Distribution of Ni and Zn in the leaves of Thlaspi japonicum growing on ultramafic soil

A Ni-hyperaccumulator, Thlaspi japonicum, grown on ultramafic soil contained large amounts of Ni and Zn in its shoot. The Ni content of the leaves was the highest (3,424 mg kg(-1)) in the lower epidermis, which had many stomata, followed by the leaf edge, upper epidermis, which had few stomata, and mesophyll. By contrast, the Zn content was the highest (615 mg kg(-1)) in the upper epidermis followed by the lower epidermis, and was the lowest at the leaf edge. Using a microscope, the dimethylglyoxime-stained Ni-compound was observed as rod-shaped crystals mainly around the stomata and the projections of the leaf edge. In addition, a considerable amount of Ni was excreted via the guttation fluid (0.67 to 1.33 mg L-1), while the concentration of Zn in the guttation fluid was very low (0.01-0.10 mg L-1). In mature leaves, the Ni content was almost constant during the growth period, while the Zn content decreased once around the summer solstice, reached the highest level in midsummer, and decreased thereafter. In young leaves, however, the Ni and Zn contents increased to a level similar to that in mature leaves in midsummer, and decreased after early September. These data suggest that Ni is translocated along with the transpiration stream and concentrated around stomata, and that the excess amount of Ni is excreted via the guttation fluid. This mechanism may not operate for Zn accumulation in this plant. Zn in mature leaves may be supplied to young leaves on demand for rapid growth.