Aluminium tolerance of root hairs underlies genotypic differences in rhizosheath size of wheat (Triticum aestivum) grown on acid soil

We found significant genetic variation in the ability of wheat (Triticum aestivum) to form rhizosheaths on acid soil and assessed whether differences in aluminium (Al(3+) ) tolerance of root hairs between genotypes was the physiological basis for this genetic variation. A method was developed to rapidly screen rhizosheath size in a range of wheat genotypes. Backcrossed populations were generated from cv Fronteira (large rhizosheath) using cv EGA-Burke (small rhizosheath) as the recurrent parent. A positive correlation existed between rhizosheath size on acid soil and root hair length. In hydroponic experiments, root hairs of the backcrossed lines with large rhizosheaths were more tolerant of Al(3+) toxicity than the backcrossed lines with small rhizosheaths. We conclude that greater Al(3+) tolerance of root hairs underlies the larger rhizosheath of wheat grown on acid soil. Tolerance of the root hairs to Al(3+) was largely independent of the TaALMT1 gene which suggests that different genes encode the Al(3+) tolerance of root hairs. The maintenance of longer root hairs in acid soils is important for the efficient uptake of water and nutrients.     

Drought-induced changes in soil contact and hydraulic conductivity for roots of Opuntia ficus-indica with and without rhizosheaths

Water movement between roots and soil can be limited by incomplete root–soil contact, such as that caused by air gaps due to root shrinkage, and can also be influenced by rhizosheaths, composed of soil particles bound together by root exudates and root hairs. The possible occurrence of air gaps between the roots and the soil and their consequences for the hydraulic conductivity of the root–soil pathway were therefore investigated for the cactus t Opuntia ficus-indica, which has two distinct root regions: a younger, distal region where rhizosheaths occur, and an older, proximal region where roots are bare. Resin-embedded sections of roots in soil were examined microscopically to determine root–soil contact for container-grown plants kept moist for 21 days, kept moist and vibrated to eliminate air gaps, droughted for 21 days, or droughted and vibrated. During drought, roots shrank radially by 30% and root–soil contact in the bare root region of nonvibrated containers was reduced from 81% to 31%. For the sheathed region, the hydraulic conductivity of the rhizosheath was the least limiting factor and the root hydraulic conductivity was the most limiting; for the bare root region, the hydraulic conductivity of the soil was the least limiting factor and the hydraulic conductivity of the root–soil air gap was the most limiting. The rhizosheath, by virtually eliminating root–soil air gaps, facilitated water uptake in moist soil. In the bare root region, the extremely low hydraulic conductivity of the root–soil air gap during drought helped limit water loss from roots to a drier soil.     

Root hair length and rhizosheath mass depend on soil porosity,strength and water content in barley genotypes

Selecting plants with improved root hair growth is a key strategy for improving phosphorus-uptake efficiency in agriculture. While significant inter- and intra-specific variation is reported for root hair length, it is not known whether these phenotypic differences are exhibited under conditions that are known to affect root hair elongation. This work investigates the effect of soil strength, soil water content (SWC) and soil particle size (SPS) on the root hair length of different root hair genotypes of barley. The root hair and rhizosheath development of five root hair genotypes of barley (Hordeum vulgare L.) was compared in soils with penetrometer resistances ranging from 0.03 to 4.45 MPa (dry bulk densities 1.2–1.7 g cm^?3). A “short” (SRH) and “long” root hair (LRH) genotype was selected to further investigate whether differentiation of these genotypes was related to SWC or SPS when grown in washed graded sand. In low-strength soil (<1.43 MPa), root hairs of the LRH genotype were on average 25 % longer than that of the SRH genotype. In high-strength soil, root hair length of the LRH genotype was shorter than that in low-strength soil and did not differ from that of the SRH genotype. Root hairs were shorter in wetter soils or soils with smaller particles, and again SRH and LRH did not differ in hair length. Longer root hairs were generally, but not always, associated with larger rhizosheaths, suggesting that mucilage adhesion was also important. The root hair growth of barley was found to be highly responsive to soil properties and this impacted on the expression of phenotypic differences in root hair length. While root hairs are an important trait for phosphorus acquisition in dense soils, the results highlight the importance of selecting multiple and potentially robust root traits to improve resource acquisition in agricultural systems.     

