Root growth, developmental changes in the apex, and hydraulic conductivity for Opuntia ficus-indica during drought

Developmental changes in the root apex and accompanying changes in lateral root growth and root hydraulic conductivity were examined for Opuntia ficus-indica (L.) Miller during rapid drying, as occurs for roots near the soil surface, and more gradual drying, as occurs in deeper soil layers. During 7 d of rapid drying (in containers with a 3-cm depth of vermiculite), the rate of root growth decreased sharply and most root apices died; such a determinate pattern of root growth was not due to meristem exhaustion but rather to meristem mortality after 3 d of drying. The length of the meristem, the duration of the cell division cycle, and the length of the elongation zone were unchanged during rapid drying. During 14 d of gradual drying (in containers with a 6-cm depth of vermiculite), root mortality was relatively low; the length of the elongation zone decreased by 70%, the number of meristematic cells decreased 30%, and the duration of the cell cycle increased by 36%. Root hydraulic conductivity ( L _P) decreased to one half during both drying treatments; L _P was restored by 2 d of rewetting owing to the emergence of lateral roots following rapid drying and to renewed apical elongation following gradual drying. Thus, in response to drought, the apical meristems of roots of O. ficus-indica near the surface die, whereas deeper in the substrate cell division and elongation in root apices continue. Water uptake in response to rainfall in the field can be enhanced by lateral root proliferation near the soil surface and additionally by resumption of apical growth for deeper roots.     

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.     

Root distribution and interactions between intercropped species

Even though ecologists and agronomists have considered the spatial root distribution of plants to be important for interspecific interactions in natural and agricultural ecosystems, few experimental studies have quantified patterns of root distribution dynamics and their impacts on interspecific interactions. A field experiment was conducted to investigate the relationship between root distribution and interspecific interactions between intercropped plants. Roots were sampled twice by auger and twice by the monolith method in wheat (Triticum aestivum L.)/maize (Zea mays L.) and faba bean (Vicia faba L.)/maize intercropping and in sole wheat, maize, and faba bean up to 100 cm depth in the soil profile. The results showed that the roots of intercropped wheat spread under maize plants, and had much greater root length density (RLD) at all soil depths than sole wheat. The roots of maize intercropped with wheat were limited laterally, but had a greater RLD than sole-cropped maize. The RLD of maize intercropped with faba bean at different soil depths was influenced by intercropping to a smaller extent compared to maize intercropped with wheat. Faba bean had a relatively shallow root distribution, and the roots of intercropped maize spread underneath them. The results support the hypotheses that the overyielding of species showing benefit in the asymmetric interspecific facilitation results from greater lateral deployment of roots and increased RLD, and that compatibility of the spatial root distribution of intercropped species contributes to symmetric interspecific facilitation in the faba bean/maize intercropping. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Sequence of drought response of maize seedlings in drying soil

Leaf elongation in monocotyledonous plants is sensitive to drought. To better understand the sequence of events in plants subjected to soil drying, leaf elongation and transpiration of maize seedlings ( Zea mays L.) of 4 cultivars were monitored continuously and the diurnal courses of the root and leaf water relations were determined. Results from this study indicate the following sequence of drought response: Leaf elongation decreased before changes in the leaf water relations of non‐growing zones of leaf blades were detected and before transpiration decreased. Reductions in leaf elongation preceded changes in the root water potential (ψ_w). Root ψ_w was not a very sensitive indicator of soil dryness, whereas the root osmotic potential (ψ_s) and root turgor (ψ_p) were more sensitive indicators. The earliest events observed in drying soil were a significant increase in the largest root diameter class (1 720 to 1 960 gm) and a decrease in leaf elongation ( P = 0.08) 2 days after withholding water. Significant increases in root length were observed 2 days later. Soil drying increased the number of fine roots with diameters of <240 µm. Slight increases in soil strength did not affect leaf elongation in the drying soil.     

