Linking above- and belowground phenology of hybrid walnut growing along a climatic gradient in temperate agroforestry systems

Background and aims

Plant phenology is a sensitive indicator of plant response to climate change. Observations of phenological events belowground for most ecosystems are difficult to obtain and very little is known about the relationship between tree shoot and root phenology. We examined the influence of environmental factors on fine root production and mortality in relation with shoot phenology in hybrid walnut trees (Juglans sp.) growing in three different climates (oceanic, continental and Mediterranean) along a latitudinal gradient in France.

Methods

Eight rhizotrons were installed at each site for 21 months to monitor tree root dynamics. Root elongation rate (RER), root initiation quantity (RIQ) and root mortality quantity (RMQ) were recorded frequently using a scanner and time-lapse camera. Leaf phenology and stem radial growth were also measured. Fine roots were classified by topological order and 0–1 mm, 1–2 mm and 2–5 mm diameter classes and fine root longevity and risk of mortality were calculated during different periods over the year.

Results

Root growth was not synchronous with leaf phenology in any climate or either year, but was synchronous with stem growth during the late growing season. A distinct bimodal pattern of root growth was observed during the aerial growing season. Mean RER was driven by soil temperature measured in the month preceding root growth in the oceanic climate site only. However, mean RER was significantly correlated with mean soil water potential measured in the month preceding root growth at both Mediterranean (positive relationship) and oceanic (negative relationship) sites. Mean RIQ was significantly higher at both continental and Mediterranean sites compared to the oceanic site. Soil temperature was a driver of mean RIQ during the late growing season at continental and Mediterranean sites only. Mean RMQ increased significantly with decreasing soil water potential during the late aerial growing season at the continental site only. Mean root longevity at the continental site was significantly greater than for roots at the oceanic and Mediterranean sites. Roots in the 0–1 mm and 1–2 mm diameter classes lived for significantly shorter periods compared to those in the 2–5 mm diameter class. First order roots (i.e. the primary or parents roots) lived longer than lateral branch roots at the Mediterranean site only and first order roots in the 0–1 mm diameter class had 44.5% less risk of mortality than that of lateral roots for the same class of diameter.

Conclusions

We conclude that factors driving root RER were not the same between climates. Soil temperature was the best predictor of root initiation at continental and Mediterranean sites only, but drivers of root mortality remained largely undetermined.

     

Spatial and temporal dynamics in the <Emphasis Type="Italic">Phialocephala fortinii</Emphasis> s.l. – <Emphasis Type="Italic">Acephala applanata</Emphasis> species complex (PAC)

Aims

We tested the hypothesis that vegetation composition and phenology drive both rhizospheric and heterotrophic soil processes, as an alternative to experimental approaches to partition these soil respiration sources.

Methods

We compared surface CO₂ efflux, depth profiles of CO₂ production, extractable SOC and MBC, and soil temperature and moisture along transects between contrasting vegetation types (deciduous riparian trees and adjacent meadow) over the snow-free growing seasons of 2005 and 2006.

Results

A dense flush of the nitrogen-fixing forb Melilotus officinalis dominated the meadow in 2005, corresponding with very high rates of CO₂ efflux (56 % higher than under trees and 82 % higher than in the diverse meadow community present in 2006). In 2006, proximity to trees was associated with greater surface CO₂ efflux and CO₂ production to 50 cm depth. SOC was higher under trees than in the meadow (p?p?=?0.01).

Conclusion

Community and phenology effects on soil carbon cycling demonstrate the interactions between plant traits and rhizospheric and heterotrophic soil respiration sources. This study highlights a unique case of a highly productive, nitrogen-fixing monoculture exhibiting vastly higher soil respiration rates than a diverse grass-forb community within a single, unmanaged site.

     

Early-spring soil warming partially offsets the enhancement of alpine grassland aboveground productivity induced by warmer growing seasons on the Qinghai-Tibetan Plateau

Aims

The response of vegetation productivity to global warming is becoming a worldwide concern. While most reports on responses to warming trends are based on measured increases in air temperature, few studies have evaluated long-term variation in soil temperature and its impacts on vegetation productivity. Such impacts are especially important for high-latitude or high-altitude regions, where low temperature is recognized as the most critical limitation for plant growth.

