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Title: Rhizotoxic effects of silver in cowpea seedlings

Author
item BLAMEY, F PAX - Queensland University - Australia
item KOPITTKE, PETER - Queensland University - Australia
item WEHR, J. BERNHARD - Queensland University - Australia
item Kinraide, Thomas
item MENZIES, NEAL - Queensland University - Australia

Submitted to: Journal of Society of Environmental Toxicology and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2010
Publication Date: 6/1/2010
Citation: Blamey, F.C., Kopittke, P.M., Wehr, J., Kinraide, T.B., Menzies, N.W. 2010. Rhizotoxic effects of silver in cowpea seedlings. Journal of Society of Environmental Toxicology and Chemistry. 29:2072-2078.

Interpretive Summary: Metal ions (charged elemental forms such as Ca 2+ or Fe 3+) are essential nutrients for all living things. Some nonessential metal ions, such as mercury (Hg 2+) and aluminum (Al 3+), occur as pollutants (Hg 2+) or as natural components of the environment (Al 3+) and can be intoxicating. Some essential metal ions, such as copper (Cu 2+) and manganese (Mn 2+), can be intoxicating at concentrations greater than those needed for good health. A feature of some intoxicating metal ions (e.g., Cu 2+), but not others (Pb 2+, lead), is that they cause anatomical lesions in plant roots. These take the form of cell rupture and tissue cracking leading to pathologies and reduced plant productivity. In order to understand the causes of these lesions, Ag + (silver) was added to the list of metal ions studied for toxic effects. Ag + is extremely intoxicating, rivaling even Hg 2+, another extremely intoxicating ion. Ag + is unusual because it binds weakly to cell wall components that bind strongly most other lesion-causing metal ions. Consequently, it was predicted that Ag + would not cause the lesions and that toxicity would be effected by other means. Contrary to expectation, Ag + caused lesions at remarkably low concentrations. This finding has forced a revision of the hypothesis that metal ions cause lesions because of strong binding to cell wall components. We interpret these results on the basis of Ag + interactions with cell-wall synthesizing enzymes.

Technical Abstract: Silver (Ag) is highly toxic to aquatic organisms, including algae, invertebrate animals and fish, but there is little information on Ag rhizotoxicity in higher plants. In two solution culture experiments with ca. 1000 uM Ca(NO3)2 and 5 uM H3BO3 (pH 5.4), 20 to 37 % of added Ag (= 2 uM) was lost from solution within ca. 30 min, with a further decrease after 48 h root growth. There was a 50 % reduction in root elongation rate of cowpea (Vigna unguiculata (L.) Walp. cv. Caloona) at 0.010 uM Ag in the first 4 h of exposure (0.021 uM in the first 8 h). Almost complete recovery of root elongation occurred in solutions initially containing = 0.015 uM Ag (probably due to gradual loss of soluble Ag), but not at higher concentrations. Rupturing of rhizodermal and outer cortical layers was evident after 48 h with 0.13 to 0.57 uM Ag initially in solution, being most severe at 0.13 or 0.25 uM Ag. An additional experiment showed that ruptures were first evident after 20 h exposure to 0.17 uM Ag, and that the severity of rupturing increased with time. The rhizotoxic effects of Ag are similar to those of some other trace metals (e.g. Al, La) that bind strongly to hard ligands and weakly to soft ligands. The reactions of Ag are likely to be different, however, since it is a cation that binds strongly to soft ligands and weakly to hard ligands. Results of this study have demonstrated that Ag is rapidly rhizotoxic to cowpea seedlings at concentrations similar to those that are toxic to freshwater biota.