The Effects of Aluminium on the Seed Germination and Seedling Growth in Hibiscus moscheutos (Malvaceae) and Wheat (Triticum aestivaum,Gramineae)

The effects of AlCl₃ on Hibisucs moscheutos seed germination and growth were investigated to evaluate its hardiness to Aluminum (Al), in compassion with Carazinho, a wheat genotype tolerant to Al, and Egret, agenotype sensitiveto Al . For H. moscheutos and two wheat genotypes, our results indicated that germination was insensitive to AlCl₃ until at 500μmol/L. AlCl₃ of 50μmol/L inhibited the elongation of primary and lateral roots significantly, but had less effect on the number of lateral roots. There was no difference of root elongation between the H. moscheutos and wheat, but the lateral roots of H. moscheutos were more tolerant to Al than that of wheat genotypes. Under AlCl₃ of 50μmol/L, the reduction of root biomass was significant in wheat genotypes, but not in H. moscheutos, comparing to their control, suggesting that H. moscheutos is more tolerant to Al than the two wheat genotypes, and that Al has different effects on the growth of primary and lateral roots in both H. moscheutos and wheat .     

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.     

Effects of water supply on root traits and biological yield of Durum (Triticum durum Desf.) and Khorasan (Triticum turanicum Jakubz) wheat

The ability of a plant to change its root characteristics to increase the acquisition of soil water is an important adaptation mechanism to water limitation. In this regard, a field study was carried out in the Pannonian region of eastern Austria with two tetraploid wheat genotypes, i.e. Durum (Triticum durum Desf.) and Khorasan (Triticum turanicum Jakubz), during a comparatively wetter and drier year, i.e. 2008 and 2009, respectively. The genotypes showed significant differences in average root diameter and fine root length. All root traits varied with soil depth. The highest root length density and root biomass were observed with Khorasan wheat in 0–10 cm soil depth. Durum wheat showed a stronger response in fine roots to water availability and produced more fine roots in the moist year. Electric root capacitance was higher with Khorasan wheat. Durum showed higher biological yield stability across years with different precipitation with respect to above- and belowground biomass. It produced more leaf area under humid conditions. Khorasan allocated more assimilates to belowground organs in dry conditions, but without positive effect on aboveground biomass.     

Seminal Root Growth in Sorghum (Sorghum bicolor) Under Allelopathic Influences from Residues of Taro (Colocasia esculenta)

The length of the seminal root (SR) axis and the number andlength of lateral roots (LRs) of sorghum (Sorghum bicolor Moench)were markedly inhibited by taro [Colocasia esculenta (L.) Schott]residues incorporated into a sand growing medium. The sand profilewas divided equally into zones with and without residues. Productionand elongation of the first-order LRs of the SR axis facingthe zone containing taro residues were severely suppressed.On the side facing the zone that was free of residues, productionand elongation of LRs was not inhibited. SR and LR growth wasdrastically impaired and many plants were killed when taro residueswere incorporated in large amounts into the uppermost 2 cm ofthe growing medium. The activity of the allelopathic substancesin the root zone appeared to be location-specific. Sorghum bicolor, seminal root, lateral root, Colocasia esculenta, taro, taro residues, allelopathic substances, root growth     

Root Growth and Water Uptake by Maize Plants in Drying Soil

Sharp, R. E and Da vies, W. J. 1985. Root growth and water uptakeby maize plants in drying soil.— J. exp. Bot. 36: 1441–1456. The influence of soil drying on maize (Zea mays L.) root distributionand use of soil water was examined using plants growing in thegreenhouse in soil columns. The roots of plants which were wateredwell throughout the 18 d experimental period penetrated thesoil profile to a depth of 60 cm while the greatest percentageof total root length was between 20–40 cm. High soil waterdepletion rates corresponded with these high root densities.Withholding water greatly restricted root proliferation in theupper part of the profile, but resulted in deeper penetrationand higher soil water depletion rates at depth, compared withthe well watered columns. The deep roots of the unwatered plantsexhibited very high soil water depletion rates per unit rootlength. Key words: Maize, roots, water deficit, soil water depletion     