Methods

We used Partial Least Squares regression to correlate long-term aboveground net primary productivity (ANPP) data of an alpine grassland on the Qinghai-Tibetan Plateau with daily air and soil temperatures during 1997–2011. We also analyzed temporal trends for air temperature and soil temperature at different depths.

Results

Soil temperatures have steadily increased at a rate of 0.4–0.9 °C per decade, whereas air temperatures showed no significant trend between 1997 and 2011. While temperature increases during the growing season (May–August) promoted aboveground productivity, warming before the growing season (March–April) had a negative effect on productivity. The negative effect was amplified in the soil layers, especially at 15 cm depth, where variation in aboveground productivity was dominated by early-spring soil warming, rather than by increasing temperature during the growing season.

Conclusions

Future warming, especially in winter and spring, may further reduce soil water availability in early spring, which may slow down or even reverse the increases in grassland aboveground productivity that have widely been reported on the Qinghai-Tibetan Plateau.

     

<Emphasis Type="Italic">Sedum</Emphasis> root foraging in layered green roof substrates

Background and aims

Layered profiles of designed soils may provide long-term benefits for green roofs, provided the vegetation can exploit resources in the different layers. We aimed to quantify Sedum root foraging for water and nutrients in designed soils of different texture and layering.

Methods

In a controlled pot experiment we quantified the root foraging ability of the species Sedum album (L.) and S. rupestre (L.) in response to substrate structure (fine, coarse, layered or mixed), vertical fertiliser placement (top or bottom half of pot) and watering (5, 10 or 20 mm week^?1).

Results

Water availability was the main driver of plant growth, followed by substrate structure, while fertiliser placement only had marginal effects on plant growth. Root foraging ability was low to moderate, as also reflected in the low proportion of biomass allocated to roots (5–13%). Increased watering reduced the proportion of root length and root biomass in deeper layers.

Conclusions

Both S. album and S. rupestre had a low ability to exploit water and nutrients by precise root foraging in substrates of different texture and layering. Allocation of biomass to roots was low and showed limited flexibility even under water-deficient conditions.

     

Variety specific relationships between effects of rhizobacteria on root exudation,growth and nutrient uptake of soybean

Aims

It has been increasingly recognized that only distal lower order roots turn over actively within the <2 mm fine root system of trees. This study aimed to estimate fine root production and turnover rate based on lower order fine roots and their relations to soil variables in mangroves.

Methods

We conducted sequential coring in five natural mangrove forests at Dongzhai Bay, China. Annual fine root production and turnover rate were calculated based on the seasonal variations of the biomass and necromass of lower order roots or the whole fine root system.

Results

Annual fine root production and turnover rate ranged between 571 and 2838 g m^?2 and 1.46–5.96 yr^?1, respectively, estimated with lower order roots, and they were increased by 0–30 % and reduced by 13–48 %, respectively, estimated with the whole fine root system. Annual fine root production was 1–3.5 times higher than aboveground litter production and was positively related to soil carbon, nitrogen and phosphorus concentrations. Fine root turnover rate was negatively related to soil salinity.

Conclusions

Mangrove fine root turnover plays a more important role than aboveground litter production in soil C accumulation. Sites with higher soil nutrients and lower salinity favor fine root production and turnover, and thus favor soil C accumulation.

     

Tree development in structural soil – an empirical below-ground in-situ study of urban trees in Copenhagen,Denmark

Aims

The purpose of this study is to evaluate root and stem growth, nutrient status and soil properties of a tree planting in structural soil.

Methods

Root and stem growth were measured at an 10-year-old urban planting of Tilia x europaea L. ‘Pallida’ established in structural soil on a paved square in Copenhagen, Denmark.Root abundance was quantified and soil profiles were established at three different distances from 10 trees (1.0, 1.9, and 2.9 m). Soil and leaf samples were analysed for nutrient concentrations.

Results

Abundant root presence was measured across the entire profile of structural soil (160?×?60 cm) at all distances, provided that structural soil layers were constructed correctly. Erroneously constructed structural soil layers however, resulted in impaired tree and root growth. Coarse root morphology was affected by the confined growing space in the voids of the stone matrix. The soil was relatively alkaline with an average pH (CaCl₂) of 7.3. Foliar analysis revealed deficiencies of K and Mn. These deficiencies might be due to a combined effect of high pH and low soil concentrations of these nutrients.

Conclusions

The study documents that correctly constructed structural soils allow and encourage root growth in load bearing layers.

