Modelling non-uniform soil water uptake by a single plant root

The assumption of uniform water flow to the root or uniform water potential at the root surface was shown by Hainsworth and Aylmore (1986, 1989) to be erroneous. The present paper demonstrates how the non-uniform uptake of water by a single root can be modeled. Differential equations are numerically solved to describe simultaneous water movement in the plant and in the soil. In the plant, boundary conditions are the water potentials at the root surface (Ψ_s) and in the xylem at the root base (Ψ_b). A set of difference equations describe the flow of water radially through the cortex to the xylem and in the xylem axially upwards to the base. For calculating the water flow in the soil and the values of Ψ_s, i.e. the boundary conditions for flow in the root, the finite element method (FEM) is used, the boundary conditions being the flux of water into the plant root and the zero flow across the wall, bottom and surface of a hypothetical soil cylinder surrounding the root. ei]Section editor: B E Clothier     

Characteristics and underlying mechanisms of plant deep soil water uptake and utilization: Implication for the cultivation of plantation trees

根系吸水是树木水分关系的重要环节, 在树木生理活动中发挥着至关重要的作用。深层土壤中的水资源含量一般相对较高, 常可为树木生长供给大量水分, 并在旱季保障其生存与正常生长。因此, 了解树木对深层土壤水的吸收利用特征与机制, 可帮助深入认识树木与环境的互作机制、树木的生长与生存策略、物种间的共存与竞争机制等内容, 同时还可帮助构建既能降低外部水资源投入, 又能避免水分生态环境负面效应的人工林绿色栽培制度。基于已有研究, 该文对树木吸收利用深层土壤水的特征与机制进行了综述。首先, 探讨了深层根系和深层土壤的界定, 指出对于除寒温带针叶林以外的其他主要森林植被类型, 可以1 m作为树木深根系和深土层的平均划分(参考)标准, 并明确了全球范围内树木深根系的成因。其次, 对已有研究中观察到的树木对深层土壤水的吸收利用特征及其影响因素进行了归纳与总结, 并从深根系性状调节、整株水力特性协调两方面探讨了树木高效吸收利用深层土壤水的机制, 如可通过深根系的空间、时间和效率调节策略来促进对深土层水分的吸收。最后, 提出了树木利用深土层水分对人工林培育的几点启示, 包括水分管理.中应使林木适度利用深层土壤水, 选用合适的灌水频率、合理的树种混交能促进深层土壤水分储库“缓冲”作用的发挥, 基于树木土壤水分利用深度的间伐木选择技术等, 并指出了该领域现有研究的不足以及今后的发展方向。     

Influence of root density on the critical soil water potential

Estimation of root water uptake in crops is important for making many other agricultural predictions. This estimation often involves two assumptions: (1) that a critical soil water potential exists which is constant for a given combination of soil and crop and which does not depend on root length density, and (2) that the local root water uptake at given soil water potential is proportional to root length density. Recent results of both mathematical modeling and computer tomography show that these assumptions may not be valid when the soil water potential is averaged over a volume of soil containing roots. We tested these assumptions for plants with distinctly different root systems. Root water uptake rates and the critical soil water potential values were determined in several adjacent soil layers for horse bean (Vicia faba) and oat (Avena sativa) grown in lysimeters, and for field-grown cotton (Gossypium L.), maize (Zea mays) and alfalfa (Medicago sativa L.) crops. Root water uptake was calculated from the water balance of each layer in lysimeters. Water uptake rate was proportional to root length density at high soil water potentials, for both horse bean and oat plants, but root water uptake did not depend on root density for horse bean at potentials lower than −25 kPa. We observed a linear dependency of a critical soil water potential on the logarithm of root length density for all plants studied. Soil texture modified the critical water potential values, but not the linearity of the relationship. B E Clothier Section editor     

