Zinc distribution and excretion in the leaves of the grey mangrove, Avicennia marina (Forsk.) Vierh

Mangroves are important as primary producers in estuarine food chains. Zinc is often a major anthropogenic contaminant in estuarine ecosystems and has potential ecotoxicological consequences for mangrove communities. Accumulation, distribution and excretion of zinc in the leaf tissue of the grey mangrove, Avicennia marina was studied using SEM X-ray microanalysis and Atomic Absorption Spectroscopy. The first leaves of A. marina grown in 500 μg Zn as ZnCl2 per g of dry soil were found to accumulate 106.3±18.5 μg Zn per g dry tissue, significantly higher than control plants, after a 7-month period. Washings from first leaves contained significantly higher amounts of zinc (0.30±0.14 μg/cm2 Zn) than control plants after 1 month, suggesting excretion of zinc from glandular trichomes. SEM X-ray microanalysis revealed salt crystals exuded from glandular tissue on the adaxial surface of first leaves to be composed of alkaline metals and zinc in zinc treated plants. SEM X-ray microanalysis of seedlings dosed with 4 g/l Zn as Zn Cl2 revealed a decreasing Zn gradient from xylem tissue, through photosynthetic mesophyll, to hypodermal (water) tissue. A subsequent increase in Zn concentration was observed in glandular tissue. Cell wall Zn concentrations were consistently higher than intracellular Zn concentrations.