Potential and actual root growth variations in root systems: modeling them with a two-step stochastic approach

Background and aims

Root elongation is essential in the determination of the root system architecture (RSA). Using experimental data, we show how it varies in the RSA and suggest a new and simple modeling approach to predict these variations.

Methods

We analyzed variation in elongation on data from pot-grown plants belonging to two different species (Helianthus annuus L. and Noccaea caerulescens (J.Presl & C.Presl) F.K.Mey). A stochastic model was designed with two successive steps to quantify and simulate these variations. The first step is the definition of a growth potential, reflected by the apical diameter, and depending on the size of the mother root. The second step, during elongation, describes the dynamic evolution of the meristem and its interaction with soil constraints.

Results

The species exhibited differences in their structured variations and very large residual (pseudo-random) variability in elongation rate and final length. The two-step model allowed us to summarize these species characteristics, and to show the interest of considering the stochastic aspects of root growth to correctly simulate the RSA.

Conclusions

Apart from being a more realistic way of simulating root development, this type of model raises new questions regarding the representation of root soil interactions during elongation.     

Interactive effects of soil nutrient heterogeneity and belowground herbivory on the growth of plants with different root foraging traits

Aims

Plants with precise root foraging patterns can proliferate roots preferentially in nutrient-rich soil patches. When nutrients are distributed heterogeneously, this trait is often competitively advantageous in pot experiments but not field experiments. We hypothesized that this difference is due to belowground herbivory under field conditions.

Methods

We performed pot experiments using seedlings of Lolium perenne (a more precise root foraging species) and Plantago lanceolata (a less precise root foraging species). The experiment had a two-way factorial randomized block design, with nutrient distribution pattern (homogeneous or heterogeneous) and belowground herbivore (present or absent) as the two factors. Each pot contained one seedling of each species.

Results

With no herbivore present, plant biomass was smaller in the heterogeneous nutrient treatment than in the homogeneous treatment in P. lanceolata, but not in L. perenne. Under homogeneous nutrient distribution, plant biomass was lower in both species with a herbivore present than with no herbivore. Under heterogeneous nutrient distribution, biomass reduction due to herbivory occurred only in L. perenne.

Conclusions

Roots of the precise root foraging species were grazed more under the heterogeneous nutrient distribution, suggesting that the herbivore more efficiently foraged for roots in nutrient-rich soil patches.     

Wheat root diversity and root functional characterization

Background and Aims

Under limited moisture conditions, roots can play an outstanding role with respect to yield stability by effective absorption of water from soil. A targeted integration of root traits into plant breeding programs requires knowledge on the existing root diversity and access to easy and cost-effective methods. This study aimed to assess wheat root diversity, root properties in relation to water regime, and the efficiency of root capacitance for in situ screening.

Methods

Root morphological, anatomical properties and root capacitance of wheat species from different ploidy levels were studied under field conditions in 2 years contrasting in water regime. Soil water content was weekly measured.

Results

Significant genotypic differences were observed for most root traits. The investigated genotypes exploited different strategies to maximize soil water depletion, e.g. high topsoil root length density, low tissue mass density, high specific root length, deep rooting and looser xylem vessels. Multivariate statistics of root traits revealed an acceptable genotypic differentiation according to regional origin, genetics and capacity to extract soil water.

Conclusions

Under supply-driven environments, dehydration avoidance via water uptake maximization can be achieved through high topsoil rooting density. In this regard, root capacitance can be useful for in situ screening.     

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.     

Effects of biochar amendment on root traits and contaminant availability of maize plants in a copper and arsenic impacted soil

Background and aims

Biochar has been proposed as a tool to enhance phytostabilisation of contaminated soils but little data are available to illustrate the direct effect on roots in contaminated soils. This work aimed to investigate specific root traits and to assess the effect of biochar amendment on contaminant availability.

Methods

Amendment with two different types of biochar, pine woodchip and olive tree pruning, was assessed in a rhizobox experiment with maize planted in a soil contaminated with significant levels of copper and arsenic.

