Root Water Uptake Rate Research Articles

Regulation of root water uptake under abiotic stress conditions

A common effect of several abiotic stresses is to cause tissue dehydration. Such dehydration is caused by the imbalance between root water uptake and leaf transpiration. Under some specific stress conditions, regulation of root water uptake is more crucial to overcome stress injury than regulation of leaf transpiration. This review first describes present knowledge about how water is taken up by roots and then discusses how specific stress situations such as drought, salinity, low temperature, and flooding modify root water uptake. The rate of root water uptake of a given plant is the result of its root hydraulic characteristics, which are ultimately regulated by aquaporin activity and, to some extent, by suberin deposition. Present knowledge about the effects of different stresses on these features is also summarized. Finally, current findings regarding how molecular signals such as the plant hormones abscisic acid, ethylene, and salicylic acid, and how reactive oxygen species may modulate the final response of root water uptake under stress conditions are discussed.

Steady Infiltration from Point Sources and Water Uptake in Confined Cylindrical Domains

This article addresses the problem of steady water flow from point sources located in the center of a confined cylindrical domain. A linearized form of the water flow equation is used to analyze soil–water regimes, assuming exponential dependence of hydraulic conductivity on the matric head. Derived exact and approximate solutions are applicable to infiltration from a continuous point source into a cylindrical lysimeter and to water extraction by a subsurface point sink coupled to a surface source. Philip's superposition theorem, developed initially for a domain that is unconfined in the radial direction is shown to hold also for a cylindrically confined domain. An approximate, tractable expression keeping the first two terms of the full solution can satisfactorily replace the full accurate solution. Solutions for buried, surface, and subsurface point sources in a confined cylinder are expressed in terms of the dimensionless matric flux potential (MFP) and the dimensionless Stokes stream function. In the presence of an impermeable cylindrical boundary, the MFP decreases toward a constant value with increasing distance from the source, and the streamlines become more vertical at these distances. The smaller the cylinder radius or the soil sorptive number, the faster the MFP approaches constancy and the faster three‐dimensional flow converges to one‐dimensional flow below the source. Explicit expressions for the MFP and for the stream functions corresponding to a coupled surface point source and subsurface point sink in a semi‐infinite cylindrical domain are presented and used to simulate root water uptake rates in lysimeters. Water extraction rates produced by a coupled source/sink in the cylindrical domain are shown to be larger than those obtained in an unconfined domain and comparable with those computed for equivalent square‐arrays of coupled sources/sinks in open fields.

Potassium deficiency affects water status and photosynthetic rate of the vegetative sink in green house tomato prior to its effects on source activity

The potassium requirement of green house tomatoes is very high for vegetative growth and fruit production. Potassium deficiency in plants takes long time for expression of visible symptoms. The objective of this study is to detect the deficiency early during the vegetative growth and define the roles of aquaporin and K-channel transporters in the process of regulation of water status and source–sink relationship. The tomato plants were grown hydroponically inside green house of Hiroshima University, Japan and subjected to different levels of K in the rooting medium. Potassium deficiency stress decreased photosynthesis, expansion and transport of 14C assimilates of the source leaf, but the effects became evident only after diameter expansion of the growing stem (sink) was down-regulated. The depression of stem diameter expansion is assumed to be associated with the suppression of water supply more than photosynthate supply to the organ. The stem diameter expansion is parameterized by root water uptake and leaf transpiration rates. The application of aquaporin inhibitor (AgNO 3) decreased leaf water potential, stem expansion and root hydraulic conductance within minutes of application. Similar results were obtained for application of the K-channel inhibitors. These observations suggested a close relationship between stem diameter expansion and activities of aquaporins and K-channel transporters in roots. The deficiency of potassium might have reduced aquaporin activity, consequently suppressing root hydraulic conductance and water supply to the growing stem for diameter expansion and leaf for transpiration. We conclude that close coupling between aquaporins and K-channel transporters in water uptake of roots is responsible for regulation of stem diameter dynamics of green house tomato plants.