Effects of Local Variations in Soil Moisture on Hydrophobic Deposits and Dye Diffusion in Corn Roots

The effects of local soil water content (SWC) alone on the development of hydrophobic deposits and on diffusivity of root tissues were studied in primary roots of maize seedlings whose top regions were growing at a constant rate in damp soil. Corn primary roots were grown through 30 cm-profiles of soil with pre-set dry (8% SWC), or wet (23% SWC), middle layers, and moist (15% SWC) top and bottom layers. When the root tissues in the soil of variable SWC were maturing (approximately 17% of the total root length), the tips of the roots were in damp soil several centimeters away and growing at the rate characteristic of this environment. Histochemistry and fluorescence microscopy on root sections of similar age showed that in these roots local SWC had no effect on hypodermal and endodermal suberization: both had equally developed Casparian bands and suberized lamellae. However, local SWC did alter the hydrophobic deposits in the walls of the epidermis; they increase in dry soil. To study tissue diffusivity, root portions from dry or wet soil were divided in the transverse direction, and half was dipped in sulphorhodamine G (SRG), sectioned, and observed with fluorescence optics. The other half was studied with a cryo-scanning electron microscope to observe the contents of the cortical intercellular spaces. The rate of SRG diffusion in a portion of root from dry or wet soil was independent of local SWC but was positively correlated with numbers of fluid-filled intercellular spaces. The implications of these observations for the movements of ions through the root cortex are discussed.     

Water uptake and storage by rhizosheaths of Oryzopsis hymenoides: a numerical simulation

Rhizosheaths (sheaths of sand grains that form around the roots of some grasses) are common in perennial grasses that colonise sandy substrates. It has been hypothesised that rhizosheaths increase water availability by increasing the efficiency of water absorption. Others have suggested that rhizosheaths act as storage reservoirs for water. In either case rhizosheaths undoubtedly play an important role in the water relations of these grasses.
In an attempt to evaluate the main function of rhizosheaths, we developed a finite element cylindrical water flow model which enabled us to simulate water uptake by Oryzopsis hymenoides (Roem, and Shult.) Ricker. This model allowed us to estimate total water uptake by root systems with and without rhizosheaths and to compare these values to the extra water stored within the rhizosheath. The results of this study suggest that the presence of rhizosheaths is more important in reducing the total resistance to water flow within the rhizosphere than in enhancing water storage.     

多糖和土壤团聚体对扁穗冰草根鞘形成的影响

根鞘是指在植物根系表面存在的由土壤颗粒与根表及根毛相互粘附、缠绕形成的土壤连续体。这种结构的形成被认为是植物根系与微生物及土壤环境相互作用的产物,是对干旱逆境环境的生理适应。以扁穗冰草(Agropyron cristatum L.)为研究对象,结合土壤质地、根鞘内粗多糖成分和根鞘表观特性三者之间的内在联系,分析探索粗多糖在扁穗冰草根鞘的形成过程中的作用。结果表明:土壤团聚体结构是根鞘形成的直接原因;多糖物质通过发挥多糖的粘性胶结作用影响土壤团聚体的结构和比例,同时发挥多糖的亲水、吸水和保水的作用从而形成根鞘。     

Importance of internal hydraulic redistribution for prolonging the lifespan of roots in dry soil

Redistribution of water within plants could mitigate drought stress of roots in zones of low soil moisture. Plant internal redistribution of water from regions of high soil moisture to roots in dry soil occurs during periods of low evaporative demand. Using minirhizotrons, we observed similar lifespans of roots in wet and dry soil for the grapevine 'Merlot' (Vitis vinifera) on the rootstock 101-14 Millardet de Gramanet (Vitis riparia x Vitis rupestris) in a Napa County, California vineyard. We hypothesized that hydraulic redistribution would prevent an appreciable reduction in root water potential and would contribute to prolonged root survivorship in dry soil zones. In a greenhouse study that tested this hypothesis, grapevine root systems were divided using split pots and were grown for 6 months. With thermocouple psychrometers, we measured water potentials of roots of the same plant in both wet and dry soil under three treatments: control (C), 24 h light + supplemental water (LW) and 24 h light only (L). Similar to the field results, roots in the dry side of split pots had similar survivorship as roots in the wet side of the split pots (P = 0.136) in the C treatment. In contrast, reduced root survivorship was directly associated with plants in which hydraulic redistribution was experimentally reduced by 24 h light. Dry-side roots of plants in the LW treatment lived half as long as the roots in the wet soil despite being provided with supplemental water (P < 0.0004). Additionally, pre-dawn water potentials of roots in dry soil under 24 h of illumination (L and LW) exhibited values nearly twice as negative as those of C plants (P = 0.034). Estimates of root membrane integrity using electrolyte leakage were consistent with patterns of root survivorship. Plants in which nocturnal hydraulic redistribution was reduced exhibited more than twice the amount of electrolyte leakage in dry roots compared to those in wet soil of the same plant. Our study demonstrates that besides a number of ecological advantages to protecting tissues against desiccation, internal hydraulic redistribution of water is a mechanism consistent with extended root survivorship in dry soils.     

