Reply to: “Comment on root orientation can affect detection accuracy of ground-penetrating radar”

Introduction

We showed that root orientation affected a parameter of ground penetrating radar (GPR), amplitude area (A) (Tanikawa et al. Plant Soil 373:317–327, 2013). The aims of this reply to Wu et al. (2014) are (i) to correct the two inaccuracies in Tanikawa et al. (2013) and (ii) to improve our method of estimating A(90°) using A(x) of root angle x.

Methods

Measured A values of Tanikawa et al. (2013) were analyzed with the modified equations.

Results

The first inaccuracy was the use of incorrect units for the coefficient b (the phase shift) in the sinusoidal waveform of A(x). The units should have been radians instead of degrees. The second inaccuracy was the mis-derivation of A(x) into A(x?+?90°). In the modified method, A(90°) was estimated by A(x) from two orthogonally intersecting transect lines and a transect line at a diagonal to them.

Conclusions

The two inaccuracies did not affect the previous main conclusions that the parameter T was suitable for estimating root diameter and that grid transects are likely to identify clear hyperbolas reflecting roots in radar profiles (Tanikawa et al. 2013). By the improved method, we could accurately estimate root diameter by scanning using three transect lines intersecting at angles of x, x?+?45°, and x?+?90°.     

Comment on: “root orientation can affect detection accuracy of ground-penetrating radar”

Introduction

In a recent paper, Tanikawa et al. Plant Soil 373:317–327, (2013) reported a considerable impact of root orientation on the accuracy of root detection and root diameter estimation by ground-penetrating radar (GPR).

Methods

In Tanikawa et al. Plant Soil 373:317–327, (2013), buried root samples in a sand box were scanned from multiple cross angles between root orientation and GPR transecting line under controlled conditions. Changes in radar waveform parameter of roots to different cross angles were investigated.

Results

Tanikawa et al. Plant Soil 373:317–327, (2013) clarified that 1) the variation in amplitude area (a signal strength related waveform parameter) to different cross angles fitted a sinusoidal waveform; and 2) the impact of root orientation on root diameter estimation by GPR could be mathematically corrected by applying a grid transect survey. However, we found that the quantitative relationship established in Tanikawa et al. Plant Soil 373:317–327, (2013) between amplitude area and cross angle was incorrect, and the application of a grid transect survey still underestimated root diameter.

Conclusion

The change in amplitude area to cross angle between transecting line and root orientation fits a sinusoidal waveform but different to that reported in Tanikawa et al. Plant Soil 373:317–327, (2013). The polarization of GPR wave may explain such sinusoidal variation in amplitude area to cross angle. The effect of root orientation on GPR-based root diameter estimation remains to be calibrated.     

Leaf litter thickness,but not plant species,can affect root detection by ground penetrating radar

Aim

Ground penetrating radar (GPR), a nondestructive tool that can detect coarse tree roots, has not yet become a mature technology for use in forests. In this study, we asked two questions concerning this technology: (i) Does the leaf litter layer influence root detection and major indices based on the time interval between zero crossings (T) and the amplitude area (A)? (ii) Can GPR images discriminate roots of different plant species?

Methods

Roots buried in a sandy bed, which was covered with different thicknesses of leaf litter, were scanned using a 900 MHz GPR antenna. Roots of four plant species in the bed were also scanned.

Results

Leaf litter decreased root reflections without distorting the shape of the hyperbolas in the radar profile. A values decreased with increasing litter thickness, whereas T was independent of litter thickness. For all species combined, GPR indices were significantly correlated with root diameter.

Conclusions

Leaf litter dramatically decreased root detection, but the influence of the litter could be ignored when the sum of T for all reflection waveforms (ΣT) is adopted to estimate root diameter. To use A values to detect roots, litter should be removed or equalized in thickness. Radar profiles could not reliably differentiate among roots belonging to plants of different species.

     

Response characteristics of seed germination and seedling growth of Acorus tatarinowii under diesel stress

Aims

Responses of typical wetland plant Acorus tatarinowii to diesel stress were investigated to provide basis of ecological monitoring system and phytoremediation for diesel-contaminated wetland.

Methods

Greenhouse experiments were established to determine the germinability of seedlings, hydrogen peroxide in leaves, and DNA damage in roots exposed to a range of potentially phytotoxic diesel.

