Evaluation of parameters describing the root system architecture of field grown maize plants (Zea mays L.)

The objective of this work was to study elongation curves of maize axile roots throughout their elongation period under field conditions. Relationships between their elongation rate and the extension rate of their branched region were also studied. Maize, early-maturing cultivar Dea, was grown on a deep, barrier-free clay loam (depth 1.80m). Trenches were dug during four periods until after silking and axile roots were excavated. Parameters measured were total length and the lengths of basal and apical unbranched zones. The rank of the bearing phytomer and general data about the carrying plant were also recorded. Results showed that axile roots from lower phytomers had similar elongation rates irrespective of the rank of the carrying phytomer. This elongation rate declined with root age. A monomolecular elongation model was fitted to the experimental data. Elongation was much slower in roots from upper phytomers. A rough linear relationship was found between the elongation rate of axile roots and the length of the apical unbranched zone. This result suggests that laterals appeared on a root segment a constant time after it was formed. Possible mechanisms with may account for the declining elongation rate with root age (increasing distance from aerial parts or adverse environmental conditions in deep soil layers) and variability between individual roots are also discussed.     

Study of differences between vertical root maps observed in a maize crop and simulated maps obtained using a model for the three-dimensional architecture of the root system

Differences between observed and simulated vertical root maps were studied in an attempt to evaluate the predictive ability of a simulation model of root system architecture under field conditions on mature plants, and to identify avenues for improvement. Some methodological problems associated with root mapping in the field are considered with a sensitivity analysis.Comparisons were made on a maize crop (early maturing hybrid F1 cultivar Dea) 15 days after silking. Four vertical root maps, perpendicular to the row and midway between two successive plants, were observed. Simulated root maps for different locations along the row showed essentially the same pattern, attesting of an approximately two-dimensional distribution of the roots in such a crop. Simulation of the intesection of roots with thin layers (thickness from 0 to 20 mm) instead of a perfect plane allowed us to assess effects due to the roughness of actual trench walls, and possible artefacts in the observation of root intersections. The simulated root profiles were very sensitive to this thickness, especially in the 0–5 mm range, in both average values, and overall shape. Actual data were close to the 3 mm thick simulations. This value seems plausible under our field conditions.Differences between simulated and actual root maps were shown to be mostly accounted for by the variations in soil bulk density. Thus, this environmental parameter appears as the most important one to include into the model for improving its predictions.     

Trajectory of the nodal roots of maize in fields with low mechanical constraints

The trajectories of seventy three nodal roots of maize were studied in two fields with loose soil structure. Their projections on horizontal and vertical planes were traced. These roots tended to remain in a vertical plane. Trajectories were related to each other by an affine transformation. Thus, all the observed trajectories could be obtained by transformation of a common root archetype. The horizontal component of the trajectories was mainly in the first 0.4 m depth of soil, in the layer where soil structure was disturbed by ploughing. This horizontal component decreased with later appearance of roots (upper internodes), but differed between the two sites. The average soil temperature during the week following root appearance accounted for differences between internodes and sites. Lungley's algorithm, which is commonly used in modelling root trajectories, was tested. A general pattern could be simulated, but the model failed to fit the trajectories in the first 100 to 200 mm of soil. As a consequence, the initial angle between the stem and the root, which is a sensitive parameter in Lungley's model, did not account for differences between root trajectories. Laboratoire d'agronomie de Colmar     

Influence of soil temperature during root appearance on the trajectory of nodal roots of field grown maize

Trajectories of maize nodal roots were studied to test the hypothesis that roots which appear on a common internode have similar geometrical characteristics, and to assess the effect of soil temperature on root trajectory. Treatments consisted of three sowing dates, a comparison between mulched and non-mulched soil, a replication of one sowing date for two years in two locations, and a comparison between two cultivars at one sowing date. All these sources of variation, except the cultivar, had an appreciable effect on the trajectories of roots which appeared on the first four internodes. The horizontal component of the trajectory differed significantly between treatments, ranging from 93 to 700 mm in roots which appeared on the second internode, and from 71 to 569 for those on the third internode. The original hypothesis had, therefore, to be rejected. Mean soil temperature during the 100°C.days after root appearance accounted for the differences in trajectory between location, year, sowing date and mulch treatments, and for the differences between internodes within each location. The critical period during which temperature affected root trajectory probably began at root appearance, and ended between 50 and 100°C days after root appearance, i.e. when the root was less than 100 mm long.     