The effect of soil strength on the growth of pigeonpea radicles and seedlings

The effect of soil strength on the growth of pigeonpea radicles and seedlings was investigated in cores of three clay soils prepared at different water contents and bulk densities in the laboratory.Radicle elongation directly into soil cores was reduced from 50–70 mm d^-1 at strengths less than 0.5 MPa to 0 mm d^-1 at 3.5–3.7 MPa. The response to soil strength was affected by the water content of the soil, presumably as a result of reduced oxygen availability in wetter soil. This effect was apparent in soils wet to air-filled porosities less than 0.15 m³ m^-3.Radicles were more sensitive to high soil strength (>1.5 MPa) than were seedling roots which encountered the same conditions at 60 mm in the profile. Radicle growth ceased at 3.5 MPa which reduced seedling root growth by only 60%.Despite a 60% reduction in root length in the high strength zone, seedling roots compensated in zones of loose soil above and below the compacted layer, and total root length and shoot growth were unaffected. There was no evidence of a root signal response which results in reduced shoot growth in some species in response to high soil strength.The proliferation of roots in surface layers and the delayed penetration of the root system to depth in compacted soil are likely to expose seedlings to a greater risk of water-deficit in the field, particularly under dryland conditions where plants rely on stored subsoil water for growth.     

Soil textures rather than root hairs dominate water uptake and soil–plant hydraulics under drought

Although the role of root hairs (RHs) in nutrient uptake is well documented, their role in water uptake and drought tolerance remains controversial. Maize (Zea mays) wild-type and its hair-defective mutant (Mut; roothairless 3) were grown in two contrasting soil textures (sand and loam). We used a root pressure chamber to measure the relation between transpiration rate (E) and leaf xylem water potential (ψ_{leaf_x}) during soil drying. Our hypotheses were: (1) RHs extend root–soil contact and reduce the ψ_{leaf_x} decline at high E in dry soils; (2) the impact of RHs is more pronounced in sand; and (3) Muts partly compensate for lacking RHs by producing longer and/or thicker roots. The ψ_{leaf_x}(E) relation was linear in wet conditions and became nonlinear as the soils dried. This nonlinearity occurred more abruptly and at less negative matric potentials in sand (ca. −10 kPa) than in loam (ca. −100 kPa). At more negative soil matric potentials, soil hydraulic conductance became smaller than root hydraulic conductance in both soils. Both genotypes exhibited 1.7 times longer roots in loam, but 1.6 times thicker roots in sand. No differences were observed in the ψ_{leaf_x}(E) relation and active root length between the two genotypes. In maize, RHs had a minor contribution to soil–plant hydraulics in both soils and their putative role in water uptake was smaller than that reported for barley (Hordeum vulgare). These results suggest that the role of RHs cannot be easily generalized across species and soil textures affect the response of root hydraulics to soil drying.

Root hairs of maize do not show evident contribution to root growth, water uptake, and soil–plant hydraulics, whereas soil textures affect the response of root hydraulics to soil drying.     

The distribution and abundance of wheat roots in a dense,structured subsoil – implications for water uptake

We analysed the abundance, spatial distribution and soil contact of wheat roots in dense, structured subsoil to determine whether incomplete extraction of subsoil water was due to root system limitations. Intact soil cores were collected to 1.6 m below wheat crops at maturity on a red Kandosol in southern Australia. Wheat roots, remnant roots, soil pores and root–soil contact were quantified at fresh breaks in the soil cores. In surface soil layers (<0.6 m) 30–40% of roots were clumped within pores and cracks in the soil, increasing to 85–100% in the subsoil (>0.6 m), where 44% of roots were in pores with at least three other roots. Most pores contained no roots, with occupancy declining from 20% in surface layers to 5% in subsoil. Wheat roots clumped into pores contacted the surrounding soil via numerous root hairs, whereas roots in cracks were appressed to the soil surface and had very few root hairs. Calculations assuming good root–soil contact indicated that root density was sufficient to extract available subsoil water, suggesting that uptake is constrained at the root–soil interface. To increase extraction of subsoil water, genetic targets could include increasing root–soil contact with denser root hairs, and increasing root proliferation to utilize existing soil pores.     