Results

The presence of diesel did not benefit the growth of A. tatarinowii. The germination ratio and germination rate decreased with the increase of diesel concentration, both the lowest value appeared when the concentration of diesel was 10,000 mg?kg^?1. The lowest diesel concentration (2,000 mg?kg^?1) in the soil significantly reduced the length, average diameter, and projected area of root, especially on the stress of the higher diesel concentration (4,000, 8,000, and 10,000 mg?kg^?1). Furthermore, H₂O₂ concentration in leaves rose with the increasing concentration of diesel. However, no DNA oxidative damage to root was observed in our experiment.

Conclusions

Diesel exposure significantly inhabited the seed germination, root elongation, and seedlings growth of A. tatarinowii. Diesel stress caused the accumulation of H₂O₂ in the leaves of A. tatarinowii.     

Root foraging capacity depends on root system architecture and ontogeny in seedlings of three Andean Chenopodium species

Aims

Morphological and ontogenetic variation in root system architecture holds ecological significance, particularly in low-resource habitats where soil rooting is critical for both seedling establishment and water and nutrient uptake. To assess this variation under contrasted agroecological backgrounds, root architecture and rooting patterns were compared in Andean populations of Chenopodium hircinum, Chenopodium pallidicaule and two ecotypes (wet- and dry-habitat) of Chenopodium quinoa.

Methods

Seedlings were grown in rhizotrons under controlled water and nutrient availability. Root branching and elongation dynamics were characterized during 6 weeks after germination, while leaf area, above and below-ground biomass, and specific root length were determined at the end of the experiment.

Results

Despite large differences in aboveground biomass, all populations showed similar herringbone root systems. The dry-habitat C. quinoa had generally the highest root trait values, with fast taproot elongation, thick roots and long root segments resulting in high total root length and deep root proliferation.

Conclusion

Irrespective of their contrasting agroecological background, the studied chenopods displayed a similar root system topology. However, from very early development stages, they showed differential root foraging patterns with two extremes: fast and vigourous rooting at depth in the dry-habitat C. quinoa, and shallow and thin root system in C. pallidicaule adapted to shallow-soil and high-altitude habitats.     

Boron delays dehydration and stimulates root growth in Eucalyptus urophylla (Blake,S.T.) under osmotic stress

Background

Water and nutritional restrictions are limiting factors for the growth of Eucalyptus trees in tropical climates. In the dry season, boron (B) uptake is severely affected.

Aims

The objectives of this study were to evaluate the phloem mobility of B and whether its deficiency can increase plant sensitivity to osmotic stress. It was also tested to what extent foliar application of B could mitigate the negative effects of drought under low B supply.

Methods

Seedlings of a drought tolerant Eucalyptus urophylla (Blake, S. T.) clone were grown in nutrient solution, subjected to low availability of B for 25 days, and then submitted to a progressive osmotic stress. After imposition of osmotic stress, B was applied to young or mature leaves.

Results

B applications, mainly to mature leaf, stimulated root growth and delayed dehydration under osmotic stress and led to an increased B translocation and carbon isotopic composition. The expression of B transporters and pectin metabolism genes were also increased in water-stressed plants supplied with B by foliar application.

Conclusions

B deficiency led to increased plant dehydration and decreased root growth under osmotic stress. The application of B to mature leaf of water-stressed plants proved effective in mitigating the negative effects of water deficit in root growth.     

Biomechanics of nodal, seminal and lateral roots of barley: effects of diameter, waterlogging and mechanical impedance

Background and aims

Biomechanical properties of cereal root systems largely control both resistance to root lodging and their ability to stabilise soil. Abiotic stresses can greatly modify root system growth and form. In this paper the effect of waterlogging and moderate mechanical impedance on root biomechanics is studied for both lateral roots and the main axes of barley.

Methods

Barley (Hordeum vulgare) plants were subjected to transient water-logging and moderate mechanical impedance in repacked soil columns. Roots were excavated, separated into types (nodal, seminal or lateral) and tested in tension to measure strength and elastic modulus.

