Examination of the Distribution of Arsenic in Hydrated and Fresh Cowpea Roots Using Two- and Three-Dimensional Techniques    

Abstract

Arsenic (As) is considered to be the environmental contaminant of greatest concern due to its potential accumulation in the food chain and in humans. Using novel synchrotron-based x-ray fluorescence techniques (including sequential computed tomography), short-term solution culture studies were used to examine the spatial distribution of As in hydrated and fresh roots of cowpea (Vigna unguiculata 'Red Caloona') seedlings exposed to 4 or 20 mm arsenate [As(V)] or 4 or 20 mm arsenite. For plants exposed to As(V), the highest concentrations were observed internally at the root apex (meristem), with As also accumulating in the root border cells and at the endodermis. When exposed to arsenite, the endodermis was again a site of accumulation, although no As was observed in border cells. For As(V), subsequent transfer of seedlings to an As-free solution resulted in a decrease in tissue As concentrations, but growth did not improve. These data suggest that, under our experimental conditions, the accumulation of As causes permanent damage to the meristem. In addition, we suggest that root border cells possibly contribute to the plant's ability to tolerate excess As(V) by accumulating high levels of As and limiting its movement into the root.

In Situ Speciation and Distribution of Toxic Selenium in Hydrated Roots of Cowpea

“…Recent advances in synchrotron-based techniques allow in situ measurement of the distribution of metal (loid)s in hydrated fresh plant tissues (Kopittke et al, 2011(Kopittke et al, , 2012Lombi et al, 2011a). In particular, the prototype Maia detector system, jointly developed by the Australian Synchrotron, the Commonwealth Scientific and Industrial Research Organization, and the Brookhaven National Laboratory, represents a newgeneration x-ray fluorescence detector and real-time processing approach that provides unprecedented capabilities in in situ element imaging and measurement (Lombi et al, 2011b).…”

“…roots) without observable damage (Lombi et al, 2011a). This has allowed us to overcome the analytical challenges of examining the two-dimensional and virtual three-dimensional distribution of lowconcentration metal(loid)s in hydrated and fresh plant tissues (Kopittke et al, 2011(Kopittke et al, , 2012Lombi et al, 2011aLombi et al, , 2011c.…”

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

“…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]. Ma et al [24] demonstrated that lateral roots are important for Si transport in rice, but the importance of these zones for As(III) i root-uptake has only recently been investigated [25].…”

Maia X-ray fluorescence imaging: Capturing detail in complex natural samples

“…Using a collimated beam, larger objects, such as works of art, have been imaged at 50-100 µm resolution [18] over areas up to 600 × 300 mm 2 . This method has been applied to studies in the earth [23][24][25][26][27][28][29][30], planetary [31], environmental [32,33], medical [19,34], biological [20][21][22][35][36][37][38][39][40][41] and material [42] sciences as well as cultural heritage [18,43]. A similar capability has been established on the P06 beamline (e.g.…”

“…5 µm tomographic data have been acquired using a KB mirror pair with ~2 µm resolution over ~1 mm specimens [37]. Single-slice data can be obtained from highly radiation-sensitive specimens, such as fresh and fully hydrated plant roots, in a matter of minutes [20,21,38,39]. Measurement of the Ge distribution in an intact rice grain with 2 µm voxels over 2-3 mm dimensions utilized high-definition 2D fluorescence tomography, with 2001 projections and a measurement time of the order of 3 hours [22] (figure 4a).…”