Roots,soil water and crop yield: tree crop interactions in a semi-arid agroforestry system in Kenya

Variations in soil water, crop yield and fine roots of 3–4 year-old Grevillea robusta Cunn. and Gliricidia sepium (Jacq.) Walp. growing in association with maize (Zea mays L.) were examined in semiarid Kenya during the long rains of 1996 and 1997. Even although tree roots penetrated more deeply than maize roots, maximum root length densities for both tree species and maize occurred in the top 200 mm of the soil profile where soil moisture was frequently recharged by rains. Populations of roots in plots containing trees were dominated by tree roots at the beginning of the growing season but because tree roots died and maize root length increased during the cropping season, amounts of tree and maize roots were similar at the end of the season. Thus, there was evidence of temporal separation of root activity between species, but there was no spatial separation of the rooting zones of the trees and crops within that part of the soil profile occupied by crop roots. Tree root length density declined with increasing distances from rows of trees and with depth in the soil profile. Although Grevillea trees were largest, plots containing G. sepium trees always contained more tree roots than plots containing G. robusta trees and Gliricidia was more competitive with maize than Grevillea. Overall, Gliricidia reduced crop yield by about 50% and Grevillea by about 40% relative to crop yield in control plots lacking trees and reductions of crop yield were greatest close to trees. There was less soil moisture in plots containing trees than in control plots. Such difference between control plots and plots containing trees were maximal at the end of the dry season and there was always less soil moisture close to trees than elsewhere in the plots. Plots containing Gliricidia trees contained less soil water than plots containing Grevillea trees.     

Effects of air-filled soil porosity and aeration on the initiation and growth of secondary roots of maize (Zea mays)

We studied the possibility whether the initiation of secondary roots is regulated by the air-filled porosity in soil, i.e. the availability of oxygen in the soil. Maize plants were grown in long PVC tubes (1 m long and 12 cm diameter) and were unwatered for different numbers of days so that variations of soil water content with depth were achieved on the same date with plants at the same age. The plants were harvested when their root systems were established in the whole soil column and watering had been withheld for 0, 15, 20, 25 days. A decrease of soil water content was significantly correlated with an increase of air-filled porosity in soil. The number of secondary lateral roots from segments of primary adventitious roots increased dramatically when soil water content decreased from field capacity to about 0.05 g water g^-1 dried soil. The total dried mass of roots at different soil depths was also positively correlated with soil air-filled porosity. It was observed that the elongation of the initiated secondary roots responded differently to the variations of soil air-filled porosity. The length of secondary roots increased initially when the soil was dried from field capacity to 0.18 g g^-1 dried soil (water potential at about−0.2 MPa, air-filled porosity 0.26 cm³ cm^-3), but was drastically reduced when the soil was dried further. Obviously elongation of secondary roots was inhibited when soil water potential began to deviate substantially from an optimum value. The present results suggested that the initiation of secondary roots was greatly promoted by the increase of air-filled soil porosity, i.e. availability of oxygen. This conclusion was further verified in a separate experiment where solution-cultured maize seedlings were subjected to different aeration treatments. An obvious increase in secondary root initiation was found in plants which were aerated with normal air (21% O₂) than in plants which were either not aerated or aerated with 5% O₂ air. ei]Section editor: B E Clothier     

缺磷胁迫下的小麦根系形态特征研究

研究了缺磷条件下不同基因型小麦（Triticum aestivum L.)苗期根系形态学适应特征,以明确环境因子对根系不同组分（根轴和侧根）生长发育调控作用的强度和根系形态与磷营养效率关系。在缺P环境中,小麦根轴数量和侧根长度明显减小,同化物向根部的分配比例增加,根轴长度、侧根数量和根系长度等均有显著提高。供试基因型小麦的根轴数量及其长度的差异在每个供磷水平及不同供磷水平之间均呈显著,说明这两种性状的差异是由基因型和环境因素共同决定的;而侧根特征的差异只在不同供磷水平间显著,表明侧根性状主要受环境因素的控制。对6种基因型小麦的研究表明,根轴数量、根轴长度、根生长角度和根系长度根角之间存在着显著的基因型差异。相关分析表明,小麦的相对产量与缺磷条件下的小麦苗期根系形态指标的交互作用之间具有显著的线性关系。这种关系说明根系形态性状可作为早期有效地筛选磷高效小麦品种的指标。     

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.     