Estimating plant root water uptake using a neural network approach

Water uptake by plant roots is an important process in the hydrological cycle, not only for plant growth but also for the role it plays in shaping microbial community and bringing in physical and biochemical changes to soils. The ability of roots to extract water is determined by combined soil and plant characteristics, and how to model it has been of interest for many years. Most macroscopic models for water uptake operate at soil profile scale under the assumption that the uptake rate depends on root density and soil moisture. Whilst proved appropriate, these models need spatio-temporal root density distributions, which is tedious to measure in situ and prone to uncertainty because of the complexity of root architecture hidden in the opaque soils. As a result, developing alternative methods that do not explicitly need the root density to estimate the root water uptake is practically useful but has not yet been addressed. This paper presents and tests such an approach. The method is based on a neural network model, estimating the water uptake using different types of data that are easy to measure in the field. Sunflower grown in a sandy loam subjected to water stress and salinity was taken as a demonstrating example. The inputs to the neural network model included soil moisture, electrical conductivity of the soil solution, height and diameter of plant shoot, potential evapotranspiration, atmospheric humidity and air temperature. The outputs were the root water uptake rate at different depths in the soil profile. To train and test the model, the root water uptake rate was directly measured based on mass balance and Darcy's law assessed from the measured soil moisture content and soil water matric potential in profiles from the soil surface to a depth of 100 cm. The ‘measured’ root water uptake agreed well with that predicted by the neural network model. The successful performance of the model provides an alternative and more practical way to estimate the root water uptake at field scale.

Relative Water Uptake Rate as a Criterion for Trickle Irrigation System Design: I. Coupled Source–Sink Steady Water Flow Model

We have developed a new modeling approach for designing the geometry of trickle irrigation systems: root water uptake rate is treated as the unknown, i.e., sought after, quantity. It is determined from solutions to the linearized, steady‐water‐flow problem that describes the interaction between surface point (or line) sources and subsurface point (or line) sinks. We postulate that a sink that creates a maximum allowable suction defines an upper bound to the relative water uptake, and propose to use this upper bound as a design criterion. In the basic scenarios of a coupled source and sink, as few as three system parameters—the soil sorptive number (inverse of the sorptive length) and the depth and radius of the conceived rooting zone—can be sufficient to characterize the competition between root water uptake and deep percolation processes. In its dimensionless form, the proposed water uptake model is general for all soil types and all source–sink geometries. Evaporation is taken into account in the basic solutions for a source–sink couple with matric‐flux‐potential‐dependent evaporative water flux at the soil surface. Local resistance can be incorporated into the model by adding a skin resistance at the soil–sink interface. In addition to evaluating the relative water uptake rate, we also plotted contours of constant matric flux potential (pressure head and saturation degree) and streamlines, including the dividing streamlines that delineate the water capture zone and separate it from the deep percolation zone and the evaporation zone. Using computations, we elucidated the effects of the above system parameters on the relative water uptake rate and water flow patterns for three‐ and two‐dimensional systems; these computations describe single drippers, drip tapes, and drip lines with closely spaced drippers.

Physical and hydraulic properties of inorganic amendments and modeling their effects on water movement in sand-based root zones

The objective of this study was to evaluate the physical and hydraulic properties of selected inorganic amendments and their mixtures with sand (85:15% v/v), and model how they affect the water movement in sand-based root zones of sports fields. The amendments are composed of: calcined diatomaceous earth materials (Axis and Isolite); zeolites (Clinolite and Ecolite); and calcined clays (Moltan Plus, Profile, and Pro’s Choice). The bulk density, particle density, porosity, particle-size distribution, saturated hydraulic conductivity, water retention and available water-holding capacity were analyzed. A numerical model was applied to simulate soil water movement for a scenario with and without amendment incorporation. The results showed that amendments significantly (P < 0.05) improved the physical and hydraulic properties of root zone. Modeling results revealed reduced surface dryness, higher volumetric water content and storage and higher initial root water uptake rate for the root zones with amendments. These results suggest there are multiple benefits of amended root zones in terms of improvement of the physical and hydraulic properties of sand-based root zones.

