Abstract
The hypothesis that uptake of metals from solution by roots is controlled only by the activity of the free metal in solution is now being challenged. We examined the hypothesis that charge/binding strength of metal complexes determine their uptake by plant roots.
Lettuce (Lactuca sativa L.) was grown in complete nutrient solution where metal activities were buffered using a range of organic ligands — nitrilotriacetate (NTA), ethylenediamine-N, N, N’ N’-tetraacetate (EDTA), trans-1, 2-cyclohexyl-diamine-N, N, N’, N’-tetraacetate (CDTA), diethylenetriaminepentaacetate (DTPA), N-2-hydroxyethyl-ethylenediamine-N, N’, N’-triacetate (HEDTA), ethylene-bis-(oxyethylenenitrilo)-tetraacetate (EGTA), hydroxyethyl-imino-diacetate (HEIDA), N, N-ethylene-diamine-diacetate (EDDA), 1, 4, 7, 10, 13-pentaazatridecane (Tetren), 8-hydroxyquinoline-5-sulfonate (Sulfoxine), ortho-phenanthroline (OP) and dithiodicarbamate (DDC). Activities of the free cadmium (Cd2+) and zinc (Zn2+) ions in solution ranged from 10-12 to 10-7 M and 10-9.9 to 10-7 M, respectively and were maintained during plant growth by labelling with 109Cd and 65Zn, followed by regular radioassay of solution 109Cd and 65Zn activities and replenishment as necessary.
Both EDDA and OP adversely affected plant growth, an effect related to the ligand concentration and not to metal toxicity. Other ligands had no effect on plant growth. In the absence of ligands, plant uptake of Cd was related linearly to the free metal ion activities in solution, with the slope of the relationship close to 1.0, as expected. At any given Cd2+ activity, uptake of Cd in the presence of ligands was greatest compared to uptake in the absence of ligand, and this difference increased as the Cd-ligand binding constant increased. Uptake of Zn was also greater in the presence of ligands, given the same solution Zn2+ activity. Shoot:root ratios of metals also increased in the presence of ligands. We conclude that chelator-buffered solution culture systems either allow uptake of intact metal-ligand complexes, or affect diffusional limitations to free metal uptake in the unstirred zone adjacent to the root and in the apoplast.
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McLaughlin, M.J., Smolders, E., Merckx, R., Maes, A. (1997). Plant uptake of Cd and Zn in chelator-buffered nutrient solution depends on ligand type. In: Ando, T., Fujita, K., Mae, T., Matsumoto, H., Mori, S., Sekiya, J. (eds) Plant Nutrition for Sustainable Food Production and Environment. Developments in Plant and Soil Sciences, vol 78. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-0047-9_20
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DOI: https://doi.org/10.1007/978-94-009-0047-9_20
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