title: Visualizing rhizosphere chemistry of legumes with mid-infrared synchrotron radiation.
abstract: A bright synchrotron light source operated by the Lawrence Berkeley National Laboratory served as an external source for infrared (IR) microscopy of plant root microcosms. Mid-IR light from synchrotrons is 2-3 orders of magnitude brighter than conventional sources, providing contrast based on the chemical information in the reflected signal at a spatial resolution near the diffraction-limit of 3-10 microm. In an experiment using plant root microcosms fitted with zinc selenide IR-transmissive windows (50 mm x 20 mm x 1 mm), we describe chemical differences and similarities within the root zone of mung bean (Vigna radiata L.), grown with or without phosphorus, and revealed by reflectance spectromicroscopy. Comparative root and root-exudate profiles are described in sand/silt culture over the wavelength range of 2.5 to 16 pm (4.000 to 650 cm(-1) ) in the mid-IR. the spectral region most useful for the analytical identification of small organic molecules. Root epidermal tissue of plants grown with low phosphorus showed a greater lipid contribution and less lignin than nutrient-sufficient plants. In the zone 200 microm from the root axis, control plants were enriched with simple sugars and monomeric lignin precursors. In low-phosphorus plants, the rhizosphere possessed IR signatures from protein and sugars. Individual soil minerals could be easily discriminated from biological material. Synchrotron IR spectromicroscopy, therefore, complements existing root imaging techniques.
authors.affiliation: Mountain Research Station, University of Colorado, Boulder 80309-0334, USA. tkraab@culter.colorado.edu
authors.forename: T K
authors.initials: TK
authors.lastname: Raab
authors.forename: M C
authors.initials: MC
authors.lastname: Martin
mesh_terms: D007887:Fabaceae / Q000737:chemistry* / Q000187:drug effects / Q000254:growth & development; D008031:Lignin / Q000378:metabolism; D010758:Phosphorus / Q000494:pharmacology; D018517:Plant Roots / Q000737:chemistry* / Q000187:drug effects / Q000254:growth & development; D012987:Soil / Q000032:analysis; D013055:Spectrophotometry, Infrared / Q000379:methods*; D013455:Sulfur / Q000378:metabolism; D017356:Synchrotrons
chemical_list: D012987:Soil; D010758:Phosphorus; D013455:Sulfur; D008031:Lignin