An Investigation of Metal Accumulation by Plants with Particular Reference to Zinc

Aerial tissues of two highly mineralized plant samples from a zinc site were examined for their treatment of the metal. For one of the species, Crotalaria nova hollandiae, extremely high zinc concentrations were found in its tissues, particularly in the Leaves. The majority of the metal was water soluble and present apparently as the Zn2+(aq) ion. Most of the insoluble zinc was bonded to pectin. For the other species, Polycarpaea glabra, the zinc levels were very much lower and less water soluble than in Crotalaria and two soluble zinc complexes were found. One, which was dominant in the stem, was identified as a zinc pectinate, the other, found particularly in flowers, was not characterized. Green stem material of Polycarpae glabra was found to possess a lower zinc concentration than woody stem but a higher proportion of water insoluble zinc associated with pectin. Treatment of zinc by the two species was found to be different in other ways. By the use of ultracentrifugation, histochemistry and the electron microprobe Crotalaria nova hollandiae was found to accumulate zinc in the phloem of leaves, Polycarpeae glabra to accumulate particularly in walls of certain cells and probably the phloem. In addition, Polycarpaea glabra accumulated zinc in new leaves and leaf nodes and the majority of the metal was associated with pectin of the cell walls. Plants from high zinc and low zinc sites treated zinc differently (a higher proportion of zinc in plants from low zinc sites was associated with proteins), but examination of cell wall-metal interaction did not reveal any corresponding differences. Mechanisms by which the two plants tolerate their high metal burdens are suggested. Other mineralized plant species were examined and findings reported here include the accumulation of iron in the leaf xylem walls of Holichrysum leptolepis, the isolation of a water soluble copper-protein complex in the same species and accumulation of lead (from car-exhaust polluted soil) in the primary branches of Ecbolium lugardae. Emphasis is made throughout of the analytical techniques employed and their applicability to plant studies.