Results

Water-logging and mechanical impedance substantially changed root system growth whilst root biomechanical properties were affected by waterlogging. Root strength was generally greater in thin roots and depended on root type. For example, seminal roots 0.4–0.6 mm in diameter were approximately seven times stronger and five times stiffer than lateral roots of the same diameter when mechanically impeded. Root sample populations typically exhibited negative power-law relationships between root strength and diameter for all root types. Mechanical impedance slowed seminal root elongation by approximately 50 % and resulted in a 15 % and 11 % increase in the diameter of in nodal and seminal roots respectively. Power-law relationships between root diameter and root biomechanical properties corresponded to the different root types. Coefficients for between root diameter, strength and elastic modulus improved when separated by root type, with R² values increasing in some roots from 0.05 to 0.71 for root strength and 0.08 to 0.74 for elastic modulus.

Conclusions

Moderate mechanical impedance did not influence the tensile strength of roots, but, waterlogging diminished the relationship between root strength and diameter. Separation of root type improved predictions of root strength and elastic modulus using power-law regressions.     

Modelling root demography in heterogeneous mountain forests and applications for slope stability analysis

Background and aims

Plant roots provide mechanical cohesion (c _r ) to soil on slopes which are prone to shallow landslides. c _r varies in heterogeneous natural forests due to the spatial, inter- and intra-annual dynamics of root demography. Characterizing root initiation density and mortality, as well as how root growth is influenced by abiotic and biotic factors is essential for exploring a root system’s capacity to reinforce soil.

Methods

In this study, root demography data were monitored using field rhizotrons during 1.5 years in two naturally regenerated mixed forests in the French Alps. These forests are composed of trees growing in groups (tree islands) with large gaps between the islands. Three categories of driving variables were measured: (i) spatial factors: altitude (1,400 m, 1,700 m), ecological patch (gap, tree island), soil depth (0.0–1.0 m divided into five layers of 0.2 m); (ii) temporal factors: month (12 months from March 2010 to February 2011), winter (winter of 2009–2010 and 2010–2011); (iii) biological factors: root diameter classes (]0, 1] mm, ]1, 2] mm, ]2, 5] mm (according to the international standard ISO 31–11, ]x, y] denotes a left half-open interval from x (excluded) to y (included)). Two types of two-part models, a Hurdle model (H) and a Zero-inflated model (ZI) were used to fit root data with a high zero population, i.e. if root initiation or mortality was zero during a given time period, or if roots were not present at all points throughout a soil profile.

Results

Root initiation quantity decreased with increasing soil depth, as well as being lower in tree islands. Both soil depth and ecological patch interacted strongly with altitude. Root dynamics were significantly less active with a lower net production and c _r increment in winter and spring than in summer and autumn. Roots which were ]1, 2] mm in diameter contributed the most to c _r compared to other diameter classes, as they had a high production but a low mortality. With regard to model selection, both H and ZI demonstrated similar outcomes and underestimated extreme values of root demography data.

Conclusion

All factors contributed towards explaining the variability of root demography and c _r . We suggest taking into consideration the seasonality of root dynamics when studying root reinforcement.     

Lolium perenne L. root systems are a collection of Gaussian curve shaped meso diameter class length distributions

Aims

Determine if the root system of Lolium perenne L. (L perenne) is a continuous distribution of diameters, or a collection of discrete diameters classes.

Methods

Plants from tillers of five clones were grown in a local soil amended with lime. Roots were excavated after they were grown in soil for 54 days, washed and imaged with both a commercial scanner (94 px mm^?1) and a high resolution, locally built, imager (204 px mm^?1). Images were converted to diameter class length data with WinRhizo.

Results

Scanned images did not have enough resolution to accurately measure fine roots diameters (<0.09 mm diam.). Therefore the high resolution images were used. The diameter class length distributions (DCLD) of these images demonstrated diameter class clusters (meso diameter classes) which could be modeled with a non-linear Gaussian (normal) curve model. Recreating the whole root system from a compilation of the DCLD, regenerated from the three parameters of each of the Gaussian curves for the root system, produced a distribution visually identical to the original whole root system curve.

Conclusions

L perenne root systems are a collection of meso diameter classes easily described by non-linear Gaussian models. The data set of the parameters from these models is much smaller than a WinRhizo data set, and can reconstruct the original whole system DCLD.     

