In Situ Distribution and Speciation of Toxic Copper, Nickel, and Zinc in Hydrated Roots of Cowpea    

Abstract

The phytotoxicity of trace metals is of global concern due to contamination of the landscape by human activities. Using synchrotron-based x-ray fluorescence microscopy and x-ray absorption spectroscopy, the distribution and speciation of copper (Cu), nickel (Ni), and zinc (Zn) was examined in situ using hydrated roots of cowpea (Vigna unguiculata) exposed to 1.5 μm Cu, 5 μm Ni, or 40 μm Zn for 1 to 24 h. After 24 h of exposure, most Cu was bound to polygalacturonic acid of the rhizodermis and outer cortex, suggesting that binding of Cu to walls of cells in the rhizodermis possibly contributes to the toxic effects of Cu. When exposed to Zn, cortical concentrations remained comparatively low with much of the Zn accumulating in the meristematic region and moving into the stele; approximately 60% to 85% of the total Zn stored as Zn phytate within 3 h of exposure. While Ni concentrations were high in both the cortex and meristem, concentrations in the stele were comparatively low. To our knowledge, this is the first report of the in situ distribution and speciation of Cu, Ni, and Zn in hydrated (and fresh) plant tissues, providing valuable information on the potential mechanisms by which they are toxic.

Synchrotron-based Techniques Shed Light on Mechanisms of Plant Sensitivity and Tolerance to High Manganese in the Root Environment

“…Kopittke et al (2011) have provided information on operating conditions for the study of fresh plant material. Briefly, x-rays were selected by a Si(111) monochromator and focused (approximately 2 mm 3 2 mm) on the specimen by a pair of Kirkpatrick-Baez mirrors.…”

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

“…The key capabilities of the Maia detector system that enable the high throughput of the XFM beamline are (i) transit times per pixel (referred to below as "dwell time") down to 50 µs [14], (ii) close detection geometry providing a solid angle of 1.3 sr [5], (iii) high sustained count rates, normally up to 12 M/s, (iv) 100 × 100 mm 2 scan range and image area (600 × 300 mm 2 on the XFM large format stage), (v) image pixel count beyond 10 8 pixels, (vi) real-time spectral deconvolution and image display, and (vii) moderate inelastic scattering signal that provides good collective images of light elements [18], especially useful for biological samples [19][20][21][22]. Typically, pixel times and image areas are not generally a limitation, and these are chosen to suit a particular application.…”

“…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).…”

“…Numerous examples of Maia imaging of light elements using scattering can be found elsewhere (e.g. [19][20][21][22]40]). …”