Spatio-temporal Variations in Axial Conductance of Primary and First-order Lateral Roots of a Maize Crop as Predicted by a Model of the Hydraulic Architecture of Root Systems

Rates at which water can be transported along plant roots (axial pathway) vary through time, in part depending on xylem maturation. Because of experimental constraints, the dynamics of root functional heterogeneity under field conditions remains mostly uncharted territory. Recent advances in mechanistic modelling offer opportunities to bypass such experimental limitations. This paper examines the dynamics of local variations in axial conductance of primary and first-order lateral roots of a maize crop using the architecture-based modelling approach developed by Doussan et al. (Annals of Botany: 81, 213–223, 1998). Specifically, we hypothesised that points of major resistance to long distance water transfers could arise from discrepancies between the hydraulic maturity (or water carrying capacity) of main axes and branch roots. To test this assumption, spatial distributions of root axial conductance were tested after 30, 60 and 100 days at soil depths of 10, 50 and 100 cm under a maize (Zea mays L.) crop sown at a density of 8 plants m⁻². As the crop developed, the corresponding root populations encompassed ever increasing amounts of hydraulically mature first-order laterals (branch roots): after a 100-day growth period, the vast majority of laterals had reached their maximum axial conductance at all soil depths down to 100 cm. In contrast, the axial conductance of a large proportion of main axes (primary roots) remained low, even at shallow soil depths and after 100 days of growth. The imbalance between the hydraulic maturity of primary and lateral roots was most conspicuous at soil depths of 100 cm, where ~10% only of the former compared to ~80% of the latter, had reached their maximum axial conductance after a 100-day growth period.     

Transient three-dimensional modeling of soil water and solute transport with simultaneous root growth,root water and nutrient uptake

A three-dimensional solute transport model was developed and linked to a three-dimensional transient model for soil water flow and root growth. The simulation domain is discretized into a grid of finite elements by which the soil physical properties are spatially distributed. Solute transport modeling includes passive and active nutrient uptake by roots as well as zero- and first-order source/sink terms. Root water uptake modeling accounts for matric and osmotic potential effects on water and passive nutrient uptake. Root age effects on root water and nutrient uptake activity have been included, as well as the influence of nutrient deficiency and ion toxicity on root growth. Examples illustrate simulations with different levels of model complexity, depending on the amount of information available to the user. At the simplest level, root growth is simulated as a function of mechanical soil strength only. Application of the intermediate level with root water and nutrient uptake simulates the influence of timing and amount of NO3 application on leaching. The most comprehensive level includes simulation of root and shoot growth as influenced by soil water and nutrient status, temperature, and dynamic allocation of assimilate to root and shoot.     

宽窄行栽植模式下三倍体毛白杨根系分布特征及其与根系吸水的关系

采用剖面法对宽窄行栽植模式下三倍体毛白杨(triploid Populus tomentosa)的根系分布特征进行了研究;采用管式TDR系统对土壤剖面含水率变化动态进行了连续观测,并据此计算林木根系吸水速率,以探讨土壤含水率、根系分布和根系吸水分布之间的相关关系。研究结果表明:毛白杨的总平均根长密度在林带两侧和不同径向距离处非常接近(P>0.05);但在不同土层间变化很大(P<0.01),其中0-20和60-150 cm土层为根系主要分布区域,其根系所占比例共达86%;不同径阶间的根长密度差异显著(P<0.01),且其比例关系会随空间位置的改变而发生变化。不同栽植方位下,林带东侧毛白杨根系分布的浅层化程度高于西侧,且在径向240-280 cm内其0-0.5 mm的极细根显著多于西侧(P<0.05)。因此,宽窄行栽植模式下,深度和径阶是毛白杨根系分布的主要影响因子,而栽植方位会对其形态构型产生影响。毛白杨根系吸水模式受细根分布的影响,但会随土壤剖面水分有效性分布的变化而变化:当表土层水分有效性增加时,根系吸水主要集中在表土层;当表土层水分有效性降低时,深层土壤根系的吸水贡献率会逐渐增加;当土壤剖面水分条件异质性较高时,根系吸水主要集中在根系密度与水分有效性均较高的区域;当土壤剖面水分分布均匀且不存在水分胁迫时,根系吸水分布与细根分布最为一致。     

