Understanding the Hydraulics of Porous Pipes: Tradeoffs Between Water Uptake and Root Length Utilization

The water uptake region in roots is several hundred times longer than the root diameter. The distributed nature of the uptake zone requires that the hydraulic design of roots be understood by analogy to flow through a “porous pipe.” Here we present results of an analytical and experimental investigation that allowed an in-depth analysis of root hydraulic properties. Measurements on nodal maize roots confirm the nonlinear distribution of water uptake predicted by the porous pipe model. The major design parameter governing the distribution of water uptake along a porous pipe is the ratio between its axial and radial hydraulic resistance. However, total flow is proportional to the pipe's overall resistance. These results suggest the existence of a tradeoff between the effective utilization of root length and the total capacity for water uptake.     

The Mechanics of Anchorage in Wheat Triticum aestivum L.: II. ANCHORAGE OF MATURE WHEAT AGAINST LODGING

The root system of mature wheat Triticum aestivum Marts Doveis dominated by the 7 to 15 adventitious roots which emergefrom the perimeter of the stem base, pointing radially outwardsand downwards. The basal, coronal region of these roots is thickand unbranched, attached to a rhizosheath of earth by a densecovering of root hairs and stiffened in bending by lignificationof outer layers of the cortex. Root lodging of plants involves bending of the coronal rootsat their base and axial movement of leeward and windward rootsthrough the soil; their resistance to these motions providemoments resisting lodging. A model of anchorage was producedby summing the resistance of each root to both forms of motionto give two anchorage components. The model was tested in aseries of mechanical experiments in which simulated lodgingwas followed by loading of individual roots; results supportedthe anchorage model and suggested that in the experimental conditionsthe two components of anchorage were approximately equal inmagnitude. The stem was about 30% stronger than the anchoragesystem. The coronal anchorage roots made up 4.4% of total dry mass;it is suggested that anchorage could be improved either by increasinginvestment in this region or by altering root orientation. Sequentialdevelopment of seminal and adventitious root systems is relatedto the changes in anchorage requirement with age.     

Water Uptake by Different Regions of the Barley Root. Pathways of Radial Flow in Relation to Development of the Endodermis

The water uptake by different lengths of lateral roots and 1.0cm or 5.0 cm lengths of the seminal axes from differentregionsof the root were measured in potometers with the shoot in airat two humidity regimes. A model of the contribution by thesedifferent regions of the root to the total water absorptionby the plant agreed well with measurements of water uptake bythe whole root system. According to this model, about one halfof the water taken up by the main axis came from the older suberizedregions further than 10 cm from the tip, and together with itsassociated lateral roots this region provided 75% of the totalwater transpired. The development of State III endodermal cellswas correlated with decreases in both the water uptake by theolder regions of the root and the translocation of calcium.Thus in the younger regions of the root where water uptake ismaximal, the flow of water is principally apoplastic althoughthere is also likely to be flow via the symplast Despite a 43%difference in transpirational demand between the two humiditytreatments, the leaf water potentials remained constant, implyinga change in root resistance. This change in resistance mightbe explained if there were an apoplastic pathway within thesuberin lamellae of State III endodermal cells. The responseto the increased transpirational demand is met by the olderregions of the root, in particular by the zone of lateral emergencewhere an apoplastic pathway is known to exist as the Casparianband in the endodermis breaks down with the emergence of thelateral roots. Key words: Endodermis, Pathways, Water Uptake     

A model for water uptake by plant roots

We present a model for water uptake by plant roots from unsaturated soil. The model includes the simultaneous flow of water inside the root network and in the soil. It is constructed by considering first the water uptake by a single root, and then using the parameterized results thereby obtained to build a model for water uptake by the developing root network. We focus our model on annual plants, in particular the model will be applicable to commercial monocultures like maize, wheat, etc. The model is solved numerically, and the results are compared with approximate analytic solutions. The model predicts that as a result of water uptake by plant roots, dry and wet zones will develop in the soil. The wet zone is located near the surface of the soil and the depth of it is determined by a balance between rainfall and the rate of water uptake. The dry zone develops directly beneath the wet zone because the influence of the rainfall at the soil surface does not reach this region, due to the nonlinear nature of the water flow in the partially saturated soil. We develop approximate analytic expressions for the depth of the wet zone and discuss briefly its ecological significance for the plant. Using this model we also address the question of where water uptake sites are concentrated in the root system. The model indicates that the regions near the base of the root system (i.e. close to the ground surface) and near the root tips will take up more water than the middle region of the root system, again due to the highly nonlinear nature of water flow in the soil.     