Modeling soil water movement with water uptake by roots

Soil water movement with root water uptake is a key process for plant growth and transport of water and chemicals in the soil-plant system. In this study, a root water extraction model was developed to incorporate the effect of soil water deficit and plant root distributions on plant transpiration of annual crops. For several annual crops, normalized root density distribution functions were established to characterize the relative distributions of root density at different growth stages. The ratio of actual to potential cumulative transpiration was used to determine plant leaf area index under water stress from measurements of plant leaf area index at optimal soil water condition. The root water uptake model was implemented in a numerical model. The numerical model was applied to simulate soil water movement with root water uptake and simulation results were compared with field experimental data. The simulated soil matric potential, soil water content and cumulative evapotranspiration had reasonable agreement with the measured data. Potentially the numerical model implemented with the root water extraction model is a useful tool to study various problems related to flow transport with plant water uptake in variably saturated soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Water storage and osmotic pressure influences on the water relations of a dicotyledonous desert succulent

Abstract Water storage and nocturnal increases in osmotic pressure affect the water relations of the desert succulent Ferocactus acanthodes, which was studied using an electrical circuit analog based on the anatomy and morphology of a representative individual. Transpiration rates and osmotic pressures over a 24-h period were used as input variables. The model predicted water potential, turgor pressure and water flow for various tissues. Plant capacitances, storage resistances and nocturnal increases in osmotic pressure were varied to determine their role in the water relations of this dicotyledonous succulent. Water coming from storage tissues contributed about one-third of the water transpired at night: the majority of this water came from the nonphotosynthetic, water storage parenchyma of the stem. Time lags of 4 h were predicted between maximum transpiration and maximum water uptake from the soil. Varying the capacitance of the plant caused proportional changes in osmotically driven water movement but changes in storage resistance had only minor effects. Turgor pressure in the chlorenchyma depended on osmotic pressure, but was fairly insensitive to doubling or halving of the capacitance or storage resistance of the plant. Water uptake from the soil was only slightly affected by osmotic pressure changes in the chlorenchyma. For this stem succulent, the movement of water from the chlorenchyma to the xylem and the internal redistribution of water among stem tissues were dominated by nocturnal changes in chlorenchyma osmotic pressure, not by transpiration.     

Effect of root anaerobiosis on the water relations of several Pyrus species

Solution culture experiments were designed to investigate the plant water relations of 3 Pyrus species subjected to root anaerobiosis. Root anaerobiosis induced partial stomatal closure prior to alterations in leaf water potential (ΨLW) or root osmotic potential (ΨRπ). In contrast, stomatal closure was accompanied by a decline in root hydraulic conductivity (Lp). Anoxia markedly reduced ΨLW for Pyrus communis L. and eventually led to wilting and defoliation. Pyrus betulaefolia Bunge and Pyrus calleryana Decne, however, were less affected by root anaerobiosis. To delineate if the increased root resistance was in the radial or longitudinal direction, 10⁻⁴ M cistrans abscisic acid (ABA) was added to detopped root systems of P. communis in solution culture after steady-state rates of Lp were established. A consistent 25 to 30% promotion of Lp was observed 1.5 h after the addition of ABA for aerobically treated plants. ABA did not influence Lp when applied to roots previously deprived of O₂ for 4 days. Additional evidence against the limiting resistance being in the radial direction was obtained when water fluxes were compared through intact P. communis roots, roots with all feeder roots detached, and stems without root systems. Severing feeder roots from anaerobically treated plants did not increase water flux to rates observed for aerobically treated plants. Resistance progressed basipetally to eventually encompass the stem itself. These results can only be explained by occlusion of the xylem vessels.     