     

Net primary production,nitrogen cycling,biomass allocation,and resource use efficiency along a topographical soil water and nitrogen gradient in a semi-arid forest near an arid boundary

Aims

The objective of this study was to investigate how plants maintain productivity under a limited supply of water and N along the topographical soil water and N gradients in semi-arid forests.

Methods

We investigated forest structure and productivity, N cycling, and water and N use by plants at three different slope positions in a forested area near an arid boundary on a loess plateau in China.

Results

Net primary production (NPP) and aboveground N uptake decreased as soil water and/or N availability decreased on upper slopes; however, NPP and aboveground N uptake were only slightly lower than those of more humid forest ecosystems. Water use efficiency (WUE), N use efficiency (NUE), and fine root biomass increased as soil water and/or N supply decreased with altitude. High NUE was linked to higher N mean residence time, caused by higher N resorption efficiency rather than increasing N productivity.

Conclusions

Our results suggest that NPP and N uptake can be maintained by increasing WUE and NUE and increasing fine root biomass in water and N co-limited semi-arid forest ecosystems near arid boundaries. Such changes in resource use and acquisition strategy can affect production and N cycling via plant-soil feedback systems.

     

archiDART: an R package for the automated computation of plant root architectural traits

Background and aims

In order to analyse root system architectures (RSAs) from captured images, a variety of manual (e.g. Data Analysis of Root Tracings, DART), semi-automated and fully automated software packages have been developed. These tools offer complementary approaches to study RSAs and the use of the Root System Markup Language (RSML) to store RSA data makes the comparison of measurements obtained with different (semi-) automated root imaging platforms easier. The throughput of the data analysis process using exported RSA data, however, should benefit greatly from batch analysis in a generic data analysis environment (R software).

Methods

We developed an R package (archiDART) with five functions. It computes global RSA traits, root growth rates, root growth directions and trajectories, and lateral root distribution from DART-generated and/or RSML files. It also has specific plotting functions designed to visualise the dynamics of root system growth.

Results

The results demonstrated the ability of the package’s functions to compute relevant traits for three contrasted RSAs (Brachypodium distachyon [L.] P. Beauv., Hevea brasiliensis Müll. Arg. and Solanum lycopersicum L.).

Conclusions

This work extends the DART software package and other image analysis tools supporting the RSML format, enabling users to easily calculate a number of RSA traits in a generic data analysis environment.

     

Determining the fate of selenium in wheat biofortification: an isotopically labelled field trial study

Background and aims

Root hair growth and development are important features of plant response to varying soil conditions and of nutrient and water uptake. Most current methods of characterizing root hairs in the field are unreliable or inefficient. We describe a method to quantify root hair area in digital images, such as those collected in situ by minirhizotron systems.

Methods

This method uses ImageJ and R open source software and is partially automated using code presented here. It requires manual tracing of a subset of root hair images (training data set) to which a multivariate logistic regression is fit with each color channel in the image as an independent variable. Thereafter the model is applied to complete sets of selected root hair sections to estimate total root hair area.

Results

There was good agreement between the training data sets and the predictions of the regression models in castor (Ricinus communis L.), maize (Zea mays L.), and papaya (Carica papaya L.).

Conclusion

This method enables time-efficient and consistent quantification of root hairs using in situ root imaging systems that are already widely in use.

     

The effect of heterogeneous soil bulk density on root growth of field-grown mangrove species

Background and aims

In Australia’s Mediterranean hyperdiverse vegetation, species that produce cluster roots to mobilise poorly-available nutrients (e.g. Banksia spp.) are an important functional and structural component. Cluster roots are only active during the wet season, indicating a strong dependence on suitable surface soil moisture conditions. Winter rainfall in this region is declining due to global climate change, with a delayed commencement of rains and a decline in precipitation. It is unknown how lower soil moisture levels will affect the root dynamics of these globally-significant plant communities.

Methods

We determined the root dynamics and root lifespan with minirhizotrons with or without irrigation to simulate reduced rainfall scenarios.

Results

We found a major effect of irrigation on the early production (0.24 m m^?2 d^?1 increase), occurrence (97% increase) of cluster roots and only slight effects on lifespan (~10 days less) of all root types. With irrigation, the resultant greater soil moisture levels increased the deployment of cluster roots. Apart from cluster roots, the dynamics of other roots did not decline at lower soil moisture levels, suggesting that this system shows some resilience to decreased rainfall.

