Thlaspi montanum var. siskiyouense occurs as an endemic on serpentine soils in southwestern Oregon, U.S.A. The leaves accumulate nickel at levels up to 3% dry mass. To determine if nickel was localized in particular cells or tissues, leaves were prepared for scanning electron microscopy and energy-dispersive X-ray microanalysis by first soaking the leaves in dimethylglyoxime to precipitate nickel. Leaves were then fixed in glutaraldehyde, dehydrated, and criticalpoint dried. Spectra of leaves of T. montanum var. siskiyouense showed accumulation of nickel, calcium, potassium, and phosphorus. On X-ray maps of the upper and lower epidermis, nickel was localized in the subsidiary cells that surround guard cells but not in guard cells or in other more elongate epidermal cells.
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Heath et al. (1997) studied this question.