Modeling and Experimental Determination of Salt Accumulation Induced by Root Water Uptake

We investigated salt accumulation processes induced by root water uptake above the groundwater table. We compared experimentally determined results from a pot experiment with those predicted by a numerical simulation. Spring wheat (Triticum aestivum L.) was grown in sand‐filled pots, and a constant groundwater table of saline water was maintained. We measured physiological factors of wheat such as daily transpiration rate, leaf water potential, and root length distribution. Water content and salt distribution in the pots were determined. We developed a numerical model for salt accumulation that was based on physical models of water and solute movement in the soil and included a root water uptake term and a stomatal behavior model. The root water uptake rate was calculated using an Ohm's law type model; the transpiration rate as a function of leaf water potential and all soil physical properties regarding water and solute movement in soil were measured independently. Observed and calculated results showed that the salt accumulation pattern changes dynamically with changes in root water uptake patterns and vice versa. In the lower root zone, salt content was at a quasi‐steady state, and salt accumulation did not proceed even though root water uptake occurred there; concentrated salt was transported upward by soil water flux induced by root water uptake in the upper zone. Salt accumulated mainly in the upper root zone where root water uptake ceased due to the lower water potential of the soil. The site of salt accumulation moved downward as salt accumulation proceeded and soil water potential decreased

Root Water Uptake and Root Nitrogen Mass of Winter Wheat and Their Simulations

In this study, winter wheat (Triticum aestivum L.) was grown in nutrient solution (Exp. 1) and soil columns (Exp. 2) under controlled environmental conditions to investigate the relationship between root water uptake (RWU) and root N mass (RNM). In Exp. 1, 15 treatments were designed to examine the influence of NO3 concentrations (0.007–0.42 kg m−3) and illumination conditions (free water surface evaporation rates of 0.25–0.47 cm d−1) on root N content (RNC) and RWU. Measurements included transpiration, dry weight, N content, and root length. In Exp. 2, RNC and RWU of wheat were measured at two NO3 levels (0.011 and 0.105 kg m−3) and low and high (80% field water capacity) soil water contents. The distributions of soil water content, NO3 concentration, RNM density, root length density, and soil temperature were observed. Results showed that the potential transpiration was proportional to RNM of wheat (R2 = 0.99), and the maximum RWU rate, Smax, which changed from 0 to 0.023 m3 m−3 d−1, was also linearly related to the RNM density (R2 = 0.78). The ratio between potential transpiration and RNM, namely the potential RWU coefficient per unit RNM, was independent of environmental NO3 level and wheat maturity, but changed linearly with the free water surface evaporation rate (R2 = 0.88). The relationship between RWU and RNM was applied to simulate the distributions of soil water content, soil NO3 concentration, and RNM density in Exp. 2. The simulated results compared well with the measured data, and the maximum RMSEs were 0.0085 m3 m−3, 0.0283 kg m−3, and 0.0009 kg m−3, respectively.

Parametric studies on bioengineering effects of tree root-based suction on ground behaviour