Root clumping may affect the root water potential and the resistance to soil-root water transport

We have appraised for clumped root systems the widely-accepted view that the resistance to water flux from soil to roots (‘soil resistance’) is low under most field conditions, so that root water potential would closely follow the mean soil water potential. Three root spatial arrangements were studied, simulating either the regular pattern generally assumed in models, or two degrees of root clumping frequently observed in the field. We used a numerical 2-dimensional model of water transfer which assumes a control of evapotranspiration by root signalling. Calculations were carried out at two evaporative demands and for two contrasting soil hydraulic properties. The rate of soil depletion, the timing of the reduction in evapotranspiration and the difference between root water potential and mean soil water potential were all affected by the root spatial arrangement, with a greater effect at high evaporative demand and low soil hydraulic conductivity. Almost all the soil water reserve was available to plants without reduction in evapotranspiration in the regular case, while only a part of it was available in clumped cases. In the regular case, calculated ‘soil resistances’ were similar to those calculated using Newman's (1969) method. Conversely they were higher by up to two orders of magnitude in clumped root spatial arrangements. These results place doubt on the generality of the view that ‘soil resistance’ is low under common field conditions. They are consistent with the results of field experiments, especially with recent data dealing with root-to-shoot communication.     

Aquaporins account for variations in hydraulic conductance for metabolically active root regions of Agave deserti in wet, dry, and rewetted soil

The importance of aquaporins for root hydraulic conductance (L_P) was investigated along roots of the desert succulent Agave deserti in wet, dry and rewetted soil. Water channel activity was inferred from HgCl₂‐induced reductions of L_P that were reversible by 2‐mercaptoethanol. Under wet conditions, HgCl₂ reduced L_P for the distal root region by 50% and for the root region near the shoot base by 36% but did not affect L_P for the mid‐root region. For all root regions, L_P decreased by 30–60% during 10 d in drying soil and was not further reduced by HgCl₂. After soil rewetting, L_P increased to pre‐drying values and was again reduced by HgCl₂ for the distal and the basal root regions but not the mid‐root region. For the distal region, water channels in the epidermis/exodermis made a disproportionately large contribution to radial hydraulic conductance of the intact segment; for the basal region, water channel activity was highest in the cortex and endodermis. The role of water channels was greatest in tissues in which cells were metabolically active both in the distal root region, where new apical growth occurs in wet soil, and in the basal region, which is the most likely root region to intercept light rainfall.     

Water relations and root activities of Buchloe dactyloides and Zoysia japonica in response to localized soil drying

Effects of localized soil drought stress on water relations, root growth, and nutrient uptake were examined in drought tolerant ‘Prairie’ buffalograss [Buchloe dactyloides (Nutt.) Engelm.] and sensitive ‘Meyer’ zoysiagrass (Zoysia japonica Steud.). Grasses were grown in small rhizotrons in a greenhouse and subjected to three soil moisture regimes: (1) watering the entire 80-cm soil profile (well-watered control); (2) drying 0–40 cm soil and watering the lower 40 cm (partially dried); (3) and drying the entire soil profile (fully dried). Drying the 0–40 cm soil for 28 days had no effect on leaf water potential (Ψ _leaf ) in Prairie buffalograss compared to the well-watered control but reduced that in Meyer zoysiagrass. Root elongation rate was greater for Prairie buffalograss than Meyer zoysiagrass under well-watered or fully dried conditions. Rooting depth increased with surface soil drying; with Prairie buffalograss having a larger proportion of roots in the lower 40 cm than Meyer zoysiagrass. The higher rates of water uptake in the deeper soil profile in the partially dried compared to the well-watered treatment and by Prairie buffalograss compared to Meyer zoysiagrass could be due to differences in root distribution. Root ¹⁵N uptake for Prairie buffalograss was higher in 0–20 cm drying soil in the partially dried treatment than in the fully dried treatment. Diurnal fluctuations in soil water content in the upper 20 cm of soil when the lower 40 cm were well-watered indicated water efflux from the deeper roots to the drying surface soil. This could help sustain root growth, maintain nutrient uptake in the upper drying soil layer, and prolong turfgrass growth under localized drying conditions, especially for the deep-rooted Prairie buffalograss. This revised version was published online in June 2006 with corrections to the Cover Date.     

