Aluminum toxicity decreases the phytoextraction capability by cadmium/zinc hyperaccumulator Sedum plumbizincicola in acid soils.

Excessive aluminum (Al) in acid soils or Al released due to acidification during repeated phytoextraction might impair the phytoextraction efficiency of hyperaccumulators but this is often neglected. Here, we investigate for the first time the toxicity of Al to the cadmium (Cd) and zinc (Zn) hyperaccumulator Sedum plumbizincicola with hydroponics experiments both in the long (7 weeks) and short terms (72 h), and in soil conditions in a pot experiment. In the long-term hydroponics experiment, observable toxic effects of Al were found even at <100 μM Al at pH 5.00 (soluble Al: 8.74 μM) which lowered shoot Cd uptake by 39.3% compared with the Al-free treatment. The scanning ion-selective electrode technique shows that root Cd2+ influx was significantly inhibited after treatment with 200 μM Al at pH 4.00 after 48 h. The pot experiment confirms that Al toxicity induced inhibition of plant growth and metal uptake in the acid soil with an exchangeable Al of 0.33 cmolc kg-1. Decreasing Cd adsorption at root surfaces induced by Al stress may be an important factor in declining shoot Cd uptake. Analysis of the chemical forms of metals shows that Al addition significantly influenced the chemical forms of Cd and Zn in stems, made them less mobile and thus restrained Cd and Zn translocation. Aluminum toxicity that potentially occurs in acid soils and in soils during repeated phytoextraction would be a primary factor limiting metal removal efficiency from contaminated soils using hyperaccumulators.