Results

Amendment was found to significantly improve root traits compared to the control soil, particularly root mass density and root length density. Copper uptake to plants and ammonium sulphate extractable copper was significantly less in the biochar amended soils. Arsenic uptake and extractability varied with type of biochar used but was not considered to be the limiting factor affecting root and shoot development.

Conclusions

Root establishment in contaminated soils can be enhanced by biochar amendment but choice of biochar is key to maximising soil improvement and controlling contaminant availability.     

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.     

PP2A mediates lateral root development under NaCl-induced osmotic stress throughout auxin redistribution in Arabidopsis thaliana

Aim

Auxin plays an important role in modulating root system architecture. The effect of salinity on root development has been extensively studied; however, evidence on how salinity affects lateral root development and its underlying molecular mechanism is scarce. Here, we analyzed the role of protein phosphatase PP2A activity in auxin redistribution during Arabidopsis root system adaptation under NaCl-induced osmotic stress.

Method

Arabidopsis Col-0 and DR5::UidA seedlings were grown in MS media containing NaCl alone or in combination with the auxin transport inhibitor naphthylphthalamic acid, the synthetic auxin α-Naphthaleneacetic acid or the phosphatase inhibitor Okadaic acid. After 8 days, primary root length and lateral root number in seedlings were quantified and the auxin distribution was analyzed.

Results

Promotion of primary root shortening and lateral root development induced by osmotic stress correlated with an increase in active auxin content and a >50 % reduction in protein phosphatase type 2A (PP2A) activity. Moreover, the observed effects on seedlings under osmotic stress are more pronounced with the PP2A inhibitor Okadaic acid.

Conclusion

Our data suggest PP2A is a positive regulator of osmotic stress-induced root system architecture modulation, involving auxin redistribution in Arabidopsis thaliana.     

Effect of species, root branching order and season on the root traits of 13 perennial grass species

Aims

Inter-specific comparisons of plant traits may vary depending on intra-specific variation. Here we examine the impact of root branching order and season on key functional root traits for grass species. We also compare root traits among co-existing grass species as a step towards defining root trait syndromes.

Methods

Monocultures of 13 grass species, grown under field conditions and subjected to intensive management, were used to record root trait values for coarse roots (1st order, >0.3?mm), fine roots (2nd and 3rd orders, <0.2?mm) and mixed root samples over three growing seasons.

Results

Branching order and species had a significant effect on root trait values, whereas season showed a marginal effect. The diameter of coarse roots was more variable than that of fine roots and, as expected, coarse roots had higher tissue density and lower specific root length values than fine roots. Principal component analysis run on eight root traits provided evidence for two trait syndromes related to resource acquisition and conservation strategies across grass species.

Conclusions

Our data show that root branching order is the main determinant of root trait variation among species. This highlights the necessity to include the proportion of fine vs coarse roots when measuring traits of mixed root samples.     

Comparative analysis of Cd and Zn impacts on root distribution and morphology of Lolium perenne and Trifolium repens: implications for phytostabilization

Backgrounds and aims

The phytostabilization potential of plants is a direct function of their root systems. An experimental design was developed to investigate the impact of Cd and Zn on the root distribution and morphology of Lolium perenne and Trifolium repens.

Methods

Seedlings were transplanted into columns filled with washed quartz and irrigated daily with Cd- or Zn-containing nutrient solutions during 1 month. Root biomass, root length density (RLD) and diameter were subsequently quantified as a function of depth. Pot experiments were also performed to quantify metal, lignin and structural polysaccharides concentrations as well as cell viability.

Results

Lolium perenne accumulated Cd and Zn in the roots whereas T. repens was unable to restrict heavy metal translocation. Cadmium and Zn reduced rooting depth and RLD but induced thick shoot-borne roots in L. perenne. Cd-induced root swelling was related to lignification occurring in the exodermis and parenchyma of central cylinder. Hemicelluloses and lignin did not play a key role in root metal retention. Cadmium slightly reduced mean root cell viability whereas Zn increased this parameter in comparison to Cd.

