64 to 0.85 cm m−3, respectively; and the coefficient of determination (R2) was greater than 0.80. Values of MAE, RMSE, and di between measured and simulated values of root length density of maize under CFI ranged from 0.01 to 0.26, 0.01 to 0.60, and 0.61 to 0.96 cm m−3, respectively; and R2 was higher than 0.86. Dynamics of root uptake at different sites were simulated using the model of root water uptake. Root growth of maize under furrow irrigation was modeled using the 2D model of root length density with reasonable accuracy. Compared with CFI, water consumption rate of the maize in APFI was lower in key water requirement period, root uptake rate was higher in the end period, and WUE in APFI was higher by 5% than that in CFI.

Dynamic Wetting on a Thin Film of Soluble Polymer: Effects of Nonlinearities in the Sorption Isotherm

The wetting dynamics of a solvent on a soluble substrate interestingly results from the rates of the solvent transfers into the substrate. When a supported film of a hydrosoluble polymer with thickness e is wet by a spreading droplet of water with instantaneous velocity U, the contact angle is measured to be inversely proportionate to the product of thickness and velocity, eU, over two decades. As for many hydrosoluble polymers, the polymer we used (a polysaccharide) has a strongly nonlinear sorption isotherm φ(a(w)), where φ is the volume fraction of water in the polymer and aw is the activity of water. For the first time, this nonlinearity is accounted for in the dynamics of water uptake by the substrate. Indeed, by measuring the water content in the polymer around the droplet φ at distances as small as 5 μm, we find that the hydration profile exhibits (i) a strongly distorted shape that results directly from the nonlinearities of the sorption isotherm and (ii) a cutoff length ξ below which the water content in the substrate varies very slowly. The nonlinearities in the sorption isotherm and the hydration at small distances from the line were not accounted for by Tay et al., Soft Matter 2011, 7, 6953. Here, we develop a comprehensive description of the hydration of the substrate ahead of the contact line that encompasses the two water transfers at stake: (i) the evaporation-condensation process by which water transfers into the substrate through the atmosphere by the condensation of the vapor phase, which is fed by the evaporation from the droplet itself, and (ii) the diffusion of liquid water along the polymer film. We find that the eU rescaling of the contact angle arises from the evaporation-condensation process at small distances. We demonstrate why it is not modified by the second process.

First steps towards an explicit modelling of ABA production and translocation in relation with the water uptake dynamics

Water flow in the plant continuum is thought to be controlled by the leaf, xylem and the root radial conductances. Leaf conductance is widely recognized as the master control in the system as it can act as a switch in the water flow, even under high transpiration demand. The control of the stomatal aperture by the plant has been the object of numerous research projects since decades and different models have been developed to study stomatal regulation. The plant hormone abscisic acid (ABA) produced in the leaves acts as an endogenous signal triggering stomatal closure, but is also thought to act as a long distance signal that is produced by roots experiencing water deficit. However, the relative contributions of leaf-supplied ABA, root-supplied ABA and xylem water potential in the regulation of water flow remains a matter of controversy. Using a novel functional structural plant model (PlaNet-Maize-ABA), we report here a first attempt to explicitly simulate ABA production, degradation, translocation in all plant organs and their effect on the transpiration flow.

Transpirational demand affects aquaporin expression in poplar roots

Isohydric plants tend to maintain a water potential homeostasis primarily by controlling water loss via stomatal conductance. However, there is accumulating evidence that plants can also modulate water uptake in a dynamic manner. The dynamics of water uptake are influenced by aquaporin-mediated changes in root hydraulics. Most studies in this area have been conducted on herbaceous plants, and less is known about responses of woody plants. Here a study was conducted to determine how roots of hybrid poplar plants (Populus trichocarpa×deltoides) respond to a step change in transpirational demand. The main objective was to measure the expression of selected aquaporin genes and to assess how transcriptional responses correspond to changes in root water flow (Q R) and other parameters of water relations. A subset of plants was grown in shade and was subsequently exposed to a 5-fold increase in light level. Another group of plants was grown at ~95% relative humidity (RH) and was then subjected to lower RH while the light level remained unchanged. Both plant groups experienced a transient drop in stem water potentials. At 28h after the increase in transpirational demand, water potentials recovered. This recovery was associated with changes in the expression of PIP1 and PIP2 subfamily genes and an increase in Q R. Stomata of plants growing at high RH were larger and showed incomplete closure after application of abscisic acid. Since stomatal conductance remained high and unchanged in these plants, it is suggested that the recovery in water potential in these plants was largely driven by the increase in Q R.