Water uptake profile response of corn to soil moisture depletion

The effects of soil moisture distribution on water uptake of drip‐irrigated corn were investigated by simultaneously monitoring the diurnal evolution of sap flow rate in stems, of leaf water potential, and of soil moisture, during intervals between successive irrigations. The results invalidate the steady‐state resistive flow model for the continuum. High hydraulic capacitance of wet soil and low hydraulic conductivity of dry soil surrounding the roots damped significantly diurnal fluctuations of water flow from bulk soil to root surface. By contrast, sap flow responded directly to the large diurnal variation of leaf water potential. In wet soil, the relation between the diurnal courses of uptake rates and leaf water potential was linear. Water potential at the root surface remained nearly constant and uniformly distributed. The slope of the lines allowed calculating the resistance of the hydraulic path in the plant. Resistances increased in inverse relation with root length density. Soil desiccation induced a diurnal variation of water potential at the root surface, the minimum occurring in the late afternoon. The increase of root surface water potential with depth was directly linked to the soil desiccation profile. The development of a water potential gradient at the root surface implies the presence of a significant axial resistance in the root hydraulic path that explains why the desiccation of the soil upper layer induces an absolute increase of water uptake rates from the deeper wet layers.     

The influence of shade and clouds on soil water potential: The buffered behavior of hydraulic lift

In the sagebrush/bunchgrass steppe of the North American Great Basin soil water potential has been shown to exhibit diel fluctuations with water potential increasing during the night as a result of water loss from roots in relatively dry soil layers. We hypothesized that environmental conditions promoting low transpiration rates (shading, cloudiness) would cause a net increase in soil water potential as a result of reduced soil water depletion during the day and continuing water efflux from roots during the night. We examined the response of soil water potential to artificial shading in sagebrush/bunchgrass plantings and used a simple model to predict how soil water potential should respond to reduced transpiration. Field measurements of soil water potential indicated that shading reduced daytime soil water depletion, but that the magnitude of the soil water potential increase during the night was related to the magnitude of the soil water potential decline during the preceding day. As a result, shading had little net effect on soil water potential. This behavior was consistent with model results and appears to result from the fact that soil water depletion during the day is largely responsible for creating the water potential gradients that drive nocturnal recharge of the shallow soil layers. The overall effect of such behavior is to buffer the seasonal course of soil water depletion in the rooting zone against day-to-day fluctuations in evapotranspiration. Despite the buffered behavior of soil water potential change, reduced evapotranspiration during light summer rains, and resulting soil water redistribution in the rooting zone, may enhance plant water status to a greater extent that would be expected on the basis of the rainfall received.     

Water relations of Capsicum genotypes under water stress

Pepper species and cultivars, Capsicum annuum cv. Bell Boy, C. annuum cv. Kulai and C. frutescens cv. Padi, differing in drought tolerance were investigated for their water relations, stomatal responses and abscisic acid (ABA) content during water stress. C. frutescens cv. Padi exhibited a greater osmotic adjustment than C. annuum cultivars. Stomatal conductance of cv. Bell Boy was more sensitive to water stress than that of cvs. Kulai and Padi. In all pepper genotypes, stomatal closure was triggered in the absence of a large decrease in leaf water status. ABA content in xylem sap and leaf was higher in C. annum cultivars compared to C. frutescens cv. Padi. This revised version was published online in July 2006 with corrections to the Cover Date.     