Conclusions

Even though plant species like Lolium perenne and Trifolium repens may appear suitable for a phytostabilization scheme based on their shoot metal tolerance, exposure to toxic heavy metals drastically impairs their root distribution. This could jeopardize the setting up of phytostabilization trials. The metal-induced alterations of root system properties are clearly metal- and species-specific. At sites polluted with multiple metals, it is therefore recommended to first test their impact on the root system of multiple plant species so as to select the most appropriate species for each site.     

Relation of fine root distribution to soil C in a Cunninghamia lanceolata plantation in subtropical China

Background and aims

Growth and distribution of fine roots closely depend on soil resource availability and affect soil C distribution in return. Understanding of relationships between fine root distribution and soil C can help to predict the contribution of fine root turnover to soil C accumulation.

Methods

A study was conducted in a subtropical Cunninghamia lanceolata plantation to assess the fine root mass density (FRMD), fine root C density (FRCD) of different fine root groups as well as their relations with soil C.

Results

The FRMD and FRCD of short-lived roots, dead roots and herb roots peaked in the 0–10 cm soil layer and decreased with soil depth, while FRMD, FRCD of long-lived roots peaked in the 10–20 cm soil layer. Soil C was positively related to FRMD and FRCD of total fine roots (across all three soil layers), dead roots (0–10 cm) and herb roots (10–20 cm) as well as FRCD of short-lived roots (20–40 cm) (P <0.05).

Conclusions

Soil C was mainly affected by herb roots in upper soil layers and by woody plant roots in deeper soil layers.     

Application of X-ray computed tomography to quantify fresh root decomposition in situ

Background and aims

Much of our understanding of plant root decomposition and related carbon cycling come from mass loss rates calculated from roots buried in litter bags. However, this may not reflect what actually happens in the soil, where the interactions between root and soil structure presents a more complex physico-chemical environment compared to organic matter isolated in a porous bag buried in disturbed soil. This work investigates the potential of using X-ray micro-computed tomography (CT) to measure root decomposition in situ.

Methods

Roots of Vicia faba L. were excised from freshly germinated seeds, buried in re-packed soil cores and cores incubated for 60 days. Changes in root volume and surface area were measured using repeated scans. Additional samples were destructively harvested and roots weighed to correlate root mass with root volume. The method was further applied to an experiment to investigate the effects of soil bulk density and soil moisture on root decomposition.

Results

Root volume (X-ray CT) and root mass (destructive harvest) decreased by 90 % over the 60 day incubation period, by which stage, root volume and mass had stabilised. There was a strong correlation (R ²?=?0.97) between root volume and root mass.

Conclusions

X-ray CT visualization and analysis provides a unique toolbox to understand root decomposition in situ.     

Assessing variability in root traits of wild Lupinus angustifolius germplasm: basis for modelling root system structure

Background and aims

Intra-specific variation in root system architecture and consequent efficiency of resource capture by major crops has received recent attention. The aim of this study was to assess variability in a number of root traits among wild genotypes of narrow-leafed lupin (Lupinus angustifolius L.), to provide a basis for modelling of root structure.

Methods

A subset of 111 genotypes of L. angustifolius was selected from a large germplasm pool based on similarity matrices calculated using Diversity Array Technology markers. Plants were grown for 6?weeks in the established semi-hydroponic phenotyping systems to measure the fine-scale features of the root systems.

Results

Root morphology of wild L. angustifolius was primarily dominated by the taproot and first-order branches, with the presence of densely or sparsely distributed second-order branches in the late growth stage. Large variation in most root traits was identified among the tested genotypes. Total root length, branch length and branch number in the entire root system and in the upper roots were the most varied traits (coefficient of variation CV >0.50). Over 94% of the root system architectural variation determined from the principal components analysis was captured by six components (eigenvalue >1). Five relatively homogeneous groups of genotypes with distinguished patterns of root architecture were separated by k-means clustering analysis.