Quantifying the effect of soil compaction on three varieties of wheat (Triticum aestivum L.) using X-ray Micro Computed Tomography (CT)

Aims

X-ray Micro Computed Tomography (CT) enables interactions between roots and soil to be visualised without disturbance. This study examined responses of root growth in three Triticum aestivum L. (wheat) cultivars to different levels of soil compaction (1.1 and 1.5?g?cm^?3).

Methods

Seedlings were scanned 2, 5 and 12?days after germination (DAG) and the images were analysed using novel root tracking software, RootViz3D?, to provide accurate visualisation of root architecture. RootViz3D? proved more successful in segmenting roots from the greyscale images than semi-automated segmentation, especially for finer roots, by combining measurements of pixel greyscale values with a probability approach to identify roots.

Results

Root density was greater in soil compacted at 1.5?g?cm^?3 than at 1.1?g?cm^?3 (P?=?0.04). This effect may have resulted from improved contact between roots and surrounding soil. Root diameter was greater in soil at a high bulk density (P?=?0.006) but overall root length was reduced (P?=?0.20). Soil porosity increased with time (P?<?0.001) in the uncompacted treatment.

Conclusions

RootViz3D? root tracking software in X-ray CT studies provided accurate, non-destructive and automated three dimensional quantification of root systems that has many applications for improving understanding on root-soil interactions.     

Variation in cadmium accumulation and translocation among peanut cultivars as affected by iron deficiency

Purpose

The current study aimed to test the hypothesis that the variations in shoot Cd accumulation among peanut cultivars was ascribed to the difference in capacity of competition with Fe transport, xylem loading and transpiration.

Methods

A hydroponics experiment was conducted to determine the plant biomass, gas exchange, and Cd accumulation in Fe-sufficient or -deficient plants of 12 peanut cultivars, at low Cd level (0.2 μM CdCl₂).

Results

Peanut varied among cultivars in morpho-physiological response to Cd stress as well as Cd accumulation, translocation and distribution. Qishan 208 and Xvhua 13 showed a higher capacity for accumulating Cd in their shoots. Fe deficiency increased the concentration and amount of Cd in plant organs, but decreased TF _{root to shoot} and TF _{root to stem}, while TF _{stem to leaf} remained unaffected. Fe deficiency-induced increase rates of Cd concentration and total Cd amount in roots and leaves were negatively correlated with the values in Fe-sufficient plants. Transpiration rate was positively correlated with leaf Cd concentration, TF _{root to shoot}, TF _{root to stem} and TF _{stem to leaf}.

Conclusions

The difference in shoot Cd concentration among peanut cultivars was mainly ascribed to the difference in Fe transport system, xylem loading capacity and transpiration.     

Fine mapping of Co-x,an anthracnose resistance gene to a highly virulent strain of Colletotrichum lindemuthianum in common bean

Key message

The Co - x anthracnose R gene of common bean was fine-mapped into a 58 kb region at one end of chromosome 1, where no canonical NB-LRR-encoding genes are present in G19833 genome sequence.

Abstract

Anthracnose, caused by the phytopathogenic fungus Colletotrichum lindemuthianum, is one of the most damaging diseases of common bean, Phaseolus vulgaris. Various resistance (R) genes, named Co-, conferring race-specific resistance to different strains of C. lindemuthianum have been identified. The Andean cultivar JaloEEP558 was reported to carry Co-x on chromosome 1, conferring resistance to the highly virulent strain 100. To fine map Co-x, 181 recombinant inbred lines derived from the cross between JaloEEP558 and BAT93 were genotyped with polymerase chain reaction (PCR)-based markers developed using the genome sequence of the Andean genotype G19833. Analysis of RILs carrying key recombination events positioned Co-x at one end of chromosome 1 to a 58 kb region of the G19833 genome sequence. Annotation of this target region revealed eight genes: three phosphoinositide-specific phospholipases C (PI-PLC), one zinc finger protein and four kinases, suggesting that Co-x is not a classical nucleotide-binding leucine-rich encoding gene. In addition, we identified and characterized the seven members of common bean PI-PLC gene family distributed into two clusters located at the ends of chromosomes 1 and 8. Co-x is not a member of Co-1 allelic series since these two genes are separated by at least 190 kb. Comparative analysis between soybean and common bean revealed that the Co-x syntenic region, located at one end of Glycine max chromosome 18, carries Rhg1, a major QTL contributing to soybean cyst nematode resistance. The PCR-based markers generated in this study should be useful in marker-assisted selection for pyramiding Co-x with other R genes.     