Genotypic variation in seedling root architectural traits and implications for drought adaptation in wheat (Triticum aestivum L.)

Root system characteristics are of fundamental importance to soil exploration and below-ground resource acquisition. Root architectural traits determine the in situ space-filling properties of a root system or root architecture. The growth angle of root axes is a principal component of root system architecture that has been strongly associated with acquisition efficiency in many crop species. The aims of this study were to examine the extent of genotypic variability for the growth angle and number of seminal roots in 27 current Australian and 3 CIMMYT wheat (Triticum aestivum L.) genotypes, and to quantify using fractal analysis the root system architecture of a subset of wheat genotypes contrasting in drought tolerance and seminal root characteristics. The growth angle and number of seminal roots showed significant genotypic variation among the wheat genotypes with values ranging from 36 to 56 (degrees) and 3 to 5 (plant⁻¹), respectively. Cluster analysis of wheat genotypes based on similarity in their seminal root characteristics resulted in four groups. The group composition reflected to some extent the genetic background and environmental adaptation of genotypes. Wheat cultivars grown widely in the Mediterranean environments of southern and western Australia generally had wider growth angle and lower number of seminal axes. In contrast, cultivars with superior performance on deep clay soils in the northern cropping region, such as SeriM82, Baxter, Babax, and Dharwar Dry exhibited a narrower angle of seminal axes. The wheat genotypes also showed significant variation in fractal dimension (D). The D values calculated for the individual segments of each root system suggested that, compared to the standard cultivar Hartog, the drought-tolerant genotypes adapted to the northern region tended to distribute relatively more roots in the soil volume directly underneath the plant. These findings suggest that wheat root system architecture is closely linked to the angle of seminal root axes at the seedling stage. The implications of genotypic variation in the seminal root characteristics and fractal dimension for specific adaptation to drought environment types are discussed with emphasis on the possible exploitation of root architectural traits in breeding for improved wheat cultivars for water-limited environments.     

Axial Resistance to Water Movement in Broad Bean (Vicia faba) Roots

A new potometer method was devised to measure the rates of wateruptake by small portions of an intact root system. An analysisof the relative effects of radial resistance (calculated fromthe potometer data) and axial resistance (calculated from thexylem dimensions) snowed that axial resistance was not likelyto be important. Root water potentials were determined by findingthe osmotic potential of the solution in the potometer whichjust prevented water uptake by the root. Root-tip water potentialswere similar in long and short roots suggesting that any effectof axial resistance was similar in both types of root.     

Understanding the Hydraulics of Porous Pipes: Tradeoffs Between Water Uptake and Root Length Utilization

The water uptake region in roots is several hundred times longer than the root diameter. The distributed nature of the uptake zone requires that the hydraulic design of roots be understood by analogy to flow through a porous pipe. Here we present results of an analytical and experimental investigation that allowed an in-depth analysis of root hydraulic properties. Measurements on nodal maize roots confirm the nonlinear distribution of water uptake predicted by the porous pipe model. The major design parameter governing the distribution of water uptake along a porous pipe is the ratio between its axial and radial hydraulic resistance. However, total flow is proportional to the pipe's overall resistance. These results suggest the existence of a tradeoff between the effective utilization of root length and the total capacity for water uptake.     

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.     

Mechanisms of waterlogging tolerance in wheat – a review of root and shoot physiology

We review the detrimental effects of waterlogging on physiology, growth and yield of wheat. We highlight traits contributing to waterlogging tolerance and genetic diversity in wheat. Death of seminal roots and restriction of adventitious root length due to O₂ deficiency result in low root:shoot ratio. Genotypes differ in seminal root anoxia tolerance, but mechanisms remain to be established; ethanol production rates do not explain anoxia tolerance. Root tip survival is short‐term, and thereafter, seminal root re‐growth upon re‐aeration is limited. Genotypes differ in adventitious root numbers and in aerenchyma formation within these roots, resulting in varying waterlogging tolerances. Root extension is restricted by capacity for internal O₂ movement to the apex. Sub‐optimal O₂ restricts root N uptake and translocation to the shoots, with N deficiency causing reduced shoot growth and grain yield. Although photosynthesis declines, sugars typically accumulate in shoots of waterlogged plants. Mn or Fe toxicity might occur in shoots of wheat on strongly acidic soils, but probably not more widely. Future breeding for waterlogging tolerance should focus on root internal aeration and better N‐use efficiency; exploiting the genetic diversity in wheat for these and other traits should enable improvement of waterlogging tolerance.     