Conclusions

Variability in the fine-scale features of root systems such as branching behaviour and taproot growth rates provides a basis for modelling root system structure, which is a promising path for selecting desirable root traits in breeding and domestication of wild and exotic resources of L. angustifolius for stressful or poor soil environments.     

A soil-free root observation system for the study of root-microorganism interactions in maize

Background and aims

The root surface of a plant usually exceeds the leaf area and is constantly exposed to a variety of soil-borne microorganisms. Root pathogens and pests, as well as belowground interactions with beneficial microbes, can significantly influence a plants' performance. Unfortunately, the analysis of these interactions is often limited because of the arduous task of accessing roots growing in soil. Here, we present a soil-free root observation system (SF-ROBS) designed to grow maize (Zea mays) plants and to study root interactions with either beneficial or pathogenic microbes.

Methods

The SF-ROBS consists of pouches lined with wet filter paper supplying nutrient solution.

Results

The aspect of maize grown in the SF-ROBS was similar to soil-grown maize; the plant growth was similar for the shoot but different for the roots (biomass and length increased in the SF-ROBS). SF-ROBS-grown roots were successfully inoculated with the hemi-biotrophic maize fungal pathogen Colletotrichum graminicola and the beneficial rhizobacteria Pseudomonas putida KT2440. Thus, the SF-ROBS is a system suitable to study two major belowground phenomena, namely root fungal defense reactions and interactions of roots with beneficial soil-borne bacteria.

Conclusions

This system contributes to a better understanding of belowground plant microbe interactions in maize and most likely also in other crops.     

Measuring variation in potato roots in both field and glasshouse: the search for useful yield predictors and a simple screen for root traits

Aims

Potatoes have an inadequate rooting system for efficient acquisition of water and minerals and use disproportionate amounts of irrigation and fertilizer. This research determines whether significant variation in rooting characteristics of potato exists, which characters correlate with final yield and whether a simple screen for rooting traits could be developed.

Methods

Twenty-eight genotypes of Solanum tuberosum groups Tuberosum and Phureja were grown in the field; eight replicate blocks to final harvest, while entire root systems were excavated from four blocks. Root classes were categorised and measured. The same measurements were made on these genotypes in the glasshouse, 2 weeks post emergence.

Results

In the field, total root length varied from 40 m to 112 m per plant. Final yield was correlated negatively with basal root specific root length and weakly but positively with total root weight. Solanum tuberosum group Phureja genotypes had more numerous roots and proportionally more basal than stolon roots compared with Solanum tuberosum, group Tuberosum genotypes. There were significant correlations between glasshouse and field measurements.

Conclusions

Our data demonstrate that variability in rooting traits amongst commercially available potato genotypes exists and a robust glasshouse screen has been developed. By measuring potato roots as described in this study, it is now possible to assess rooting traits of large populations of potato genotypes.     

Spatio-temporal mapping of local soil pH changes induced by roots of lupin and soft-rush

Aims

The rhizosphere is a dynamic system strongly influenced by root activity. Roots modify the pH of their surrounding soil causing the soil pH to vary as a function of distance from root surface, location along root axes, and root maturity. Non-invasive imaging techniques provide the possibility to capture pH patterns around the roots as they develop.

Methods

We developed a novel fluorescence imaging set up and applied to the root system of two lupin (Lupinus albus L., Lupinus angustifolius L.) and one soft-rush (Juncus effusus L.) species. We grew plants in glass containers filled with soil and equipped with fluorescence sensor foils on the container side walls. We gained highly-resolved data on the spatial distribution of H⁺ around the roots by taking time-lapse images of the samples over the course of several days.

Results

We showed how the soil pH in the vicinity of roots developed over time to different values from that of the original bulk soil. The soil pH in the immediate vicinity of the root surface varied greatly along the root length, with the most acidic point being at 0.56–3.36 mm behind the root tip. Indications were also found for temporal soil pH changes due to root maturity.

