IRON PLAQUE FORMATION ON THE ROOTS OF MANGROVE SEEDLINGS

W. Machado, L.D. Lacerda* (Dept. Geoquímica, Universidade Federal Fluminense, Niterói 24020-007, RJ, Brazil, e-mail: geowmac@vm.uff.br, Fax: 55 21 7174189)

The formation of metal coatings on the roots of plants in waterlogged sediments, denominated iron plaques, may mediate the plant uptake of potentially toxic elements. We estimate and compare the iron plaque formation on the fine (nutritive) roots of mangrove seedlings growing in two different environmental conditions ( oxic sand flat sediments and anoxic mangrove forest sediments. Our results indicate that seedlings of the mangrove species Avicennia schaueriana, Laguncularia racemosa and Rhizophora mangle developed efficient mechanisms of Fe, Mn and Zn exclusion, directly linked to plaque formation. These processes seem to be highly influenced by plant species-specific responses to environmental conditions, associated to the geochemical behavior of each element. The metal exclusion in the plaques was estimated as the percentage of metal concentration on the root surface in relation to total root (with plaque cover) concentration. A high plaque exclusion was observed for Fe (averages of 62% to 87%) and Mn (65% to 88%), while for Zn a more strong variability was observed (39% to 89%). Fe and Zn translocation to leaves was highly suppressed by accumulation into the fine root tissues, while leaf tissues showed similar or higher Mn concentrations than fine root tissues. This suggests that root tissues are not able to suppress Mn translocation to the leaves. The iron plaque formation and the Mn precipitation in the sediments (both associated with root O2 release to rhizosphere) may be the only significant exclusion mechanisms of this element from the whole organism level. The results presented here suggest that iron plaque formation is an useful mechanism presented by seedlings of mangrove species and seems to be a key adaptive strategy in the seedling colonization of intertidal environments.