Water use of winter cereals under well-watered and drought-stressed conditions

A reduction in the water available to plants will lend increasing importance to the dynamics of water uptake and to the water use efficiency (WUE) of cereals. The effect of drought on the water use efficiency of winter cereals was investigated in a greenhouse experiment in the Centre for Agricultural Research. The effect of water deficiency on the water use properties was studied by measuring changes in the grain weight, thousand-kernel weight and aboveground biomass. The water use efficiency of wheat varieties generally ranged from 1.5–2.3 kg/m<sup>3</sup> and 1.06–2.0 kg/m<sup>3</sup> in the case of optimum and limited water supplies, respectively, while these figures were 1.4 kg/m<sup>3</sup> and 0.8 kg/m<sup>3</sup> for winter barley and 0.8 kg/m<sup>3</sup> and 0.5 kg/m<sup>3</sup> for winter oat. Investigation on the relationship between harvest index (HI) and WUE was found that the harvest index is only one indicator of drought tolerance; but the stability of HI under non-optimum environmental conditions also needs to be determined.

Root Water Uptake and Rhizosphere Dynamics

Root Water Uptake and Rhizosphere Dynamics

Plant water uptake strategies to cope with stochastic rainfall

We develop and compare two hydraulically-based schemes of the xylem structure of an individual plant. The dynamics of water uptake are analyzed under random conditions through the modeling of rainfall as a stochastic process. The two hydraulic schemes differ in the modeling of the root ability to cooperate, i.e., the capacity of roots to increase the water uptake from the wetter soil layers when other parts of the soil are dry (compensation effect) and to transfer water from moister into drier soil layers (water redistribution effect). Both compensation and water redistribution are direct consequences of the hydraulic structure of the root system, which is modeled here considering two contrasting cases corresponding to non-interacting uptake paths from different soil layers, and converging uptake paths. Only the latter hydraulic architecture allows for compensation and water redistribution. Another important difference between the hydraulic schemes is the stomatal response to soil dryness. When the soil water is unevenly distributed in the soil layers, the differences in the hydraulic schemes emerge. In semi-arid climates, plants characterized by a cooperative root system are shown to be less prone to water stress. In contrast, plants with non-interacting roots result to be better fit to humid climates, where the probability of droughts is small.

Root Water Uptake Dynamics of Cichorium intybus var. sativum Under Water-Limited Conditions

Chicory (Cichorium intybus L.) is a cash crop cultivated in Western Europe for inulin production. Due to actual and future climate changes, this plant could be exposed to severe water stress at the end of its growing period, leading to a decrease of its yield. The aim of this work was to investigate the chicory root water uptake dynamics and the plant ability to compensate a lack of water in the upper horizons. We performed a controlled experiment with 3 replicates under contrasted irrigation scenarios. We observed that, in case of drought, total root length decreased and root profiles developed deeper. We successfully used a one-dimensional Richards-based model with a stress function and a compensation mechanism (Hydrus 1-D) to inversely characterize the dynamics of the actual sink-term profiles under both irrigation scenarios. We could also use the model to assess the compensation thanks to a weighted stress index that is consistent between replicates. The extraction profiles evolved differently under water-deficit and controlled situations. The passive compensation mechanism allowed chicory roots under water-limited conditions to take water deeper in the soil, where they had only few lateral roots. We found that, in case of drought, compensation started before the plants had to reduce their transpiration rate. Because the soil kept drying out, compensation was not sufficient anymore, and the plants had to decrease their transpiration some days later. However, chicories maintained their metabolism and continued to transpire and to growth slowly. This allowed them to adapt thanks to an active compensation mechanism, by generating new lateral roots in wetter horizons. This study also showed that there was no unique Feddes stress parameter set able to describe plant behavior under contrasted irrigation conditions or even under different plant development stages.