Leaf age and salinity influence water relations of pepper leaves

Plant growth is reduced under saline conditions even when turgor in mature leaves is maintained by osmotic adjustment. The objective of this study was to determine if young leaves from salt-affected plants were also osmotically adjusted. Pepper plants (Capsicum annuum L. cv. California Wonder) were grown in several levels of solution osmotic potential and various components of the plants' water relations were measured to determine if young, rapidly growing leaves could accumulate solutes rapidly enough to maintain turgor for normal cell enlargement. Psychrometric measurements indicated that osmotic adjustment is similar for both young and mature leaves although osmotic potential is slightly lower for young leaves. Total water potential is also lower for young leaves, particularly at dawn for the saline treatments. The result is reduced turgor under saline conditions at dawn for young but not mature leaves. This reduced turgor at dawn, and presumably low night value, is possibly a cause of reduced growth under saline conditions. No differences in leaf turgor occur at midday. Porometer measurements indicated that young leaves at a given salinity level have a higher stomatal conductance than mature leaves, regardless of the time of day. The result of stomatal closure is a linear reduction of transpiration.     

A biophysical model of fruit growth: simulation of seasonal and diurnal dynamics of mass

A model of fruit growth was developed, based on a biophysical representation of water and dry material transport, which is coupled with cell wall extension stimulated by turgor pressure. The fluxes of materials connect the growing fruit with the parent plant (by phloem and xylem transport) and with the ambient atmosphere (by respiration and transpiration). The sugars are transported from the phloem to the fruit mesocarp by mass flow, passive diffusion and an active (and/or facilitated) mechanism. The stages after cell division has ceased and when fruit growth is due mainly to cell enlargement were modelled. This enabled us to consider the fruit as a cell community with a constant number of cells and to apply directly the equation describing the effect of hydrostatic pressure on the irreversible cell wall expansion elaborated originally for a single cell. The model was applied to the peach [ Prunus persica (L.) Batsch] fruit. Seasonal and diurnal fruit growth, expressed in terms of dry and fresh mass changes, was calculated for conditions of water stress with various crop loads. Simulation of the diurnal patterns of fruit fresh mass variation revealed, in agreement with observations, intensive growth by night and midday fruit shrinkage, which depend on plant water status and on crop load.     

Water relations of Picea abies. II. Determination of the apoplastic water content and other water relations parameters of needles by means of capillary microcryoscopy and the pressure-volume

Using the pressure volume analysis (PV analysis) on the shoots of Norway spruce (Picea abies [L.] Karst.) and the here presented capillary microcryoscopy of the needle press sap of the same shoots, it was possible to determine the amount of apoplastic water in the needles (W_an) as well as in the defoliated shoots (W_as). Additionally, the bulk osmotic pressure at full water saturation in the symplast of the needles and defoliated shoots (π_on and π_os) was determined. The dependence of the bulk-averaged turgor pressure (P_t) on the water content and the relationship between the bulk modulus of elasticity of the needles (?_n) and the bulk-averaged needle turgor pressure (P_tn) was shown with help of the PV analysis on the whole shoots and defoliated shoots. The study was conducted at the end of the vegetation period in 1987 and during winter 1988. The proportion of W_an in the total needle water content (W_tn) was 14% in September 1987 and 12.5% in March 1988. The respective percentage of W_as in W_ts were 27% and 25%. The amount of apoplastic water depended on the ratio of the dry weight of defoliated shoots to the dry weight of the whole shoots. A standard mean value for the amount of W_an in the total water content of the shoots (W_t) was therefore not possible. The bulk osmotic pressure at full water saturation in the needle symplasts was –1.9 MPa in September 1987 and –2.2 MPa in winter 1988. The respective values of the bulk osmotic pressures in the symplast of the defoliated shoots (π_os) were –1.5 MPa and –1.7 MPa. Thus π_on was 0.1 MPa lower and π_os 0.3–0.4 MPa higher than the average osmotic pressure during full water saturation in the symplast of the whole shoots (π_o). The relation between bulk-averaged turgor pressure and water content showed that during water loss P_tn dropped more rapidly than the turgor pressure of defoliated shoots (P_ts). Consequently the needles were less elastic than the defoliated shoots. The turgor values of whole shoots followed an intemediate course between P_tn and P_ts. The flat course of P_ts seems to be the main reason for the often observed “plateau” of ψ-isotherms of whole shoots near full turgor.     