The contribution of different regions of the seminal roots of wheat to uptake of nitrate from soil

A technique was developed to determine the physiological activity of defined sections of seminal roots of wheat grown in sand. Wheat plants were grown for 2 weeks in narrow columns of N-deficient sand to which all other nutrients had been added. The columns were split longitudinally and ¹⁵N-labelled nitrate, in an agar medium, supplied to 2 cm sections of root. Shoots and roots were analysed after 24 h to determine the uptake of ¹⁵N. Three sections were examined on either the secondary or tertiary seminal root: 1 cm from the seed (basal segment), 35 cm from the seed (middle segment) and 4 cm from the root apex (apical segment). Total uptake was greatest from the basal and middle segments, declining by 50% from the apical segment. However, uptake per unit root length, including exposed sections of lateral roots, was not significantly different along the root.     

The Direction of Growth of Seminal Roots of Triticum aestivum L. and Experimental Modification Thereof

The seminal root system of wheat (Triticum aestivum L.) consistsof a primary seminal root and the first and second pair of seminalroots, counting upwards. These roots are plagiotropic. The directionof growth was estimated as the angle from the vertical for eachof the three types of seminal roots that protruded from a hemispherical,soil-filled basket buried in the field. The angle of growthvaried with cultivar. It was smallest in the primary seminalroot and largest in the second pair of roots in all 12 cultivarsgrown in the field. Attempts to modify the angle of growth weremade under controlled environmental conditions. When the grainwas sown with its embryo face down, the angle of growth of thefirst pair of roots became smaller in the cultivars with inherentlylarger angles. The excision of the primary seminal root affectedthe first pair of roots and the excision of the first pair affectedthe second pair. These effects comprised a decrease in the angleof growth and an increase in root diameter. These changes inthe angle of growth caused by root excision is interpreted asa kind of compensatory growth. The direction of root growthand its impact on shaping wheat root system is discussed.Copyright1994, 1999 Academic Press Compensatory growth, direction of growth, gravitropism, liminal angle, plagiotropism, seminal roots, Triticum aestivum L., wheat     

Growth, Nitrogen Uptake and Flow in Maize Plants Affected by Root Growth Restriction

The objective of the present study was to investigate the influence of a reduced maize root-system size on root growth and nitrogen (N) uptake and flow within plants. Restriction of shoot-borne root growth caused a strong decrease in the absorption of root: shoot dry weight ratio and a reduction in shoot growth. On the other hand, compensatory growth and an increased N uptake rate in the remaining roots were observed. Despite the limited long-distance transport pathway in the mesocotyl with restriction of shoot-borne root growth, N cycling within these plants was higher than those in control plants, implying that xylem and phloem flow velocities via the mesocotyl were considerably higher than in plants with an intact root system. The removal of the seminal roots in addition to restricting shoot-borne root development did not affect whole plant growth and N uptake, except for the stronger compensatory growth of the primary roots. Our results suggest that an adequate N supply to maize plant is maintained by compensatory growth of the remaining roots, increased N uptake rate and flow velocities within the xylem and phloem via the mesocotyl, and reduction in the shoot growth rate.     

Resistance to water flow in the intracoleoptile internode of wheat

Summary Measurements of xylem vessel number and radii in the seminal roots and intra-coleoptile internode (I.C.I.) of five wheat genotypes showed that the conducting capacity of the I.C.I. was close to the main seminal axis, and would restrict flow when the usual 3–5 seminal axes contributed to uptake. The length and hence resistance of the I.C.I. increased with sowing depth, whilst xylem diameter also fell in two genotypes, which would further restrict flow. The resistance per unit length of I.C.I., assuming Poiseuille flow, was 4×10^–4 cm^–4 day MPa. A pressure drop of 0.15 MPa along an I.C.I. 5 cm long would be required to maintain transpiration under typical field conditions in southern Australia in spring.In a second study of eleven wheat varieties sown up to 10 cm deep, maximum I.C.I. length ranged from 3.6–6.8 cm amongst varieties with similar maximum coleoptile lengths (6–8 cm). Thus considerable variation in hydraulic resistance may be achieved by the appropriate combination of genotype and planting depth. It was concluded that potentially useful differences in the rate of subsoil water use could result.     

