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Abstract
Mangroves have been observed to possess a tolerance to high levels of heavy metals,
yet accumulated metals may induce subcellular biochemical changes, which can impact
on processes at the organism level. Six month-old seedlings of the grey mangrove,
Avicennia marina (Forsk.) Vierh, were exposed to a range of Cu (0-800 micrograms/g),
Pb (0-800 micrograms/g) and Zn (0-1000 micrograms/g) concentrations in sediments under
laboratory conditions, to determine leaf tissue metal accumulation patterns, effects
on photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoids), and the
activity of the antioxidant enzyme peroxidase. Limited Cu uptake to leaves was observed
at low sediment Cu levels, with saturation and visible toxicity to Cu at sediment
levels greater than 400 micrograms/g. Leaf Pb concentrations remained low over a range
of Pb sediment concentrations, up to 400 micrograms/g Pb, above which it appeared
that unrestricted transport of Pb occurred, although no visible signs of Pb toxicity
were observed. Zn was accumulated linearly with sediment zinc concentration, and visible
toxicity occurring at the highest concentration, 1000 micrograms/g Zn. Significant
increases in peroxidase activity and decreases in photopigments were found with Cu
and Zn at concentrations lower than those inducing visible toxicity. Significant increases
in peroxidase activity only, were found when plants were exposed to Pb. Positive linear
relationships between peroxidase activity and leaf tissue metal concentrations were
found for all metals. Significant linear decreases in photosynthetic pigments with
increasing leaf tissue metal concentrations were observed with Cu and Zn only. Photosynthetic
pigments and peroxidase activity may be applicable as sensitive biological indicators
of Cu and Zn stress, and peroxidase activity for Pb stress in A. marina.