|Mcnear Jr., David|
Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2005
Publication Date: 4/1/2005
Citation: Mcnear Jr., D.H., Peltier, E., Everhart, J., Chaney, R.L., Newville, M., Rivers, M., Sutton, S., Sparks, D.L. 2005. The novel application of fluorescence and absorption edge computed microtomography to image metal distribution in alyssum murale. Environmental Science and Technology. 39(7):2210-2218. Interpretive Summary: Plants which hyperaccumulate soil metals can be used for phytoremediation or phytomining, in which these species are farmed to clean-up contaminated soils, or as a crop to produce an alternative metal ore. Study of the fundamental processes by which hyperaccumulators achieve over 1% Ni or Zn in leaves is needed in order to discover what needs to be combined by plant breeding for improved cultivars. The Ni hyperaccumulator, Alyssum murale, which was developed for commercial use as a new phytomining crop, was studied using novel methods of using synchrotron radiation which can characterize the location and kinds of chemical bonds to the metal in living plant tissues. Tomography is measurement of the three-dimensional distribution of elements which can be measured by X-ray absorption or fluorescence. In earlier studies with scanning electron microscopy and X-ray analysis, we had shown that particular leaf cells accumulated Ni and Mn while trichomes accumulated Ca. Using microtomography, one can show that all trichomes have high Ca levels, while no other leaf cells or cell parts have high Ca. Epidermal (both upper and lower) cells hyperaccumulated the highest levels of Ni, and particular cells at the base of trichomes accumulated both Ni and Mn. Because the X-ray tomography examines an area with numerous trichomes, the large pattern of localization of Ni, Mn and Ca was obvious. Because of the sensitivity of this method, and the ability to examine cells throughout a plant tissue, Ni was observed in the xylem transport system (vascular tissues), but not the phloem. These techniques add important new information to our understanding of how hyperaccumulator plants achieve this remarkable metal accumulation.
Technical Abstract: Conventional techniques (SEM, TEM etc.) used to determine metal partitioning in hyperaccumulating plant tissues suffer from sample pretreatment and ultra high vacuum sampling conditions which have the potential to alter the chemical and physical elemental distributions within plant tissues. Additionally, because of the relatively high detection limits, less concentrated, but potentially relevant elemental concentrations may be overlooked. In this paper we examine the ability of novel synchrotron based fluorescence and absorption edge computed microtomography (CMT) techniques to determine the metal partitioning behavior in the nickel hyperaccumulator Alyssum murale 'Kotodesh'. Plants were grown in a glass house in Ni enriched soils collected from an area adjacent to a Ni refinery in Port Colborne, Ontario Canada. Fluorescence and absorption edge CMT images were collected at the GeoSoilEnviroCARS (GSECARS) beamline of the Advanced Photon Source, Chicago, Ill. Fluorescence CMT enabled us to determine the metal distribution within a virtual slice through leaf, stem and root samples with high spatial resolution and a detection limit of ~5 ppm (Ni). Ni was concentrated throughout the upper and lower epidermis with discrete spots of concentration correlating with Mn in the vicinity of the Ca rich trichomes. Vascular enrichment was also observed and attributed to the minimal sample treatment and increased sensitivity of the fluorescence CMT technique, as no other study has observed this trend. Ni was also seen concentrated in the epidermis and xylem and absent from the phloem and cortex tissues of the stem. Enrichment of Ni within the stele of the secondary roots examined indicates that these roots may contribute more to Ni uptake as these patterns were not observed in coarser roots. Absorption edge CMT revealed that the epidermal Ni distribution seen in the fluorescence CMT virtual slice was present throughout the entire leaf. Discrete regions of Ni enrichment were detected within the epidermis which corresponded to the Mn and Ca associations observed in the fluorescence CMT images. Distinct, but discontinuous regions of Ni enrichment were seen within the interior of the leaf and attributed to Ni within the vascular system. Ni was also concentrated in the leaf tip compared to the remainder of the leaf possibly serving as a detoxification or anti-herbivory response of the plant. Together these techniques provided a highly resolved, highly sensitive and all-inclusive picture of the metal partitioning within selected leaf stem and root samples from the Ni hyperaccumulator Alyssum murale with relatively little sample preparation and should prove to be a useful tool for exploring metal partitioning in similar systems.