Using native vegetation to improve soil stiffness, stabilise slopes and control erosion is a rapidly evolving process. A theoretical model previously developed by the authors for the rate of tree root water uptake together with an associated numerical simulation is used to study the effects of a wide range of soil, tree, and atmospheric parameters on partially saturated ground. The influence of different parameters on the maximum initial rate of root water uptake is investigated through parametric and sensitivity analyses. Field measurements taken from previously published literature are compared with numerical predictions for validation. The rate of selected parameters such as potential transpiration and its distribution, suction at wilting point, the coefficient of permeability and the distribution of root length density are studied in detail. The analysis shows that the rate of potential transpiration increases the soil matric suction and ground settlement, while the potential transpiration rate has an insignificant effect on the distribution of soil suction. Root density distribution factors affect the size of the influence zone. Suction at the wilting point increases the soil matric suction and ground settlement, whereas the saturation permeability decreases the maximum soil matric suction generated. The analysis confirms that the most sensitive parameters, including the coefficients of the tree root system, the transpiration rate, the permeability of the soil and its suction at the wilting point should be measured or estimated accurately for an acceptable prediction of ground conditions in the vicinity of trees.

Bioengineering ground improvement considering root water uptake model

Bioengineering features of native vegetation are currently being evolved to enhance soil stiffness, slope stabilisation and erosion control. The effects of tree roots on soil moisture content and ground settlement are discussed in this paper. Matric suction induced by tree roots is a key factor, governing the properties of unsaturated soils, directly imparting stability to slopes and resistance for yielding behaviour. A mathematical model for the rate of root water uptake that considers ground conditions, type of vegetation and climatic parameters has been developed. This study highlights the inter-related parameters contributing to the development of a conceptual evapo-transpiration and root moisture uptake equilibrium model that is then incorporated in a comprehensive numerical finite element model. The developed model considers fully coupled-flow-deformation behaviour of soil. Field measurements obtained by the Authors from a site in Victoria, South of Australia, are used to validate the model. In this study, the active tree root distribution has been predicted by measuring soil organic content distribution. The predicted results show acceptable agreement with the field data in spite of the assumptions made for simplifying the effects of soil heterogeneity and anisotropy. The results prove that the proposed root water uptake model can reliably predict the region of the maximum matric suction away from the tree axis.

Absorption and Transport of Water and Ions in Corn and Sunflower Plants Grown Under Saline Conditions

In order to analyze absorption and transport of water and ions in plants affecting the salinization in the root zone through physical and physiological processes, we measured water and ion uptake by roots, transpiration rate, leaf conductance, and ion concentrations in root xylem sap and other plant tissues of corn and sunflower grown under saline conditions using a nutrient film technique system. The rate of root water uptake was lower in corn than in sunflower, where the daytime stomatal closure in response to the excessive water stress induced by the saline solution was occurred in corn but not in sunflower. NO3-, PO43- and K+ were highly concentrated in the xylem sap as a result of the active and selective uptake of nutrients by roots of both corn and sunflower. Na+, which is not an essential element for plant growth, was not highly concentrated in the xylem sap or plant tissues of corn because corn exhibited a poor Na+ absorption and transportation ability. On the other hand, sunflower had a greater ability to absorb water, Na+, Mg2+, Ca2+, Cl-, and SO42-, and to transport these ions from the roots to the shoot. These differences in absorption and transport of water and ions indicate that sunflower is more salt tolerant than corn. The effects of these absorption and transport characteristics on plant growth and soil salinity should be taken into account for sustainable and effective plant production in salinized crop fields.

An optimality-based model of the coupled soil moisture and root dynamics

Abstract. The main processes determining soil moisture dynamics are infiltration, percolation, evaporation and root water uptake. Modelling soil moisture dynamics therefore requires an interdisciplinary approach that links hydrological, atmospheric and biological processes. Previous approaches treat either root water uptake rates or root distributions and transpiration rates as given, and calculate the soil moisture dynamics based on the theory of flow in unsaturated media. The present study introduces a different approach to linking soil water and vegetation dynamics, based on vegetation optimality. Assuming that plants have evolved mechanisms that minimise costs related to the maintenance of the root system while meeting their demand for water, we develop a model that dynamically adjusts the vertical root distribution in the soil profile to meet this objective. The model was used to compute the soil moisture dynamics, root water uptake and fine root respiration in a tropical savanna over 12 months, and the results were compared with observations at the site and with a model based on a fixed root distribution. The optimality-based model reproduced the main features of the observations such as a shift of roots from the shallow soil in the wet season to the deeper soil in the dry season and substantial root water uptake during the dry season. At the same time, simulated fine root respiration rates never exceeded the upper envelope determined by the observed soil respiration. The model based on a fixed root distribution, in contrast, failed to explain the magnitude of water use during parts of the dry season and largely over-estimated root respiration rates. The observed surface soil moisture dynamics were also better reproduced by the optimality-based model than the model based on a prescribed root distribution. The optimality-based approach has the potential to reduce the number of unknowns in a model (e.g. the vertical root distribution), which makes it a valuable alternative to more empirically-based approaches, especially for simulating possible responses to environmental change.