A Novel Approach to the Water Uptake Dynamics in Roots of Maize, Wheat and Barley Under Salt Stress

The water uptake dynamics in maize, wheat, and barley under salt stress were investigated with a xylem pressure probe. The average xylem pressure responses to salt stress in the three plants were 36, 93, and 89% of the osmotic stresses for maize, wheat, and barley, respectively, which are significantly smaller than the magnitude of the osmotic stresses being applied. In order to explain the thermodynamic discrepancies among the water potential changes in the root xylem of the three plants, a novel approach, tentatively named the “symplastic flow dilution model” was proposed in this paper. The model was presented in an attempt to give answers to the problem of how the roots under salt stress could absorb water when the water potential of the xylem sap is considerably higher than that of the solution in the root ambient. According to the model, the salt solution in the microenvironment of the endodermis of a root was diluted to some extent by the efflux from cells so the central stele of the root is not exposed to the same solution bathing the root with the same salt concentration. In contrast, we also presented another approach, the “reflection coefficient progression approach”, which was less likely to be true because it requires a considerable amount of solute to be transported into the root xylem when the salt stress is severe.

Water uptake dynamics under progressive drought stress in diverse accessions of the OryzaSNP panel of rice (Oryza sativa).

In addition to characterising root architecture, evaluating root water uptake ability is important for understanding drought response. A series of three lysimeter studies were conducted using the OryzaSNP panel, which consists of 20 diverse rice (Oryza sativa L.) genotypes. Large genotypic differences in drought response were observed in this genotype panel in terms of plant growth and water uptake. Total water uptake and daily water uptake rates in the drought-stress treatment were correlated with root length density, especially at depths below 30cm. Patterns of water uptake among genotypes remained consistent throughout the stress treatments: genotypes that initially extracted more water were the same genotypes that extracted more water at the end of the study. These results suggest that response to drought by deep root growth, rather than a conservative soil water pattern, seems to be important for lowland rice. Genotypes in the O. sativa type aus group showed some of the greatest water uptake and root growth values. Since the OryzaSNP panel has been genotyped in detail with SNP markers, we expect that these results will be useful for understanding the genetics of rice root growth and function for water uptake in response to drought.

Capillary filling with giant liquid/solid slip: Dynamics of water uptake by carbon nanotubes

This article discusses the way the standard description of capillary filling dynamics has to be modified to account for liquid/solid slip in nanometric pores. It focuses, in particular, on the case of a large slip length compared to the pore size. It is shown that the liquid viscosity does not play a role, and that the flow is only controlled by the friction coefficient of the liquid at the wall. Moreover, in the Washburn regime, the filling velocity does not depend on the tube radius. Finally, molecular dynamics simulations suggest that this standard description fails to describe the early stage of capillary filling of carbon nanotubes by water, since viscous dissipation at the tube entrance must be taken into account.

A one-dimensional model of water flow in soil-plant systems based on plant architecture

The estimation of root water uptake and water flow in plants is crucial to quantify transpiration and hence the water exchange between land surface and atmosphere. In particular the soil water extraction by plant roots which provides the water supply of plants is a highly dynamic and non-linear process interacting with soil transport processes that are mainly determined by the natural soil variability at different scales. To better consider this root-soil interaction we extended and further developed a finite element tree hydro-dynamics model based on the one-dimensional (1D) porous media equation. This is achieved by including in addition to the explicit three-dimensional (3D) architectural representation of the tree crown a corresponding 3D characterisation of the root system. This 1D xylem water flow model was then coupled to a soil water flow model derived also from the 1D porous media equation. We apply the new model to conduct sensitivity analysis of root water uptake and transpiration dynamics and compare the results to simulation results obtained by using a 3D model of soil water flow and root water uptake. Based on data from lysimeter experiments with young European beech trees (Fagus silvatica L.) is shown, that the model is able to correctly describe transpiration and soil water flow. In conclusion, compared to a fully 3D model the 1D porous media approach provides a computationally efficient alternative, able to reproduce the main mechanisms of plant hydro-dynamics including root water uptake from soil.