Assessment of a rapid method,using soil cores,for estimating the amount and distribution of crop roots in the field

Summary A study was made of the relationship between the number of roots (N_r) observed on unit area of the freshly exposed, horizontal faces of soil cores, and the amounts of roots (per unit volume) present in the same cores. Soil cores, 7 cm diameter, were extracted to depths of 1 m from cereal crops in 1976 at three field sites located on clay soils. Sampling was either at the start of stem elongation, or at anthesis. Estimates of root length per unit soil volume (L) were derived from N_r by assuming random orientation of roots in the soil.Values of L were found to be highly correlated with the measured lengths of both the main roots (root axes) and the total roots (axes and laterals) washed from the soil at a given growth stage, for each of the soils. On average, L was 3.3 times the length of root axes washed from the soil, and was 0.42 times the length of total roots, but there was appreciable variation between different growth stages and field sites. Possible factors giving rise to differences between L and the measured lengths of roots are discussed. Estimates of root length from observation of soil cores may nonetheless provide a suitable basis for rapidly comparing therelative distribution of roots down the soil profile under field conditions.     

树木不同着生位置1级根的形态、解剖结构和氮含量

树木根系中1级根在养分和水分吸收方面发挥着重要作用。研究1级根的形态结构与功能的联系,对了解1级根的生理功能和寿命,以及森林生态系统碳和养分的循环具有重要的理论意义。但是,1级根在根系统中,因着生的位置不同,可能表现出不同的生理生态功能。该研究以胡桃楸(Juglans mandshurica)、黄波罗(Phellodendron amurense)和水曲柳(Fraxinus mandshurica)人工林根系的1级根为研究对象,研究了不同着生位置的1级根的形态、解剖结构和组织化学特征。将1级根按着生位置的不同分成3类:Ar类根为2级根上的1级根;Br类根为2级–4级根的根尖;Cr类根为3级–5级根上的1级根。结果表明:不同着生位置的1级根,形态、解剖结构和组织化学方面都具有高度的异质性。3类1级根中,Ar类根数量多、根长较短、直径细,N含量高,皮层比例高、维根比低,主要由二原型原生木质部的根组成;Br类和Cr类根数量较少、单根较长、直径粗,N含量低,皮层比例低、维根比高,主要由多原型原生木质部的根组成。研究结果对了解不同着生位置的1级根的吸收功能和寿命具有重要的理论意义。     

Maize lateral root developmental plasticity induced by mild water stress. II: Genotype-specific spatio-temporal effects on determinate development

Maize lateral roots exhibit determinate growth, whereby the meristem is genetically programmed to stop producing new cells. To explore whether lateral root determinacy is modified under water deficits, we studied two maize genotypes (B73 and FR697) with divergent responses of lateral root growth to mild water stress using an experimental system that provided near-stable water potential environments throughout lateral root development. First-order laterals of the primary root system of FR697 exhibited delayed determinacy when grown at a water potential of −0.28 MPa, resulting in longer and wider roots than in well-watered (WW) controls. In B73, in contrast, neither the length nor width of lateral roots was affected by water deficit. In water-stressed FR697, root elongation continued at or above the maximum rate in WW roots for 3 days longer, and was still 45% of maximum when WW roots approached their determinate length. Maintenance of root elongation was associated with sustained rates of cell production. In addition, kinematic analyses showed that reductions in tissue expansion rates with aging were delayed in the longitudinal, radial and tangential planes throughout the root growth zone. Thus, this study reveals large genotypic differences in the interaction of water stress with developmental determinacy of maize lateral roots.     

