Localized Flux Maxima of Arsenic, Lead, and Iron around Root Apices in Flooded Lowland Rice

Abstract

In wetland-adapted plants, such as rice, it is typically root apexes, sites of rapid entry for water/nutrients, where radial oxygen losses (ROLs) are highest. Nutrient/toxic metal uptake therefore largely occurs through oxidized zones and pH microgradients. However, the processes controlling the acquisition of trace elements in rice have been difficult to explore experimentally because of a lack of techniques for simultaneously measuring labile trace elements and O2/pH. Here, we use new diffusive gradients in thin films (DGT)/planar optode sandwich sensors deployed in situ on rice roots to demonstrate a new geochemical niche of greatly enhanced As, Pb, and Fe(II) mobilization into solution immediately adjacent to the root tips characterized by O2 enrichment and low pH. Fe(II) mobilization was congruent to that of the peripheral edge of the aerobic root zone, demonstrating that the Fe(II) mobilization maximum only developed in a narrow O2 range as the oxidation front penetrates the reducing soil. The Fe flux to the DGT resin at the root apexes was 3-fold higher than the anaerobic bulk soil and 27 times greater than the aerobic rooting zone. These results provide new evidence for the importance of coupled diffusion and oxidation of Fe in modulating trace metal solubilization, dispersion, and plant uptake.

Evidence for the Root-Uptake of Arsenite at Lateral Root Junctions and Root Apices in Rice (Oryza sativa L.)

“…The elevated As(III) we observed in the less mature root apex corroborate previous findings that show elevated As [18][19][20][21] or Se (as selenite) [22] localized to apical regions in rice. However, the image (Figure 2) alone does not reveal whether the elevated As in apical regions equates to transport into the root interior because the image is a 2-D projection of a 3-D root.…”

“…Advanced imaging techniques used by different researchers have revealed elevated levels of As [18][19][20][21] or Se (as selenite) [22] associated with rice root apices where the Casparian bands are purportedly absent [12,13]. However, because some of these images [18,19] were generated from whole root mounts and not cross-sections, it was unclear if the elevated As signal in those studies was predominantly on the inside of the roots or on the outside of roots associated with Fe plaques (or both). Other researchers have successfully used computed tomography or modeling approaches to estimate the influx of As in root apices in rice, wheat, and cowpea roots in short-term studies [20,23].…”

High-resolution measurement and mapping of tungstate in waters, soils and sediments using the low-disturbance DGT sampling technique

“…For sediment dwelling organisms in contact with microniche zones [31,35,60], exposures to W could be significantly higher than predicted by averaged 1D data. High resolution 2D DGT captures these chemical processes (Fig.…”

“…In situ precipitation of ferrihydrite and zirconia within a precast hydrogel (precipitated/high-resolution gels) has been validated as a reliable technique to meet these requirements [25,33]. Laser-ablation (LA)-ICPMS remains one of only a few techniques with sufficiently high sensitivities to image the multi-elements distribution patterns captured by DGT [34,35]. The precipitated ferrihydrite (PF) gel based DGT analyzed by LA−ICP−MS has been validated for the measurement of oxyanion transfer across sedimentwater interfaces (SWI) at sub-mm scales [23].…”

Two decades of chemical imaging of solutes in sediments and soils – a review

“…The DGT technique has since been adopted as an analytical tool for the determination of the availability of nutrients and contaminants to terrestrial [42,43] and aquatic organisms [44], complex dissociation kinetics [45] and also for 2D chemical imaging of the distribution of labile chemical species in both, soils and sediments [11][12][13]15,36,[46][47][48][49][50][51][52][53][54][55][56][57][58][59].…”