A comprehensive analysis of root morphological changes and nitrogen allocation in maize in response to low nitrogen stress

The plasticity of root architecture is crucial for plants to acclimate to unfavourable environments including low nitrogen (LN) stress. How maize roots coordinate the growth of axile roots and lateral roots (LRs), as well as longitudinal and radial cell behaviours in response to LN stress, remains unclear. Maize plants were cultivated hydroponically under control (4 mm nitrate) and LN (40 μm ) conditions. Temporal and spatial samples were taken to analyse changes in the morphology, anatomical structure and carbon/nitrogen (C/N) ratio in the axile root and LRs. LN stress increased axile root elongation, reduced the number of crown roots and decreased LR density and length. LN stress extended cell elongation zones and increased the mature cell length in the roots. LN stress reduced the cell diameter and total area of vessels and increased the amount of aerenchyma, but the number of cell layers in the crown root cortex was unchanged. The C/N ratio was higher in the axile roots than in the LRs. Maize roots acclimate to LN stress by optimizing the anatomical structure and N allocation. As a result, axile root elongation is favoured to efficiently find available N in the soil.     

Maize root system growth and development as influenced by phosphorus deficiency

Effects on leaf growth, biomass accumulation and root morphogenesis associated with the establishment of phosphorus (P) deficiency were studied on maize in order to test the hypothesis that the root system response can be accounted for by the effect of P deficiency on the carbon budget of the plant. P deprivation had a large and rapid negative effect on leaf expansion. For 7 d after P deprivation, the total dry matter production per plant was almost fully accounted for by the effect of P starvation on leaf growth and its subsequent effect on photosynthetically active radiation (PAR) interception. No strong effect of P deficiency was observed on the radiation use efficiency during this first period, although it was reduced thereafter. Root growth was slightly enhanced a few days after P starvation, but strongly reduced thereafter. The elongation rate of axile roots was maintained throughout the experiment, whereas emergence of new axile roots and elongation of first-order laterals were drastically reduced. The density of first-order laterals was not severely affected. These morphological responses are very similar to what is observed when root growth is limited by the availability in carbohydrates. The results are therefore compatible with the hypothesis that P deficiency mainly affects the root system morphology through its effect on the carbon budget of the plant with no additional specific effect of P deficiency on root morphogenesis. The drastic and early reduction of shoot growth after P deprivation may explain that more carbohydrates were available for root growth which was observed a few days after P starvation and reported by several authors. Later on, however, because of the reduced leaf area of P-deprived plants, their capacity to intercept light was severely reduced so that root growth was finally reduced.Keywords: Zea mays L., maize, phosphorus, root, root morphogenesis.      

A simulation model of the three-dimensional architecture of the maize root system

In order to study the nutrient and water uptake of rootsin situ, we need a quantitative three-dimensional dynamic model of the root system architecture. The present model takes into account current observations on the morphogenesis of the maize root system. It describes the root system as a set of root axes, characterised by their orders and their inter-node of origin. The evolution of the simulated pattern is achieved by three processes, occuring at each time step: emission of new primary root axes from the shoot, growth and branching of existing root axes. The elongation of an axis depends on its order, inter-node and local growing conditions. Branches appear acropetally at a specified distance from the apex and from former branches, along ranks facing xylem poles, with a branching angle specific to their order and inter-node. From the three-dimensional branched patterns simulated by the model, various outputs, such as root profiles or cross-section maps can be computed, compared to observed data and used as inputs in uptake models. A number of examples of such possible outputs are presented.     