Contribution of Cd-EDTA complexes to cadmium uptake by maize: a modelling approach

Background and aims

Chelant-enhanced phytoextraction has given variable and often unexplained experimental results. This work was carried out to better understand the mechanisms of Cd plant uptake in the presence of EDTA and to evaluate the contributions of Cd-EDTA complexes to the uptake.

Method

A 1-D mechanistic model was implemented, which described the free Cd²⁺ root absorption, the dissociation and the direct absorption of the Cd-EDTA complexes. It was used to explain Cd uptake by maize in hydroponics and in soil.

Results

In hydroponics, the addition of EDTA caused a decrease in Cd uptake by maize, particularly when the ratio of total EDTA ([EDTA] _T ) to total Cd ([Cd] _T ) was greater than 1. At [Cd] _T = 1 μM, when [EDTA] _T /[Cd] _T < 1, the model indicated that Cd uptake was predominantly due to the absorption of free Cd²⁺, whose pool was replenished by the dissociation of Cd-EDTA. When [EDTA] _T /[Cd] _T > 1, the low Cd uptake was mostly due to Cd-EDTA absorption. In soil spiked with 5 mg Cd kg^?1, Cd uptake was not affected by the various EDTA additions, because of the buffering capacity of the soil solid phase.

Conclusions

Addition of EDTA to soil increases Cd solubility but dissociation of Cd-EDTA limits the availability of the free Cd²⁺ at the root surface, which finally reduces the plant uptake of the metal.     

Net root growth and nutrient acquisition in response to predicted climate change in two contrasting heathland species

Background and aims

Accurate predictions of nutrient acquisition by plant roots and mycorrhizas are critical in modelling plant responses to climate change.

Methods

We conducted a field experiment with the aim to investigate root nutrient uptake in a future climate and studied root production by ingrowth cores, mycorrhizal colonization, and fine root N and P uptake by root assay of Deschampsia flexuosa and Calluna vulgaris.

Results

Net root growth increased under elevated CO₂, warming and drought, with additive effects among the factors. Arbuscular mycorrhizal colonization increased in response to elevated CO₂, while ericoid mycorrhizal colonization was unchanged. The uptake of N and P was not increased proportionally with root growth after 5 years of treatment.

Conclusions

While aboveground biomass was unchanged, the root growth was increased under elevated CO₂. The results suggest that plant production may be limited by N (but not P) when exposed to elevated CO₂. The species-specific response to the treatments suggests different sensitivity to global change factors, which could result in changed plant competitive interactions and belowground nutrient pool sizes in response to future climate change.     

Impact of root water content on root biomass estimation using ground penetrating radar: evidence from forward simulations and field controlled experiments

Background and aims

The GPR indices used for predicting root biomass are measures of root radar reflectance. However, root radar reflectance is highly correlated with root water content. The objectives of this study are to assess the impact of root water content on GPR-based root biomass estimation and to develop more reliable approaches to quantify root biomass using GPR.

Methods

Four hundred nine roots of five plant species in a sandy area of northern China were examined to determine the general water content range of roots in sandy soils. Two sets of GPR simulation scenarios (including 492 synthesized radargrams in total) were then conducted to compare the changes of root radar signal and the accuracies of root biomass estimation by GPR at different root gravimetric water content levels. In the field, GPR transects were scanned for Ulmus pumila roots buried in sandy soils with three antenna center frequencies (0.5, 0.9, and 2.0 GHz). The performance of two new GPR-based root biomass quantification approaches (one using time interval GPR index and the other using a non-linear regression model) was then tested.

Results

All studied roots exhibited a broad range of gravimetric water content (>125 %), with the water contents of most roots ranging from 90 % to 150 %. Both field experiments and forward simulations indicated that 1) waveforms of root radar reflection, radar-reflectance related GPR indices, and root biomass estimation accuracy were all affected by root water content; and 2) using time interval index and establishing a nonlinear regression model of root biomass on GPR indices improved the accuracy of root biomass estimation, decreasing the prediction error (RMSE) by 4 to 30 % under field conditions.