Root length density and water uptake distributions of winter wheat under sub-irrigation

As the critical information to study flow transport in soil–plant systems, root distributions and root-water-uptake (RWU) patterns have been studied extensively. However, most root distribution data in the past were collected under surface irrigation. Less research has been conducted to characterize root distributions under sub-irrigation. The objectives of this study were to (1) test if the generalized function of normalized root length density (NRLD) in the literature was applicable to root distributions of winter wheat under natural sub-irrigation, which provides water from subsurface by capillary rise from the water table, and (2) estimate RWU distributions of winter wheat under natural sub-irrigation. Column experiments were conducted to study the distributions of root length density (RLD) and RWU of winter wheat (Triticum aestivum L. cv. Nongda 189) during a growing period of 57 days from planting to tillering stages under surface irrigation and natural sub-irrigation. Data of root distributions and soil water content were collected in the experiments with different treatments of irrigation levels. Results showed that the RLD distributions of winter wheat under both surface irrigation and natural sub-irrigation were of similar patterns. The NRLD distributions under sub-irrigation were adequately characterized by the generalized function. An inverse method was employed to estimate the average RWU rate distributions of winter wheat. In addition, based on the potential RWU coefficient and the NRLD function, a simple approach was developed to predict RWU rates at different depths. The predicted RWU rates had a good agreement with the estimated RWU rate distributions using the inverse method.

Numerical analysis of matric suction effects of tree roots

The use of native vegetation in the coastal regions of Australia has become increasingly popular for stabilising railway corridors built over expansive clays and compressive soft soils. The tree roots provide three stabilising functions: (a) they reinforce the soil; (b) they dissipate excess pore pressures; and (c) they establish sufficient matric suction to increase the shear strength. The matric suction generated within the tree root zone propagates radially into the soil matrix, as a function of the moisture content change. Considering soil conditions, the type of vegetation and atmospheric conditions, a mathematical model for the rate of root water uptake is developed. A conical shape is considered to represent the geometry of the tree root zone. Based on this model for the rate of root water uptake, the pore water pressure distribution and the movement of the ground adjacent to the tree are numerically analysed. Field measurements taken from the previously published literature are compared with the authors’ numerical predictions. It is found that, given the approximation of the assumed model parameters, the agreement between the predicted results and field data is still promising. The study indicates that native vegetation improves the shear strength of the soil by increasing the matric suction, and also curtails soil movements.

THE FLEXIBLE TOLERANCE METHOD FOR ESTIMATING HYDROLOGIC PARAMETERS IN THE ROOT ZONE

: In this study, a constrained minimization method, the flexible tolerance method, was used to solve the optimization problems for determining hydrologic parameters in the root zone: water uptake rate, spatial root distribution, infiltration rate, and evaporation. Synthetic soil moisture data were first generated using the Richards' equation and its associated initial and boundary conditions, and these data were then used for the inverse analyses. The results of inverse simulation indicate the following. If the soil moisture data contain no noise, the rate of estimated water uptake and spatial root distribution parameters are equal to the true values without using constraints. If there is noise in the observed data, constraints must be used to improve the quality of the estimate results. In the estimation of rainfall infiltration and surface evaporation, interpolation methods should be used to reduce the number of unknowns. A fewer number of variables can improve the quality of inversely estimated parameters. Simultaneous estimation of spatial root distribution and water uptake rate or estimation of evaporation and water uptake rate is possible. The method was used to estimate the water uptake rate, spatial root distribution, infiltration rate, and evaporation using long-term soil moisture data collected from Nebraska's Sand Hills.