Nitrate-uptake capacity of different root zones of Zea mays (L.) in vitro and in situ

The close relationship between nitrate depletion of the subsoil and root-length densities found in field experiments could not be explained by mathematical models simulating nitrate uptake (Wiesler and Horst, 1994). The objective of the present study was the validation of some of the assumptions made in these models namely uniform nitrate-uptake rates (NURs) independent on root age and daytime.Different techniques were developed and compared for the measurement of NUR of different root zones: (i) isolated root segments, (ii) compartmented uptake cuvettes, (iii) depletion of nitrate (water) from agarose blocks placed on specific zones of roots growing in nutrient solution and (iv) in rhizotrones filled with soil over the whole growing cycle of maize plants. All methods yielded a similar magnitude of NUR (10 - 30 pmol cm-2 s-1). However, only intact plants growing in nutrient solution as well as in soil, but not isolated root segments, showed higher NUR at apical root zones compared to more mature branching root zones by a factor of 2 - 8. The NUR of the root apex was particularly sensitive to the nitrogen demand of the plant and the assimilate supply from the shoots as affected by light intensity. At suboptimal, but not at optimal light conditions during preculture, NUR was lower in the dark than in the light. As plants matured, NUR of soil grown plants became increasingly dependent on water uptake. But even if nitrate uptake by mass flow was subtracted from total nitrate uptake, mature roots showed a surprisingly high nitrate-uptake capacity.The results indicate that the formation of root-age classes with different NUR and the assumption of lower NUR at night could improve the modelling of nitrate uptake.     

种间互作和施氮对蚕豆/玉米间作生态系统地上部和地下部生长的影响

为河西走廊绿洲灌区豆科/禾本科间作体系的养分管理提供科学依据,于2007年在武威绿洲农业试验站应用田间原位根系行分隔技术研究了蚕豆/玉米种间互作和施氮对玉米抽雄期的根系空间分布、根系形态和作物地上部生长的影响。研究结果表明:种间互作和施氮均增加了玉米和蚕豆在纵向和横向两个尺度上的根重密度、根长密度、根表面积、根系体积。根长密度和根表面积与两种作物产量和氮素吸收均呈正相关,而与蚕豆的根瘤重呈负相关;抽雄期的土壤含水量与玉米产量和养分吸收呈显著的负相关。玉米根系可以占据蚕豆地下部空间,但蚕豆的根却较少到间作玉米的地下部空间,也就是间作后增加了玉米根系水平尺度的生态位。蚕豆和玉米根系主要分布分别在0-40 cm浅土层和0-60 cm 土层,且间作玉米根系在60-120 cm比单作和分隔的多。因此,种间互作和施氮扩大了两作物根系纵向和横向的空间生态位,改变了作物根系形态,即扩展了两者水分和养分吸收的生态位,增加了作物吸收养分的有效空间,从而提高了间作生态系统的生产力。     

Effect of heterogeneous distribution of nutrients on root growth, ABA content and drought resistance of wheat plants

Heterogeneous distribution of nutrients influenced morphology of wheat roots; they were shorter and had more laterals at the site of high nutrient content. The roots outside the nutrient-rich patch were longer and penetrated deeper in the soil than those of the plants supplied with the same quantity of nutrients but having them homogeneously distributed. The ABA content of roots sampled from the site of high nutrient content was greater than in those elsewhere. It is suggested that this might be important for inhibition of their extension growth and stimulation of laterals in the nutrient rich patch. Heterogeneously distributed nutrients beneficially influenced accumulation of ¹⁵N and assimilation of CO₂ by the plants, enabling an increased supply of nutrients to the parts of the root system beyond the nutrient rich patch and sustained their rapid extension. Enhanced penetration of roots into the deeper soil layers could contribute to the observed increase in drought resistance of plants grown with localised placement of fertilisers under field conditions.     

An integrated method for quantifying root architecture of field-grown maize

Background and Aims

A number of techniques have recently been developed for studying the root system architecture (RSA) of seedlings grown in various media. In contrast, methods for sampling and analysis of the RSA of field-grown plants, particularly for details of the lateral root components, are generally inadequate.

Methods

An integrated methodology was developed that includes a custom-made root-core sampling system for extracting intact root systems of individual maize plants, a combination of proprietary software and a novel program used for collecting individual RSA information, and software for visualizing the measured individual nodal root architecture.

Key Results

Example experiments show that large root cores can be sampled, and topological and geometrical structure of field-grown maize root systems can be quantified and reconstructed using this method. Second- and higher order laterals are found to contribute substantially to total root number and length. The length of laterals of distinct orders varies significantly. Abundant higher order laterals can arise from a single first-order lateral, and they concentrate in the proximal axile branching zone.