Conclusions

The magnitude of GPR indices depends on both root biomass and root water content, and root water content affects root biomass estimation using GPR indices. Using a linear regression model of root biomass on radar-reflectance related GPR index for root biomass estimation would only be feasible for roots with a relative narrow range of water content (e.g., when gravimetric water contents of studied roots vary within 20 %). Appropriate GPR index and regression models should be selected based on the water content range of roots. The new protocol of root biomass quantification by GPR presented in this study improves the accuracy of root biomass estimation.     

Genotypic variation in the ability of wheat roots to penetrate wax layers

Background and aims

The role of the root system in mediating crop yields has recently been emphasised, resulting in several laboratory approaches for phenotyping root traits. We aimed to determine the existence of, and reasons for, genotypic variation in wheat (Triticum aestivum L.) root penetration of strong wax layers.

Methods

Three contrasting groups (UK elite lines, CIMMYT lines and near-isogenic lines of cv Mercia containing dwarfing and semi-dwarfing Rht alleles) comprising 18 different genotypes with contrasting phenologies were studied. We determined the ability of roots of these genotypes to penetrate strong wax layers and the angular spread of the root systems.

Results

There were no intrinsic differences in root system ability to penetrate strong wax layers (consistent with the similar root diameter of all lines) since greater root penetration was simply related to more root axes. Recording root penetration of concentric zones of the wax layer demonstrated that cv. Battalion had a root system with a smaller angular spread than cv. Robigus, which had the root system with the greatest angular spread.

Conclusions

There was limited genotypic variability in root penetration of strong layers within the wheat cultivars studied. A key challenge will be to determine the physiological and agronomic significance of the variation in root angular spread.     

Estimating the parameters of a 3-D root distribution function from root observations with the trench profile method: case study with simulated and field-observed root data

Background and aims

Root length density (RLD) is a parameter that is difficult to measure, but crucial to estimate water and nutrient uptake by plants. In this study a novel approach is presented to characterize the 3-D root length distribution by supplementing data of the 3-D distribution of root intersections with data of root length density from a limited number of soil cores.

Methods

The method was evaluated in a virtual experiment using the RootTyp model and a field experiment with cauliflower (Brassica oleracea L. botrytis) and leek (Allium porrum, L.).

Results

The virtual experiment shows that total root length and root length distribution can be accurately estimated using the novel approach. Implementation of the method in a field experiment was successful for characterizing the growth of the root distribution with time both for cauliflower and leek. In contrast with the virtual experiment, total root length could not be estimated based upon root intersection measurements in the field.

Conclusions

The novel method of combining root intersection data with root length density data from core samples is a powerful tool to supply root water uptake models with root system information.     

Growth, carboxylate exudates and nutrient dynamics in three herbaceous perennial plant species under low, moderate and high phosphorus supply

Background and aims

Australian herbaceous native species have evolved in phosphorus (P) impoverished soils. Our objective was to explore shoot and root adaptations of two of these species with potential to be developed as pasture plants, at low, moderate and high P supply after 4 and 7?weeks of growth.

Methods

A glasshouse experiment examined the effect of 5, 20 and 80?mg?P?kg^?1 air-dry soil on growth, rhizosphere carboxylate content, and mineral nutrition of two Australian native perennials, Kennedia nigricans (Fabaceae) and Ptilotus polystachyus (Amaranthaceae), and the exotic Medicago sativa (Fabaceae).

Key results

Leaf P concentrations at P80 were 6, 14 and 52?mg?P?g^?1 leaf dry weight for M. sativa, K. nigricans and P. polystachyus, respectively. As soil P concentration increased, rhizosphere carboxylate content decreased for M. sativa, increased and then decreased for K. nigricans and was unchanged for P. polystachyus. For all species, the contribution of malic acid declined at the second harvest. For all species and P treatments, the amount of rhizosphere carboxylates per unit root length decreased as root length of a plant increased. Plant P content was determined more by P uptake rate per unit root length and time than by root length. Uptake of Mo for all species, and uptake of K, Mg and Mn for P. polystachyus, increased with soil P concentration. Uptake of Fe and S was higher when the content of carboxylates in the rhizosphere was higher.

Conclusion

Root physiological adaptations (i.e. rhizosphere carboxylate content and P-uptake rate) are more important than morphological adaptations (i.e. root length and diameter) to enhance the uptake of P and cations.