非均匀水势下作物根系吸水模型分析

根据土壤-根系统中水分守恒和水势对水分运输作用的原理, 建立了土壤中非均匀水势作物根系吸水模型。在该模型中, 分别对一次函数和指数函数两种不同的非均匀土壤水势的表达形式建立模型, 并对非均匀水势和均匀水势下模型的解析解之间的关系进行了探讨; 利用该模型讨论根系的吸收阻力和木质部传导阻力的比率对根吸水的影响; 运用阻力比率的合理生理范围确定根生长的优化长度。结果表明: 在特定情况下, 非均匀水势下的根系吸水模型可以用于均匀水势, 对Poiseuille公式进行修正后得到的根的优化长度接近实际值。     

Morphological plasticity of wheat and barley roots in response to spatial variation in soil strength

Root systems of individual crop plants may encounter large variations in mechanical impedance to root penetration. Split-root experiments were conducted to compare the effects of spatial variation in soil strength on the morphological plasticity of wheat and barley roots, and its relationship to shoot growth. Plants of spring barley (Hordeum vulgare cv Prisma) and spring wheat (Triticum aestivum cv Alexandria) were grown for 12 days with their seminal roots divided between two halves of a cylinder packed with sandy loam soil. Three treatment combinations were imposed: loose soil where both halves of the cylinder were packed to 1.1 g cm^–3 (penetrometer resistance 0.3 MPa), dense soil where both halves were packed to 1.4 g cm^–3 (penetrometer resistance 1 MPa), and a split-root treatment where one half was packed to 1.1 and the other to 1.4 g cm^–3. In barley, uniform high soil strength restricted the extension of main seminal root axes more than laterals. In the split-root treatment, the length of laterals and the dry weight of main axes and laterals were increased in the loose soil half and reduced in the dense soil half compared with their respective loose and dense-soil controls. No such compensatory adjustments between main axis and laterals and between individual seminal roots were found in wheat. Variation in soil strength had no effect on the density of lateral roots (number per unit main axis length) in either barley or wheat. The nature and extent of wheat root plasticity in response to variation in soil strength was very different from that in response to changes in N-supply in previous experiments. In spite of the compensatory adjustments in growth between individual seminal roots of barley, the growth of barley shoots, as in wheat, was reduced when part of the root system was in compacted soil.     

Activation of seminal root primordia during wheat domestication reveals underlying mechanisms of plant resilience

Seminal roots constitute the initial wheat root system and provide the main route for water absorption during early stages of development. Seminal root number (SRN) varies among species. However, the mechanisms through which SRN is controlled and in turn contribute to environmental adaptation are poorly understood. Here, we show that SRN increased upon wheat domestication from 3 to 5 due to the activation of 2 root primordia that are suppressed in wild wheat, a trait controlled by loci expressed in the germinating embryo. Suppression of root primordia did not limit water uptake, indicating that 3 seminal roots is adequate to maintain growth during seedling development. The persistence of roots at their primordial state promoted seedling recovery from water stress through reactivation of suppressed primordia upon rehydration. Our findings suggest that under well‐watered conditions, SRN is not a limiting factor, and excessive number of roots may be costly and maladaptive. Following water stress, lack of substantial root system suppresses growth and rapid recovery of the root system is essential for seedling recovery. This study underscores SRN as key adaptive trait that was reshaped upon domestication. The maintenance of roots at their primordial state during seedling development may be regarded as seedling protective mechanism against water stress.     