Rhizosheath formation and involvement in foxtail millet(Setaria italica) root growth under drought stress

The rhizosheath, a layer of soil particles that adheres firmly to the root surface by a combination of root hairs and mucilage, may improve tolerance to drought stress. Setaria italica(L.) P. Beauv.(foxtail millet), a member of the Poaceae family, is an important food and fodder crop in arid regions and forms a larger rhizosheath under drought conditions. Rhizosheath formation under drought conditions has been studied, but the regulation of root hair growth and rhizosheath size in response to soil moisture remains unclear. To address this question, in this study we monitored root hair growth and rhizosheath development in response to a gradual decline in soil moisture. Here, we determined that a soil moisture level of 10%–14%(w/w)stimulated greater rhizosheath production compared to other soil moisture levels. Root hair density and length also increased at this soil moisture level, which was validated by measurement of the expression of root hair-related genes.These findings contribute to our understanding of rhizosheath formation in response to soil water stress.     

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.     

Response of millet and sorghum to a varying water supply around the primary and nodal roots

Background and Aims

Cereals have two root systems. The primary system originates from the embryo when the seed germinates and can support the plant until it produces grain. The nodal system can emerge from stem nodes throughout the plant''s life; its value for yield is unclear and depends on the environment. The aim of this study was to test the role of nodal roots of sorghum and millet in plant growth in response to variation in soil moisture. Sorghum and millet were chosen as both are adapted to dry conditions.

Methods

Sorghum and millet were grown in a split-pot system that allowed the primary and nodal roots to be watered separately.

Key Results

When primary and nodal roots were watered (12 % soil water content; SWC), millet nodal roots were seven times longer than those of sorghum and six times longer than millet plants in dry treatments, mainly from an 8-fold increase in branch root length. When soil was allowed to dry in both compartments, millet nodal roots responded and grew 20 % longer branch roots than in the well-watered control. Sorghum nodal roots were unchanged. When only primary roots received water, nodal roots of both species emerged and elongated into extremely dry soil (0·6–1·5 % SWC), possibly with phloem-delivered water from the primary roots in the moist inner pot. Nodal roots were thick, short, branchless and vertical, indicating a tropism that was more pronounced in millet. Total nodal root length increased in both species when the dry soil was covered with plastic, suggesting that stubble retention or leaf mulching could facilitate nodal roots reaching deeper moist layers in dry climates. Greater nodal root length in millet than in sorghum was associated with increased shoot biomass, water uptake and water use efficiency (shoot mass per water). Millet had a more plastic response than sorghum to moisture around the nodal roots due to (1) faster growth and progression through ontogeny for earlier nodal root branch length and (2) partitioning to nodal root length from primary roots, independent of shoot size.

Conclusions

Nodal and primary roots have distinct responses to soil moisture that depend on species. They can be selected independently in a breeding programme to shape root architecture. A rapid rate of plant development and enhanced responsiveness to local moisture may be traits that favour nodal roots and water use efficiency at no cost to shoot growth.     

Heterogeneity in Water Availability Alters Cellular Development and Hydraulic Conductivity along Roots of a Desert Succulent

Plants of the desert succulent Agave deserti were grown in partitionedcontainers to determine whether heterogeneity in soil moistureleads to differences in cellular development and hydraulic conductivityalong individual roots. Roots from containers with a dry distalcompartment (furthest from the shoot), a wet middle compartment,and a dry proximal compartment had distal regions (includingthe root tips) that were more suberized and lignified in theendodermis and adjacent cell layers than were root regions fromthe wet middle compartment. Proximal root regions about 40 mmfrom the succulent shoot base were also relatively unsuberized,suggesting that both external and internal supplies of waterdelayed tissue maturation. Root segments from wet middle compartmentsand from dry proximal compartments had higher hydraulic conductivitythan did the more suberized root segments from dry distal compartments.Unlike distal root segments from wet compartments, segmentsfrom dry compartments suffered no decrease in hydraulic conductivityafter immersion in mercuric chloride, suggesting that aquaporinactivity diminished for roots during drought. The possible closureof water channels could help limit root water loss to a dryingsoil. The delayed development of suberized cell layers may allowroot regions to maximize water uptake from wet soil patches(such as under rocks), and the relatively immature, absorptiveroot region near the base of the shoot may help A. deserti capturewater from a briefly wetted surface soil. Copyright 2000 Annalsof Botany Company Agave deserti, root plasticity, water uptake, aquaporins, suberization, endodermis, divided pots.     