High Resolution Stroboscopic Neutron Radiography at the FRM-II ANTARES Facility

The rapidly developing field of high resolution neutron radiography primarily concentrates on non-destructive studies of stationary objects with relatively long exposure times required to achieve adequate neutron statistics. The combination of a high intensity neutron beam with a high temporal and spatial resolution detector, enables the investigation of dynamic processes in a stroboscopic mode, where image frames are synchronized with the sample or acquired continuously at high acquisition frame rates. Although neutron statistics in the acquisition frames as short as <; 10 μs is considered quite low (typically <; 1000 n/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at the sample), repetitive processes can still be studied with high resolution by integrating a large number of frames synchronized to the process. In this paper we demonstrate the stroboscopic imaging capabilities of the highly collimated thermal neutron beamline ANTARES together with a high resolution detector with neutron-sensitive microchannel plates and the Medipix2 readout. The dynamics of water uptake due to capillary forces as well as the two-phase flow of an air-water mixture is investigated, and stroboscopic imaging of an operating beam chopper and a spinning fan is performed, with sub-100-μm spatial resolution and with acquisition frames varying between 10 μs and 200 ms. The results of these experiments demonstrate the future potential for performing high resolution neutron radiography of fast and/or repetitive processes, such as water flow and uptake, operation of fuel injection nozzles, as well as many others.

Eggs under Pressure: Components of Water Potential of Chameleon Eggs during Incubation

Water exchange of squamate eggs is driven by the difference between the water potentials of eggs and of their nest environment. While osmotic potential is generally assumed to dominate the net water potential of eggs, resistance of the eggshell to stretching also affects egg water potential. We therefore determined osmotic potentials and pressure potentials (mechanical pressure) of eggs of the veiled chameleon Chamaeleo calyptratus over the course of incubation. Because embryos are diapausing gastrulae when eggs are laid and diapause persists several months, the water potential of eggs can be evaluated before it is influenced by the developing embryo. Water uptake during the first 2 wk of incubation was rapid as a result of the large difference between the total water potential of the egg (-848 kPa) and that of its incubation substrate. After about 2 wk, water potential of the egg stabilized at -460 kPa. By day 80 of incubation, the developing embryo and allantois affected water exchange of the egg. The allantoic fluid was initially very dilute, but its osmotic potential decreased to about -200 kPa by the end of incubation. Pressure potential of the egg averaged 25 kPa, with no systematic trend during incubation. The pressure potential exerted by the eggshell reduced the difference between the water potential of the egg and the water potential of the environment, that is, the ability of eggs to take up water. At the time of oviposition, this effect was relatively small, producing a 4%-6% reduction in water potential difference. Once the yolk osmotic potential stabilized, however, the reduction was 12% or more. This observation means that the dynamics of water uptake by squamate eggs cannot be fully understood without consideration of the pressure that is exerted on the contents of eggs by their shells.

Expansins expression is associated with grain size dynamics in wheat (Triticum aestivum L.)

Grain weight is one of the most important components of cereal yield and quality. A clearer understanding of the physiological and molecular determinants of this complex trait would provide an insight into the potential benefits for plant breeding. In the present study, the dynamics of dry matter accumulation, water uptake, and grain size in parallel with the expression of expansins during grain growth in wheat were analysed. The stabilized water content of grains showed a strong association with final grain weight (r2=0.88, P <0.01). Grain length was found to be the trait that best correlated with final grain weight (r2=0.98, P <0.01) and volume (r2=0.94, P <0.01). The main events that defined final grain weight occurred during the first third of grain-filling when maternal tissues (the pericarp of grains) undergo considerable expansion. Eight expansin coding sequences were isolated from pericarp RNA and the temporal profiles of accumulation of these transcripts were monitored. Sequences showing high homology with TaExpA6 were notably abundant during early grain expansion and declined as maturity was reached. RNA in situ hybridization studies revealed that the transcript for TaExpA6 was principally found in the pericarp during early growth in grain development and, subsequently, in both the endosperm and pericarp. The signal in these images is likely to be the sum of the transcript levels of all three sequences with high similarity to the TaExpA6 gene. The early part of the expression profile of this putative expansin gene correlates well with the critical periods of early grain expansion, suggesting it as a possible factor in the final determination of grain size.