Chemical forms of calcium in Ca,Zn- and Ca,Cd-containing grains excreted by tobacco trichomes

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Tobacco (Nicotiana tabacum L. cv. Xanthi) plants exposed to toxic levels of zinc and cadmium excrete metals through their leaf trichomes (epidermal hairs) as Zn,Caand Cd,Ca-containing grains. Little is known about the nature and formation mechanism of these precipitates. The chemical, crystalline, and non-crystalline compositions of individual grains produced by tobacco were studied by scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDX), micro-X-ray diffraction (μXRD) and calcium K-edge micro X-ray absorption near edge structure (μXANES) spectroscopy. Zinc is predominantly incorporated in calcite, and cadmium in calcite and vaterite. Aragonite, which occurs occasionally, does not seem to contain trace metals. In addition to being precipitated in its three possible polymorphic forms, calcite, aragonite and vaterite, calcium also is speciated as amorphous CaCO3 and possibly organic Ca in some grains. Most often, a particular grain consists of two or more crystalline and non-crystalline phases. The observed variability of intraand inter-grain elemental and phase composition suggests that this biomineralization process is not constrained by biological factors, but instead results from thermodynamically and kinetically controlled reactions. This study illustrates the potential of laterally resolved X-ray synchrotron radiation techniques (μXRD and μXANES) to study biomineralization and metal immobilization processes in plants.