Simulation of Phytoremediation of a TNT‐Contaminated Soil Using the CTSPAC Model

Knowledge of water movement in the plant-xylem system and contaminant bioavailability in the soil environment is crucial to evaluate the success of phytoremediation practices. This study investigated the removal of 2,4,6-trinitrotoluene (TNT) from a contaminated sandy soil by a single poplar (Populus fastigiata) tree through the examinations of temporal variations of xylem water potential, root water uptake, and soil TNT bioavailability. A mathematical model, CTSPAC (Coupled Transport of water, heat, and solutes in the Soil-Plant-Atmosphere Continuum), was modified for the purpose of this study. The model was calibrated using laboratory measurements before its application. Our simulations show that the xylem water potential was high in the roots and low in the leaves with a potential head difference of 3.55 cm H2O, which created a driving force for water flow and chemical transport upward from the roots through the stem to the leaves. The daily average root water uptake rate was 25 cm3 h(-1) when an equilibrium condition was reached after 24 h. Our simulations further reveal that no TNT was found in the stem and leaves and only about 1% of total TNT mass was observed in the roots due to the rapid biodegradation and transformation of TNT into its daughter products. About 13% of the soil TNT was removed by the poplar tree, resulting mainly from root uptake since TNT is a recalcitrant compound. In general, the soil TNT bioavailability decreased with time due to the depletion of soil solution TNT by the poplar tree. A constant bioavailability (i.e., 3.1 x 10(-6)) was obtained in 14 d in which the soil TNT concentration was about 10 mg L(-1). Our study suggests that CTSPAC is a useful model to simulate phytoremediation of TNT-contaminated sites.

USE OF A CROP MODEL TO EVALUATE SOIL IMPEDANCE AND ROOT CLUMPING EFFECTS ON SOIL WATER EXTRACTION IN THREE ARGENTINE SOILS

Argentina is the third largest producer of grain soybean in the world and the largest producer of soybean meal and soybean oil exports. Soybean is grown mostly in the Pampas of central–eastern Argentina. Argillic soils (typic and vertic argiudolls) and typic pelluderts are present in approximately 50% of the soils used for soybean production. Those soils have swelling and cracking behavior that may result in root clumping, and thus the root system’s ability to extract water may be restricted due to limited proliferation into the surrounding soil. The CROPGRO–Soybean model’s root growth and root water uptake routines were modified to improve its ability to simulate root growth and water uptake in soils where root penetration is restricted or roots are clumped together. Two empirical soil parameters, referred to as a soil impedance factor and a root clumping factor, were incorporated into this “revised” model. Data sets from a vertic argiudoll (Oliveros, Santa Fe), a typic argiudoll (Balcarce, Buenos Aires), and an enthic hapludoll (Manfredi, Cordoba) were used to evaluate the ability of the model to predict soil water extraction by soybean crops. An adaptive simulated annealing technique was used to estimate the soil impedance and root clumping factors. The revised CROPGRO–Soybean model was able to predict soil water extraction for three Argentine soils with different soil textures. The effectiveness of the model was expressed by low percent errors of estimation, ranging from 3.5% to 7.0% among different sites. The incorporation of soil impedance and root clumping factors allowed us to assess the effects of the argillic horizons in restricting root depth growth and root proliferation and thus in decreasing the potential root water uptake rates. The argillic horizons had more significant effects on root clumping than on restrictions of root depth growth. The clumping of roots caused by the presence of cracking horizons also reduced the rate of water uptake below those layers. Critical clay content thresholds as predictors of soil impedance and root clumping were found, providing a method for predicting soil impedance and root clumping factors for argillic soils.