Conclusions

The new method allows more meaningful sampling than conventional methods because of its easily opened, wide corer and sampling machinery, and effective analysis of RSA using the software. This provides a novel technique for quantifying RSA of field-grown maize and also provides a unique evaluation of the contribution of lateral roots. The method also offers valuable potential for parameterization of root architectural models.     

Nutrient uptake relationship to root characteristics of rice

Data on root parameters and distribution are important for an improved understanding of the factors influencing nutrient uptake by a crop. Therefore, a study was conducted on a Crowley silt loam at the Rice Research and Extension Center near Stuttgart, Arkansas to measure root growth and N, P and K uptake by three rice (Oryza sativa L.) cultivars at active tillering (36 days after emergence (DAE)), maximum tillering (41 DAE), 1.25 cm internode elongation (55 DAE), booting (77 DAE) and heading (88 DAE). Soil-root core samples were taken to a depth of 40 cm after plant samples were removed, sectioned into 5 cm intervals, roots were washed from soil and root lengths, dry weights and radii were measured. Root parameters were significantly affected by the soil depth × growth stage interaction. In addition, only root radius was affected by cultivar. At the 0- to 5-cm soil depth, root length density ranged from 38 to 93 cm cm^-3 throughout the growing season and decreased with depth to about 2 cm cm^-3 in the 35- to 40-cm depth increment. The increase in root length measured with each succeeding growth stage in each soil horizon also resulted in increased root surface area, hence providing more exposed area for nutrient uptake. About 90% of the total root length was found in the 0- to 20-cm soil depth throughout the season. Average root radius measured in the 0- to 5-cm and 35- to 40-cm depth increments ranged from 0.012 to 0.013 cm and 0.004 to 0.005 cm, respectively throughout the season. Total nutrient uptake by rice differed among cultivars only during vegetative growth. Differences in total nutrient uptake among the cultivars in the field appear to be related to absorption kinetics of the cultivars measured in a growth chamber study. Published with permission of the Arkansas Agricultural Experiment Station.     

柠条人工林细根不同分枝根序寿命估计

植物细根在发育结构上表现的形态特征和生理功能异质性影响细根寿命的准确估计,因此了解分枝根序细根寿命差异对于深入认识细根的周转过程和陆地生态系统碳分配具有重要意义。采用微根管(Minirhizotron)技术对晋西北黄土区的五年生柠条(Caragana Korshinskii Kom.)人工林细根的生长过程进行了为期3a(2007—2009年)的追踪观测,分析了不同因素(土层深度、季节变化、空间位置)对一级根和高级根寿命的影响。结果表明:(1)在各土层深度处,一级根的中值寿命均低于高级根中值寿命,其中一级根中值寿命表现随土层深度增加而增加趋势,而高级根除表层0—20 cm中值寿命较短外,各土层间变化趋势不明显,40—60 cm、80—100 cm土层高级根在观测期结束时其累积存活率仍在50%以上;(2)不同季节出生一级根和高级根的中值寿命季节性表现为:秋季夏季春季,并且在各个季节均表现,高级根寿命显著大于一级根寿命(P0.01);一级根仅夏季与秋季差异性不显著(P0.05),而高级根仅春季与秋季存在极显著差异(P0.01);(3)一级根和高级根距树干基部0 cm处细根中值寿命均大于50 cm处一级根和高级根细根的中值寿命。同一位置处高级根寿命要大于一级根寿命。在距树干基部0 cm处和50 cm处,一级根和高级根的寿命均存在极显著差异(P0.01),但高级根却在距树干基部0 cm和50 cm处差异不明显,而一级根却表现极显著差异(P0.01)。     

Relationships between root diameter,root length and root branching along lateral roots in adult,field-grown maize