Conclusions

Future research should focus on assessing if climate-altered cluster-root activity may be promoting compositional shifts in plant communities with additional restraining effects on root trait diversity.

     

Responses of switchgrass soil respiration and its components to precipitation gradient in a mesocosm study

Aims

The objective of this study was to investigate the effects of the precipitation changes on soil, microbial and root respirations of switchgrass soils, and the relationships between soil respiration and plant growth, soil moisture and temperature.

Methods

A mesocosm experiment was conducted with five precipitation treatments over two years in a greenhouse in Nashville, Tennessee. The treatments included ambient precipitation, ?50%, ?33%, +33% and +50% of ambient precipitation. Soil, microbial, and root respirations were quantified during the growing seasons.

Results

Mean soil and root respirations in the +50% treatment were the highest (2.48 and 0.93 μmol CO₂ m^?2 s^?1, respectively) among all treatments. Soil microbial respiration contributed more to soil respiration, and had higher precipitation sensitivity mostly than root respiration. Increases in precipitation mostly enhanced microbial respiration while decreases in precipitation reduced both microbial and root respirations. Across precipitation treatments, soil respiration was significantly influenced by soil moisture, soil temperature, and aboveground biomass.

Conclusions

Our results showed that microbial respiration was more sensitive to precipitation changes, and precipitation regulated the response of soil respiration to soil temperature. The information generated in this study will be useful for model simulation of soil respiration in switchgrass fields under precipitation changes.

     

Aboveground biomass and root/shoot ratio regulated drought susceptibility of ecosystem carbon exchange in a meadow steppe

Background and aims

Nitrogen deposition and altered precipitation regime are likely to change plant growth, biomass allocation and community structure, which may influence susceptibility of ecosystem functions (i.e. ecosystem carbon exchange) to extreme climatic events, such as drought.

Methods

In a meadow steppe, we deployed a drought treatment on a long-term water and nitrogen addition experiment to investigate resource abundance changes induced variation in the sensitivity of ecosystem carbon exchange to extreme drought.

Results

Compared to the control plots, long-term water and nitrogen addition caused a strong increase in biomass, and a reduction in diversity and root/shoot ratio. Net ecosystem CO₂ exchange (NEE) in water and nitrogen addition plots were more sensitive to drought stress than the control plots. The enhanced NEE drought sensitivity (S_NEE) in nitrogen fertilization habitat is associated with changes in aboveground biomass and root/shoot ratio, rather than variation in species diversity, while S_NEE in the unfertilized plots was controlled by root/shoot ratio. Compared to the water and nitrogen addition plots, the control plots had the highest percentage recovery of ecosystem carbon exchange (R_NEE) during the rehydration period. R_NEE is likely determined by aboveground biomass and level of damage in the photosynthetic organ.

Conclusion

These findings suggest that long-term changes in precipitation regimes and nitrogen deposition may significant alter the susceptibility of key ecosystem processes to drought stress.

     

Zinc uptake by roots and accumulation in maize plants as affected by phosphorus application and arbuscular mycorrhizal colonization

Background and aims

Phosphorus (P) application reduces the zinc (Zn) concentration of cereal grain, but the mechanisms, including root Zn accumulation, remain controversial.

Methods

Field and pot experiments were conducted to determine the degree to which root Zn accumulation, root arbuscular mycorrhizal (AM) colonization, and other factors contribute to the negative interaction between P and Zn.

Results

Root Zn accumulation was positively related to shoot Zn accumulation. In responding to P application, root Zn accumulation was more affected by changes in AM colonization than by changes in root dry weight (RDW). In the pot experiment without Zn supply, root Zn concentration (RZnC), RDW, and AM colonization together explained 98% (adjusted R² value) of the decrease in root Zn accumulation with P application, while AM colonization and RDW explained 66% (adjusted R² value) of decrease in total Zn accumulation. In the pot experiment with Zn sufficient supply, RZnC and RDW explained 89% (adjusted R² value) of the decrease in root Zn accumulation with increasing P application, while RDW, RZnC, and AM colonization explained 53% (adjusted R² value) of the decrease in total Zn accumulation.

Conclusion

Especially in Zn-deficient soil, root Zn accumulation explains much of the negative interaction between P and Zn, and root Zn accumulation is greatly affected by AM colonization.