Conclusion

In conclusion, this study shows that this novel optical fluorescence imaging set up is a powerful tool for studying pH developments around roots in situ.     

Fine root dynamics and soil carbon accretion under thinned and un-thinned Cupressus lusitanica stands in, Southern Ethiopia

Background and aims

Forest management activities influences stand nutrient budgets, belowground carbon allocation and storage in the soil. A field experiment was carried out in Southern Ethiopia to investigate the effect of thinning on fine root dynamics and associated soil carbon accretion of 6-year old C. lusitanica stands.

Methods

Fine roots (≤2 mm in diameter) were sampled seasonally to a depth of 40 cm using sequential root coring method. Fine root biomass and necromass, vertical distribution, seasonal dynamics, annual turnover and soil carbon accretion were quantified.

Results

Fine root biomass and necromass showed vertical and temporal variations. More than 70 % of the fine root mass was concentrated in the top 20 cm soil depth. Fine root biomass showed significant seasonal variation with peaks at the end of the major rainy season and short rainy season. Thinning significantly increased fine root necromass, annual fine root production and turnover. Mean annual soil carbon accretion, through fine root necromass, in the thinned stand was 63 % higher than that in the un-thinned stand.

Conclusions

The temporal dynamics in fine roots is driven by the seasonality in precipitation. Thinning of C. lusitanica plantation would increase soil C accretion considerably through increased fine root necromass and turnover.     

Spatial root distribution of plants growing in vertical media for use in living walls

Background and Aims

For plants growing in living walls, the growth potential is correlated to the roots ability to utilize resources in all parts of the growing medium and thereby to the spatial root distribution. The aim of the study was to test how spatial root distribution was affected by growing medium, planting position and competition from other plants.

Methods

Five species (Campanula poscharskyana cv. ‘Stella’, Fragaria vesca cv. ‘Småland’, Geranium sanguineum cv. ‘Max Frei’, Sesleria heufleriana and Veronica officinalis cv. ‘Allgrün’) were grown in three growing media (coir and two of rockwool) in transparent boxes under greenhouse conditions. Root frequency was registered and the activity of individual root systems was studied via ¹⁵N uptake and plant dry weight was measured.

Results

Plants in coir had stronger root growth in all parts of the medium than plants in rockwool. Upwards root growth was limited for plants in the middle or lower parts of the medium and ¹⁵N measurements confirmed that only plants in the bottom of the box had active roots in the bottom of the medium. The species differed in root architecture and spatial root distribution.

Conclusions

The choice of growing medium, plant species and planting position is important for a living wall as it affects the spatial root growth of the plants.     

Interactions between root hair length and arbuscular mycorrhizal colonisation in phosphorus deficient barley (Hordeum vulgare)

Aims

Phosphorus (P) limits crop yield and P-fertilisers are frequently applied to agricultural soils. However, supplies of quality rock phosphate are diminishing. Plants have evolved mechanisms to improve P-acquisition and understanding these could improve the long-term sustainability of agriculture. Here we examined interactions between root hairs and arbuscular mycorrhizal (AM) colonisation in barley (Hordeum vulgare L.).

Methods

Barley mutants exhibiting different root hair phenotypes, wild type barley and narrowleaf plantain (Plantago lanceolata L.) were grown in the glasshouse in P-sufficient and P-deficient treatments and allowed to develop AM colonization from the natural soil community. Plants were harvested after 6 weeks growth and root hair length, AM-fungal colonisation, shoot biomass and P-accumulation measured.

Results

Under P-deficient conditions, root hair length and AM colonisation were negatively related suggesting that resources are allocated to root hairs rather than to AM fungi in response to P-deficiency. There was evidence that barley and narrowleaf plantain employed different strategies to increase P-acquisition under identical conditions, but root hairs were more effective.

Conclusions

This research suggests future barley breeding programmes should focus on maintaining or improving root hair phenotypes and that pursuing enhancements to AM associations under the prevalent agricultural conditions tested here would be ineffectual.     

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.