Modelling of the Hydraulic Architecture of Root Systems: An Integrated Approach to Water Absorption--Distribution of Axial and Radial Conductances in Maize

The ‘Hydraulic Tree Model’ of the root system simulateswater uptake through root systems by coupling a root architecturemodel with laws for water flow into and along roots (Doussan,Pagès and Vercambre,Annals of Botany81: 213–223,1998). A detailed picture of water absorption in all roots comprisingthe root system is thus provided. Moreover, the influence ofdifferent distributions of radial and axial hydraulic conductancesin the root system on the patterns of water uptake can be analysed.Use of the model with Varney and Canny's data (1993) for flowalong maize roots demonstrated that a constant conductance inthe root system cannot reproduce the observed water flux profiles.Taking into account the existing data on hydraulic conductancesin maize roots, we fitted the distribution of conductances inthe root system to the observed flux data. The result is that,during root tissue maturation, the radial conductivity decreasesby one order of magnitude while the axial conductance increasesby about three orders of magnitude. Both types of conductanceexhibit abrupt changes in their evolution. Due to the conductancedistribution in the root system, appreciable water potentialgradients may develop in the roots, in both the branch rootsand main axes. An important point is that the conductance distributionin the branch roots described by the model should be relatedto the age of the tissue (and not the distance from the branchroot tip) and is therefore closely related to the developmentprocess. Thus for branch roots, which represent about 90% ofthe calculated total water uptake in 43-d-old maize, water absorptionwill depend on the opening of the metaxylem in the axes, andon the time dependent variation of the conductances in the branchroots.Copyright 1998 Annals of Botany Company Water; absorption; root system; architecture; model; hydraulic conductance;Zea maysL.     

Water Movement Through Plant Roots

Mathematical analysis of the hydraulics of water movement throughplant roots, in terms of radial and axial resistances, has ledto equations which provide new insights into the effects ofthe component resistan ces on water uptake by and movement throughindividual roots and root systems. The ratio of axial to radialresistance determines the optimum length of a root and its totalresistance to water movement. The equations permit direct calculationsof the plant water potentials necessary, at the base of theplant, to sustain given flow rates through root systems withgiven characteristics. Lateral spacing and the resistance ofindividual laterals are the dominant factors determining totalflux per unit area into a root. When soil water potential increases with depth (surface layersdrier) root resistance tends to decrease with increasing flowrate; the reverse occurs when the surface is wetter than thelower layers. Calculated patterns of water movement into andthrough roots, in relation to soil water potential and flowrate through the root, indicate efflux from root to soil undercertain conditions. This is considered to reflect reality, althoughthe fluxes are probably transient or intermittent. The equations presented should be combined with equations describingwater movement through soil to define the behaviour of the wholeroot-soil system adequately.     

Changes in Root Hydraulic Conductivity During Wheat Evolution

A better understanding of the mechanisms of water uptake by plant roots should be vital for improving drought resistance and water use efficiency (WUE). In the present study, we have demonstrated correlations between root system hydraulic conductivity and root characteristics during evolution using six wheat evolution genotypes (solution culture) with different ploidy chromosome sets (Triticum boeoticum Bioss., T. monococcum L.: 2n = 2x = 14; T. dicoccides Koern., T. dicoccon (Schrank) Schuebl.: 2n = 4x = 28;T. vulgare Vill., T. aestivum L. cv. Xiaoyan No. 6: 2n = 6x = 42). The experimental results showed that significant correlations were found between root system hydraulic conductivity and root characteristics of the materials with the increase in ploidy chromosomes (2x→6x) during wheat evolution. Hydraulic conductivity of the wheat root system at the whole-plant level was increased with chromosome ploidy during evolution, which was positively correlated with hydraulic conductivity of single roots, whole plant biomass,root average diameter, and root growth (length, area), whereas the root/shoot ratio had an inverse correlation with the hydraulic conductivity of root system with increasing chromosome ploidy during wheat evolution. Therefore, it is concluded that that the water uptake ability of wheat roots was strengthened from wild to modern cultivated species during evolution, which will provide scientific evidence for genetic breeding to improve the WUE of wheat by genetic engineering.     

Water and Sulphate Uptake at Root Reduction in Ricinus communis

The aim of the investigation was to study the influence of the rate of water uptake on the uptake of sulphate at supernormal rates of water flow. This was achieved by reducing the size of the root system of 42 days old Ricinus plants. The rate of water flow through the root increased 3 times by reducing the root system to 20 percent. This did not change the retention of sulphate in the roots. The uptake of sulphate was proportional to the size of the root system and thus independent of the rate of water flow while the water uptake (transpiration) was a function of the size of the shoot and the resistance of the root. This was contrary to the conditions at a moderate rate of water flow, when water and sulphate uptake followed each other. The results are discussed in terms of the salt uptake as a series of active and passive processes.