     

Root fungal colonisations of the understory grass <Emphasis Type="Italic">Deschampsia flexuosa</Emphasis> after top-canopy harvesting

Aims

Root fungal relationships in forest understory may be affected by tree harvesting. Deschampsia flexuosa forms a mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi functioning in nutrient uptake, and a more loose association with dark septate endophytic (DSE) fungi. We asked how harvesting affects fungal colonisations and whether DSE is more prone to change than AM.

Methods

Deschampsia flexuosa plants were sampled close to a control or a cut tree after top-canopy harvesting in a primary successional site. Colonisations were studied using light microscopy. Shoot N%, vegetation cover and soil nutrients were determined.

Results

Tree harvesting did not affect vegetation and soil parameters, except potassium (K⁺) increasing near cut trees. AM colonisation did not change, while DSE increased. Shoot N% increased with increasing DSE near cut trees. Hyaline septate (HSE) hyphae and soil K⁺ and magnesium (Mg²⁺) were positively correlated near control trees. Lichen cover and HSE correlated negatively.

Conclusions

DSE colonisation increased but AM did not change after harvesting. Positive correlation of DSE with shoot N% near cut trees may suggest a role for DSE in favouring plant nitrogen uptake after disturbance in an open microsite. HSE may play a role in K⁺ and Mg²⁺ uptake.

     

Fine-root growth in a forested bog is seasonally dynamic,but shallowly distributed in nutrient-poor peat

Background and aims

Fine roots contribute to ecosystem carbon, water, and nutrient fluxes through resource acquisition, respiration, exudation, and turnover, but are understudied in peatlands. We aimed to determine how the amount and timing of fine-root growth in a forested, ombrotrophic bog varied across gradients of vegetation density, peat microtopography, and changes in environmental conditions across the growing season and throughout the peat profile.

Methods

We quantified fine-root peak standing crop and growth using non-destructive minirhizotron technology over a two-year period, focusing on the dominant woody species in the bog: Picea mariana, Larix laricina, Rhododendron groenlandicum, and Chamaedaphne calyculata.

Results

The fine roots of trees and shrubs were concentrated in raised hummock microtopography, with more tree roots associated with greater tree densities and a unimodal peak in shrub roots at intermediate tree densities. Fine-root growth tended to be seasonally dynamic, but shallowly distributed, in a thin layer of nutrient-poor, aerobic peat above the growing season water table level.

Conclusions

The dynamics and distribution of fine roots in this forested ombrotrophic bog varied across space and time in response to biological, edaphic, and climatic conditions, and we expect these relationships to be sensitive to projected environmental changes in northern peatlands.

     

Embedded rock fragments affect alpine steppe plant growth,soil carbon and nitrogen in the northern Tibetan Plateau

Background and Aims

Rock fragments within topsoil have important effects on soil properties and plant growth. This study mainly aimed to investigate the relationships between rock fragments, soil carbon (C) and nitrogen (N) densities and vegetation biomass in an alpine steppe.

Methods

Rock fragments, plant and soil samples were collected from four topographic positions (top, upper, lower, and bottom) on a hillslope.

Results

Volumetric rock fragment content within the 0–30 cm soil profile varied from 17.8 to 30.5%, the upper position value was significantly greater (P < 0.05) than those at other positions. The highest aboveground biomass was observed at the lower position (921 kg ha^?1), while the highest belowground biomass within the 0–30 cm profile was found at the upper position (4460 kg ha^?1). More fine earth and plant litter input accompanied by lower C and N losses induced by rainfall erosion resulted in higher soil organic C and total N densities (28.6 Mg C ha^?1 and 2.87 Mg N ha^?1) at the lower position.

Conclusions

Rock fragments may promote root growth but limit aboveground biomass production, and can therefore change the biomass distribution pattern. Our findings provide more evidence for scientifically assessing alpine steppe productivity.

     

Unimodal pattern of soil hydrophobicity along an altitudinal gradient encompassing Mediterranean,temperate, and alpine ecosystems

Background and Aims

Soil water repellency (SWR, i.e. the reduced affinity for water due to the presence of hydrophobic coatings on soil particles) has relevant hydrological implications on the rate of water infiltration, surface runoff, and overland flow. Here, we test how SWR varies along a 2490 m altitudinal gradient encompassing six ecosystems including Mediterranean, Temperate, and Alpine vegetation types.