Background and Aims Root diameter, especially apical diameter, plays an important role in root development and function. The variation in diameter between roots, and along roots, affects root structure and thus the root system’s overall foraging performance. However, the effect of diameter variation on root elongation, branching and topological connections has not been examined systematically in a population of high-order roots, nor along the roots, especially for mature plants grown in the field.Methods A method combining both excavation and analysis was applied to extract and quantify root architectural traits of adult, field-grown maize plants. The relationships between root diameter and other root architectural characteristics are analysed for two maize cultivars.Key Results The basal diameter of the lateral roots (orders 1–3) was highly variable. Basal diameter was partly determined by the diameter of the bearing segment. Basal diameter defined a potential root length, but the lengths of most roots fell far short of this. This was explained partly by differences in the pattern of diameter change along roots. Diameter tended to decrease along most roots, with the steepness of the gradient of decrease depending on basal diameter. The longest roots were those that maintained (or sometimes increased) their diameters during elongation. The branching density (cm^–1) of laterals was also determined by the diameter of the bearing segment. However, the location of this bearing segment along the mother root was also involved – intermediate positions were associated with higher densities of laterals.Conclusions The method used here allows us to obtain very detailed records of the geometry and topology of a complex root system. Basal diameter and the pattern of diameter change along a root were associated with its final length. These relationships are especially useful in simulations of root elongation and branching in source–sink models.     

不同土壤深度落叶松细根分解及N动态变化

细根分解受根序和土壤深度的潜在影响。使用根序法分根,将落叶松Larix gmelini根系分为两类:一级根、二级根为一类(1—2级根),即低级根;三级根和四级跟为另一类(3—4级根),即高级根。采用埋袋法对落叶松低级根和高级根在不同土壤深度(0—10、10—20 cm和20—30 cm)进行了为期862 d的分解实验,探讨不同根序细根分解规律,养分释放及其影响。结果表明:1—2级根的分解速率比3—4级分解速率慢,这种规律同时存在于不同深度的土壤中。在空间上,低级根和高级根的分解速率均随土壤深度的增加而降低,高级根的降低趋势更明显。随着分解时间的进行,各个土层之间的分解率在低级根之间差异更大。细根分解过程中,落叶松不同根序养分的释放特征不同。N释放速率总体上随细根根序增加而增大,随土壤深度的增加而降低。     

Root distribution and morphology of maize seedlings as affected by tillage and fertilizer placement

Suboptimal soil conditions are known to result in poor early growth of maize (Zea mays L.) in no-tillage (NT) systems in contrast with conventional tillage (CT) systems. However, most studies have generally focused on maize roots at later growth stages and/or do not give details on root morphology. In a 2-year field study at two locations (silt loam and loam soils) in the Swiss midlands, we investigated the impacts of tillage intensity, NT vs. CT, and NP-fertilizer sidebanding on the morphology, vertical and horizontal distribution, and nutrient uptake of maize roots at the V6 growth stage. The length density (RLD) and the length per diameter-class distribution (LDD) of the roots were determined from soil cores taken to a depth of 0.5 m and at distances of 0.05 and 0.15 m from both sides of the maize row. The temperature of the topsoil was lower, and the bulk density and penetration resistance were greater in the topsoil of NT compared with CT. The growth and the development of the shoot were slower in NT. RLD was greater and the mean root diameter smaller in CT than in NT, while the vertical and horizontal distribution of roots did not differ between CT and NT. RLD increased in the zone enriched by the sidebanded fertilizer, independent of the tillage system, but LDD did not change. The poorer growth of the roots and shoots of maize seedlings was presumably caused by the lower topsoil temperature in NT rather than by mechanical impedance. The placement of a starter fertilizer at planting under NT is emphasized.     

Scaling of root length density of maize in the field profile

Root length density is an important parameter in crop growth simulation and in evaluating consequences of root pattern on crop water and nutrient uptake. In this study, a scaling model was presented for estimating the profile distribution of root length density of maize (Zea mays L.). The model inputs are root length data of a reference profile and bulk densities of soil layers, as well as root length data in the first soil layer of a field profile to be investigated. Using the root length data of 10 soil profiles investigated over 2 years, the model was examined. The results show that the proposed scaling approach is effective in estimating the root length density of each layer of soil in the field profile. The relative root mean square error (RRMSE) of the developed scaling model was 25.28%, while that of the traditional exponential model was 39.53%. The scaling approach would facilitate determination of heterogeneous distributions of root length densities in the field.