The toxicity of cadmium to barley plants as affected by complex formation with humic acid |
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Authors: | D Cabrera S D Young D L Rowell |
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Institution: | (1) Department of Soil Science, University of Reading, RG1 5AQ Reading, Berks, UK;(2) Present address: Department of Physiology and Environmental Science, Nottingham University School of Agriculture, LE12 5RD Loughborough, Leics |
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Abstract: | An ‘alternating solution’ culture method was used to study the effects of chloride ions and humic acid (HA) on the uptake
of cadmium by barley plants. The plants were transferred periodically between a nutrient solution and a test solution containing
one of four levels of HA (0, 190, 569 or 1710 μg cm−3) and one of five levels of Cd (0, 0.5, 1.0, 2.5 or 5.0 μg cm−3) in either a 0.006M NaNO3 or 0.006M NaCl medium. Harvest and analysis of shoots and roots was after nineteen days. The distribution of Cd in the test solutions
between Cd2+, CdCl+ and HA-Cd was determined in a separate experiment by dialysis equilibrium.
In the nitrate test solutions Cd uptake was clearly controlled by Cd2+ concentration and was therefore reduced by HA complex formation. In the absence of HA, chloride suppressed Cd uptake indicating
that Cd2+ was the preferred species. However complex formation with Cl− enhanced uptake when HA was present because of an increase in the concentration of inorganic Cd species relative to the nitrate
system.
The ratio root-Cd/shoot-Cd remained at about 10 across a wide range of shoot-Cd concentrations, from about 3 μg g−1 (sub-toxic) up to 85 μg g−1 (80% yield reduction). The ability of the barley plants to accumulate ‘non-toxic’ Cd in their roots was thus very limited.
Humic acid also had no effect on Cd translocation within the plant and the root/shoot weight ratio did not vary with any treatment.
At shoot-Cd concentrations in excess of 50 μg g−1, K, Ca, Cu and Zn uptake was reduced, probably the result of root damage rather than a specific ion antagonism. The highest
concentration of HA also lowered Fe and Zn uptake and there was a toxic effect with increasing HA concentration at Cd=0. However
the lowest HA level, comparable with concentrations found in mineral soil solutions, only reduced yield (in the absence of
Cd) by <5% while lowering Cd uptake across the range of Cd concentrations by 66%–25%. |
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Keywords: | barley cadmium chelation complexation humic acid plant uptake solution culture toxicity |
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