Non-uniform soil water extaction by plant roots

Using the technique of Computer Assisted Tomography applied to gamma ray attenuation measurement of soil water content, it has been shown that the assumption of uniform absorption of soil water along a plant root is clearly erroneous and that drawdown distance is a function of time. The results suggest that the plant sequentially removes water from the top to the bottom of the root as soil hydraulic resistance becomes a major limiting factor in the upper layers, even at the high soil water potential (–0.30 MPa) used.     

Water relations of Picea abies. I. Comparison of water relations parameters of spruce shoots examined at the end of the vegetation period and in winter

Seasonal changes of some water relations parameters of Norway spruce shoots ( Picea abies [L.] Karst.) were studied during two experiments using the pressure-volume analysis. For each experiment only shoots of a single tree were used.
During the first study, the course of the turgor loss point (as bulk osmotic pressure when turgor first reaches zero, πp) of shoots developed in late 1986 vegetation period, were measured in 1987. The turgor loss point decreased temporarily from –2.5 MPa at the beginning of the year to –3.3 MPa at the end of March, but then increased to the original level for the rest of the year.
During the second study, water relations parameters were measured in late summer 1987 and in late winter 1988. Winter shoots at full water saturation contained up to 20% less water than in late summer. Accordingly, the bulk osmotic pressure at full water saturation (π_p) decreased from –1.7 MPa in late summer to –1.9 MPa in winter, π_p decreased also from –2.2 MPa to –2.8 MPa. However, the amount of osmotically active substances (mOsmol, N) remained unchanged. The relative amount of apoplastic water in the total shoot water content appeared to drop insignificantly from 17% to 15%.
The results show that the decrease in π_o and π_p in late winter is not due to an accumulation of osmotically active substances in the vacuoles but is due to a decrease in tissue water content. The temporary reduction of the symplastic volume by deposition of osmotically inert substances seems to be the most probable cause of this phenomenon.     

Comparison between gradient-dependent hydraulic conductivities of roots using the root pressure probe: the role of pressure propagations and implications for the relative roles of parallel radial pathways

Hydrostatic pressure relaxations with the root pressure probe are commonly used for measuring the hydraulic conductivity (Lp(r)) of roots. We compared the Lp(r) of roots from species with different root hydraulic properties (Lupinus angustifolius L. 'Merrit', Lupinus luteus L. 'Wodjil', Triticum aestivum L. 'Kulin' and Zea mays L. 'Pacific DK 477') using pressure relaxations, a pressure clamp and osmotic gradients to induce water flow across the root. Only the pressure clamp measures water flow under steady-state conditions. Lp(r) determined by pressure relaxations was two- to threefold greater than Lp(r) from pressure clamps and was independent of the direction of water flow. Lp(r) (pressure clamp) was two- to fourfold higher than for Lp(r) (osmotic) for all species except Triticum aestivum where Lp(r) (pressure clamp) and Lp(r) (osmotic) were not significantly different. A novel technique was developed to measure the propagation of pressure through roots to investigate the cause of the differences in Lp(r). Root segments were connected between two pressure probes so that when root pressure (P(r)) was manipulated by one probe, the other probe recorded changes in P(r). Pressure relaxations did not induce the expected kinetics in pressure in the probe at the other end of the root when axial hydraulic conductance, and probe and root capacitances were accounted for. An electric circuit model of the root was constructed that included an additional capacitance in the root loaded by a series of resistances. This accounted for the double exponential kinetics for intact roots in pressure relaxation experiments as well as the reduced response observed with the double probe experiments. Although there were potential errors with all the techniques, we considered that the measurement of Lp(r) using the pressure clamp was the most unambiguous for small pressure changes, and provided that sufficient time was allowed for pressure propagation through the root. The differences in Lp(r) from different methods of measurement have implications for the models describing water transport through roots and the potential role of aquaporins.