The role of soil hydraulic conductivity on the spatial and temporal variation of root water uptake in drip-irrigated corn

A multiplexed TDR system and a heat-pulse system for stem sap flow measurements were used to determine the spatial and temporal pattern of root water uptake in field-grown corn. The TDR probes, 0.15 and 0.30 m in length, were buried vertically in the soil profile to a depth of 0.95 m below the soil surface and heat-pulse sensors were installed on the plant base. Nocturnal readings from TDR probes were used successfully to differentiate the two components of moisture change: root uptake and net drainage. The instantaneous rate of water extraction by the plant measured by the heat-pulse system agreed well with the integrated rate of root water uptake measured frequently (at half-hour or hourly intervals) by the TDR probes. This agreement enabled further exploration into the cause of the evolution of the spatial and temporal patterns of root water uptake during a drying cycle. The results indicated that right after irrigation in the well-watered soil profile, it is the spatial distribution of the roots that mainly determines the typical pattern of root extraction, in addition to the fact that the roots near the plant base are more effective than those farther away. The higher density and effectiveness of the roots near the plant base dry the soil rapidly so that soil hydraulic conductivity soon becomes a limiting factor for water uptake. Further analysis revealed that a decrease in root uptake occurs near the plant base under a given atmospheric demand when the relative bulk soil hydraulic conductivity decreases to 0.002Kr. This suggests that low conductivity (high resistance) in the soil near the plant base is the initial cause for downward and lateral shifting of the root uptake pattern. Note that this critical value of hydraulic conductivity is not universal since it depends on the soil type and atmospheric water demand during the period under observation. Therefore, prior to the application of moisture content or suction head as measures of water availability or to control irrigation scheduling, it is suggested that these parameters be calibrated by the soil K(θ) or K(ι) curves, respectively, for the expected atmospheric water demand for the specific crop and growing period.

Estimation of root water uptake of maize: an ecophysiological perspective

An understanding of movement and distribution of soil water has been long known essential to the optimization of plant biomass productivity. This study provides an ecophysiologically based analytic model for estimating root water uptake rate of maize ( Zea mays L.). The model can be run with readily available inputs, such as water potentials of leaf, soil and air, solar radiation, potential evapotranspiration, root length, and some soil physical properties, such as bulk density and particle-size distribution. Comparison with measured data showed that the model described root water extraction with acceptable accuracy. Analysis of the response of the model to changes in the input parameters revealed that the model is most sensitive to leaf water potential and root length. Examination of the model response also showed that the model could be improved with more information about the root hydraulic conductance and effective root length responsible for water extraction.

Root growth and water uptake by wheat during drying of undisturbed and repacked soil in drainage lysimeters

The dynamics of root development and water uptake in undisturbed and repacked clay soil were studied during increasing soil water deficit pre- and post-anthesis to find how root distribution, soil type and plant development affect plant available water (PAW). Volumetric water fraction was measured with a neutron probe, while root distribution was measured non-destructively using a series of horizontal perspex observation tubes. Soil modification affected the downward root growth which changed little over time for undisturbed (U) soil (6.7-13.6 mm/d) compared with the repacked (R) soil (7.5-42.9 mm). In the well-watered treatments root distribution was mostly above the 0.55 m depth in U soil, while there was a reasonably uniform vertical distribution in R soil. The rate of root growth during post-anthesis drying was 61% of that during pre-anthesis drying. The specific root water uptake rate (vol./unit root length/time) was linearly related to the relative root growth rate, indicating the importance of root growth in supplying water during soil drying. Estimated PAW values reflected effects of soil hydraulic properties, root distribution and a propensity to grow new roots during soil drying. Differences in grain yield between treatments were thought to result from the effects of different root distributions and the propensity to grow roots during soil drying.