Vertical distribution of maize roots in relation to permanent soil characteristics

The vertical distribution of maize roots was studied in four contrasting soils, (arenosols, luvisols, planosols and vertisols) by using in-situ root mapping on vertical planes. The relationship between root contact frequency and depth was different for each soil, with a relatively low field-to-field variability within each soil type. The general aspect of this relationship did not change appreciably for three years in arenosols, with a low colonization in sandy layers probably being due to mechanical barriers. The relationship was consistently non-monotonic in luvisols and planosols, because of the sparse colonization of sandy layers. In planosols, these layers were traversed by some primary roots, which were essentially clustered in animal burrows. The distribution of root contact frequency was closer to an exponential function in vertisols. In these soils rooting depth and colonization of deep soil layers showed a marked increase during two dry years compared with a wet year. This was probably due to a denser net of shrinkage cracks and slickensides, where roots were essentially located in dry years. These results raise the possibility of modelling the decrease in root distribution with depth using soil information and climatic characteristics.     

Nutrient uptake estimates for woody species as described by the NST 3.0, SSAND, and PCATS mechanistic nutrient uptake models

The effect of soil acidity on root and rhizosheath development in wheat and barley seedlings was investigated in an acid Ferrosol soil to which various amounts of lime (CaCO₃) were applied to modify soil Al concentrations (pH (CaCl₂): 4.22 to 5.35 and Al (CaCl₂ extract): 17.7 to 0.4 mg kg^?1 soil; respectively), and Ferrosol soil from an adjacent location at the same site which had a higher Al concentration (pH 4.19; 29.2 mg kg^?1 Al). The cereal lines were selected on the basis of differences in their rate of root growth, Al-resistance and root hair morphology. Root morphology was assessed after 7 days of growth. The length of fine (mainly lateral) roots of Al-sensitive genotypes was more sensitive to soil Al concentrations than that of the coarse (mainly primary) roots. The experiments demonstrated that even where root growth was protected by expression of the TaALMT1 gene for Al-resistance, root-soil contact was diminished by soil acidity because root hair length (in many lines), and root hair density and rhizosheath formation (all lines) were adversely affected by soil acidity. In the case of Al-sensitive lines, fine root growth and rhizosheath mass were reduced over much the same range of soil Al concentrations (i.e. >3–6 mg kg^?1 Al). Although Al-resistant lines could maintain fine root length under these conditions, they were similarly unable to maintain rhizosheath mass. This finding may help to explain why Al-resistant wheats which yield relatively well in deep acid soils, may also benefit from application of lime to the surface layers of the soil.     

Significance of root hairs for plant performance under contrasting field conditions and water deficit

Background and AimsPrevious laboratory studies have suggested selection for root hair traits in future crop breeding to improve resource use efficiency and stress tolerance. However, data on the interplay between root hairs and open-field systems, under contrasting soils and climate conditions, are limited. As such, this study aims to experimentally elucidate some of the impacts that root hairs have on plant performance on a field scale.MethodsA field experiment was set up in Scotland for two consecutive years, under contrasting climate conditions and different soil textures (i.e. clay loam vs. sandy loam). Five barley (Hordeum vulgare) genotypes exhibiting variation in root hair length and density were used in the study. Root hair length, density and rhizosheath weight were measured at several growth stages, as well as shoot biomass, plant water status, shoot phosphorus (P) accumulation and grain yield.Key ResultsMeasurements of root hair density, length and its correlation with rhizosheath weight highlighted trait robustness in the field under variable environmental conditions, although significant variations were found between soil textures as the growing season progressed. Root hairs did not confer a notable advantage to barley under optimal conditions, but under soil water deficit root hairs enhanced plant water status and stress tolerance resulting in a less negative leaf water potential and lower leaf abscisic acid concentration, while promoting shoot P accumulation. Furthermore, the presence of root hairs did not decrease yield under optimal conditions, while root hairs enhanced yield stability under drought.ConclusionsSelecting for beneficial root hair traits can enhance yield stability without diminishing yield potential, overcoming the breeder’s dilemma of trying to simultaneously enhance both productivity and resilience. Therefore, the maintenance or enhancement of root hairs can represent a key trait for breeding the next generation of crops for improved drought tolerance in relation to climate change.     