Numbers and locations of native bacteria on field-grown wheat roots quantified by fluorescence in situ hybridization (FISH)

Native bacteria, Pseudomonas and filamentous bacteria were quantified and localized on wheat roots grown in the field using fluorescence in situ hybridization (FISH). Seminal roots were sampled through the season from unploughed soil in a conservation farming system. Such soils are spatially heterogeneous, and many roots grow slowly through hard soil with cracks and pores containing dead roots remnant from previous crops. Root and rhizosphere morphology, and contact with soil particles were preserved, and autofluorescence was avoided by observing sections in the far-red with Cy5 and Cy5.5 fluorochromes. Spatial analyses showed that bacteria were embedded in a stable matrix (biofilm) within 11 microm of the root surface (range 2-30 microm) and were clustered on 40% of roots. Half the clusters co-located with axial grooves between epidermal cells, soil particles, cap cells or root hairs; the other half were not associated with visible features. Across all wheat roots, although variable, bacteria averaged 15.4 x 10(5) cells per mm(3) rhizosphere, and of these, Pseudomonas and filaments comprised 10% and 4%, respectively, with minor effects of sample time, and no effect of plant age. Root caps were most heavily colonized by bacteria along roots, and elongation zones least heavily colonized. Pseudomonas varied little with root development and were 17% of bacteria on the elongation zone. Filamentous bacteria were not found on the elongation zone. The most significant factor to rhizosphere populations along a wheat root, however, was contact with dead root remnants, where Pseudomonas were reduced but filaments increased to 57% of bacteria (P < 0.001). This corresponded with analyses of root remnants showing they were heavily colonized by bacteria, with 48% filaments (P < 0.001) and 1.4%Pseudomonas (P = 0.014). Efforts to manage rhizosphere bacteria for sustainable agricultural systems should continue to focus on root cap and mucilage chemistry, and remnant roots as sources of beneficial bacteria.     

Apical diameter and branching density affect lateral root elongation rates in banana

Thick roots elongate faster than thinner ones. However, within one species, the growth achieved by roots of a given diameter can be very variable, and root diameter can only be considered as a determinant of root potential elongation rate. As root elongation is highly correlated to carbon availability, it can be hypothesized that local competition for resources, expressed as the number of lateral roots per unit length (i.e. the branching density), modulates root elongation. Using novel methods in field conditions, we have estimated apical diameters, elongation rates and growth durations of nearly 3500 banana lateral roots, in a field experiment with high radiations and a shaded glasshouse experiment with low radiations. Apical diameters and branching densities were lower in the experiment with low radiation, but elongation rates were higher. In both experiments, mean elongation rates of first-order laterals and thick second-order laterals were negatively correlated with bearing root branching densities. It is hypothesized that, even though apical diameters were lower, low branching densities in the shaded glasshouse allowed enhanced lateral root elongation. In both experiments, second-order laterals elongated more slowly than first-order laterals of similar diameter. A specific effect of root order, independent of branching density and apical diameter, contributed to explain these slow second-order lateral elongation rates. Most lateral roots elongated between 9 and 21 days and growth duration was mainly correlated with root diameter.     

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.     

Immunohistological localization and quantification of IAA in studies of root growth regulation

The content and distribution of auxins were studied in gravistimulated roots of maize (Zea mays L.) and primary roots of 7-day-old wheat (Triticum durum Desf.) seedlings, which branching was enhanced by excision of adventitious roots. IAA localization was observed immunohistochemically, using specific anti-IAA antibody in combination with second (anti-species) antibody labeled with colloidal gold. Differences in the IAA content (staining intensity) were found between upper and lower parts of gravistimulated maize roots. We also observed IAA accumulation in the primary wheat root after adventitious root excision; the cells of lateral root primordia were characterized by more intense IAA staining. The role of auxin redistribution in plants for lateral root initiation and development is discussed.