Abstract
Elevated levels of many metals are toxic to plant roots, but their modes of action are not well understood. We investigated the toxicities of aluminium (Al), copper (Cu), and lanthanum (La) in solution on the growth and external morphology of 3-d-old cowpea (Vigna unguiculata L.) roots for periods of up to 48 h. Root elongation rate decreased by 50% at ca. 30 µM Al, 0.3 µM Cu, or 2.0 µM La, accompanied by a decrease in the distance from the root tip to the proximal lateral root. Kinks developed in some roots 2.0 ± 0.4 mm from the root apex on exposure to Al or La (but not Cu). Light and scanning electron microscopy showed that soluble Al, Cu, or La caused similar transverse ruptures to develop > 1 mm from the root apex through the breaking and separation of the rhizodermis and outer cortex from inner-layers. The metals differed, however, in the range in concentration at which they had this effect; developing in solutions containing 54 to‑600 µM Al, but only from 0.85 to 1.8 µM Cu or 2.0 to 5.5 µM La. These findings suggest that Al, Cu, and La bind to the walls of cells, causing increased cell wall rigidity and eventual cell rupturing of the rhizodermis and outer cortex in the elongating zone. We propose that this is a major toxic effect of Al, and that Cu and La also have additional toxic effects.
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Abbreviations
- ICP-OES/MS:
-
Inductively coupled plasma-optical emission spectrometry/mass spectroscopy
- SEM:
-
scanning electron microscopy
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Acknowledgments
The authors thank Dr Kim Sewell, Rick Webb, Robyn Webb, and Rob Gould from the Centre for Microscopy and Microanalysis at The University of Queensland for their assistance with the electron microscopy. This research was funded through the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC-CARE) Project 3-3-01-05/6 and through the Australian Research Council’s Discovery funding scheme (DP0665467).
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Kopittke, P.M., Blamey, F.P.C. & Menzies, N.W. Toxicities of soluble Al, Cu, and La include ruptures to rhizodermal and root cortical cells of cowpea. Plant Soil 303, 217–227 (2008). https://doi.org/10.1007/s11104-007-9500-5
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DOI: https://doi.org/10.1007/s11104-007-9500-5