沙丘多枝柽柳灌丛根层土壤含水量变化特征与根系水力提升证据

分析干旱区深根型荒漠植物的根层土壤水分是揭示荒漠植物与土壤水分关系机理的重要方面。在黑河中游一片风沙侵蚀区域的多枝柽柳(Tamarix ramosissima)人工林地中, 对表层0.3 m到3 m深的土壤不同深度的含水量进行了连续的动态观测。结果显示, 多枝柽柳根系层土壤含水量可以分为明显不同的3层: 浅层(0.2-1.7 m深)相对湿润层、中间(1.7-2.7 m深)相对干层和深层(2.7 m以下)有效含水层。在多枝柽柳生长盛期, 浅层相对湿润层土壤含水量呈现明显的昼夜变化特征, 同时, 在晚上植物根系与浅层土壤之间存在正水势梯度, 这说明存在根系水力提升现象。水力提升是干旱气候下根层浅层土壤含水量保持相对湿润的主要原因, 并因此维系浅层根系的发育, 也为多枝柽柳具备的防风固沙功能提供了可能的解释。据初步估算, 多枝柽柳根系水力提升占每天耗水量的5%-8%, 耗水的主要水分来源仍然是充足的土壤深层有效含水层。     

What are the implications of variation in root hair length on tolerance to phosphorus deficiency in combination with water stress in barley (Hordeum vulgare)?

Background and Aims

Phosphorus commonly limits crop yield and is frequently applied as fertilizer; however, supplies of quality rock phosphate for fertilizer production are diminishing. Plants have evolved many mechanisms to increase their P-fertilizer use efficiency, and an understanding of these traits could result in improved long-term sustainability of agriculture. Here a mutant population is utilized to assess the impact of root hair length on P acquisition and yield under P-deficient conditions alone or when combined with drought.

Methods

Mutants with various root hair phenotypes were grown in the glasshouse in pots filled with soil representing sufficient and deficient P treatments and, in one experiment, a range of water availability was also imposed. Plants were variously harvested at 7 d, 8 weeks and 14 weeks, and variables including root hair length, rhizosheath weight, biomass, P accumulation and yield were measured.

Key Results

The results confirmed the robustness of the root hair phenotypes in soils and their relationship to rhizosheath production. The data demonstrated that root hair length is important for shoot P accumulation and biomass, while only the presence of root hairs is critical for yield. Root hair presence was also critical for tolerance to extreme combined P deficit and drought stress, with genotypes with no root hairs suffering extreme growth retardation in comparison with those with root hairs.

Conclusions

The results suggest that although root hair length is not important for maintaining yield, the presence of root hairs is implicit to sustainable yield of barley under P-deficient conditions and when combined with extreme drought. Root hairs are a trait that should be maintained in future germplasm.     

Modelling the effect of varying soil water on root growth dynamics of annual crops

We present a simple framework for modelling root growth and distribution with depth under varying soil water conditions. The framework considers the lateral growth of roots (proliferation) and the vertical extension of roots (root front velocity). The root front velocity is assumed to be constant when the roots descend into an initially wet soil profile. The lateral growth of roots is governed by two factors: (1) the current root mass or root length density at a given depth, and (2) soil water availability at that depth.Under non-limiting soil water conditions, the increase in root mass at any depth is governed by a logistic equation so that the root length density (R _v) cannot exceed the maximum value. The maximumR _v, is assumed to be the same for all depths. Additional dry matter partitioned to roots is initially distributed according to the current root mass at each depth. As the root mass approaches the maximum value, less dry matter is partitioned to that depth.When soil water is limiting, a water deficit factor is introduced to further modify the distribution of root dry matter. It is assumed that the plant is an energy minimiser so that more root mass is partitioned to the wetter regions of the soil where least energy will be expended for root growth. Hence, the model allows for enhanced root growth in areas where soil water is more easily available.Simulation results show that a variety of root distribution patterns can be reproduced due to varying soil water conditions. It has been demonstrated that broad patterns of root distribution reported in the literature can also be simulated by the model.