Methods

Water repellency, measured by the Molarity of an Ethanol Droplet (MED) test, was quantified in 80 soil samples collected for 16 different elevations. Soil quality was assessed by measuring soil texture, pH, organic carbon, salinity, and nutrient availability.

Results

SWR showed a unimodal pattern along the 2490 m transect, peaking at intermediate elevations. Unexpectedly, SWR was the highest under broad-leaf deciduous forests, and the lowest under evergreen, sclerophyllous Mediterranean vegetation types. The soil organic carbon content, and the pH were the main determinants of water repellency, showing respectively a positive, and a negative correlation with the SWR. In contrast, soil texture and salinity resulted unrelated to the SWR.

Conclusions

With this study we demonstrated a linkage between SWR, vegetation type and soil pH and organic carbon content along the elevation gradient. Further studies are needed to explicitly evaluate the impact SRW on erosion risk at catchment scale in the context of climatic change.

     

Nitric oxide-mediated cytosolic glucose-6-phosphate dehydrogenase is involved in aluminum toxicity of soybean under high aluminum concentration

Aims

Glucose-6-phosphate dehydrogenase (G6PDH) has been reported to be involved in resistance to various environmental stresses. However, the role of G6PDH in aluminum (Al) toxicity remains unclear.

Methods

Physiological and biochemical methods together with histochemical analysis were used to investigate the participation of G6PDH in Al-induced inhibition of root growth.

Results

Exposure to high Al concentration caused a significant increase in the activities of total and cytosolic G6PDH in roots of soybean. Al-induced inhibition of root growth and oxidative stress were alleviated by a G6PDH inhibitor. Reactive oxygen species (ROS) accumulation in Al-treated root apexes could be abolished by a NADPH oxidase inhibitor. Furthermore, treatment with a G6PDH inhibitor reduced NADPH content and NADPH oxidase activity in Al-treated root apexes. Further investigation demonstrates that nitric oxide (NO) mediates Al-induced increase in cytosolic G6PDH activity by modulating the expression of genes encoding cytosolic G6PDH. In addition, nitrate reductase pathway is mainly responsible for Al-induced NO production in root apexes.

Conclusions

These results indicate that NADPH produced by NO-modulated cytosolic G6PDH in root apexes is responsible for ROS accumulation mediated by NADPH oxidase under Al stress, subsequently suffering from oxidative stress and thus causing the inhibition of root elongation.

     

Optimization of fecal sample preparation for untargeted LC-HRMS based metabolomics

Introduction

Collecting feces is easy. It offers direct outcome to endogenous and microbial metabolites.

Objectives

In a context of lack of consensus about fecal sample preparation, especially in animal species, we developed a robust protocol allowing untargeted LC-HRMS fingerprinting.

Methods

The conditions of extraction (quantity, preparation, solvents, dilutions) were investigated in bovine feces.

Results

A rapid and simple protocol involving feces extraction with methanol (1/3, M/V) followed by centrifugation and a step filtration (10 kDa) was developed.

Conclusion

The workflow generated repeatable and informative fingerprints for robust metabolome characterization.

     

Trapping of lead (Pb) by corn and pea root border cells

Aims

Most plants produce a root tip extracellular matrix that includes viable border cell populations programmed to disperse into soil. Like neutrophils, border cells export structures that trap pathogens and prevent root tip infection. Border cells also trap metals. The goal of this study was to determine if border cells trap Pb.

Methods

Border cell responses to Pb were observed microscopically. Border cell impact on Pb-induced injury to roots was assessed using root growth assays. Pb removal from solution was measured using inductively coupled plasma mass spectrometry (ICP-MS). Speciation of Pb associated with border cells was evaluated by synchrotron X-ray absorption spectroscopy (XAS).

Results

Increased border cell trap size and number occurred within minutes in response to Pb but not silicon (Si). Transient immersion of root tips into Pb after border cells were removed resulted in growth inhibition. Immersion of root tips and border cells into Pb solution resulted in significant removal of Pb. Si levels in the presence of root tips remained unchanged. The Pb speciation, measured with Pb L_III XAS, altered when reacted with border cells, indicating that direct binding by extracellular traps occurred.

Conclusions

Border cells can trap Pb and prevent damage to the root tip.