Lead and Zn uptake and chemical changes in rhizosphere soils of four emergent-rooted wetland plants;Aneilema bracteatum,Cyperus alternifolius,Ludwigia hyssopifolia and Veronica serpyllifolia were investigated by two experiments:(1) rhizobag filled with "clean" or metal-contaminated soil for analysis of Pb and Zn in plants and rhizosphere soils;and (2) applied deoxygenated solution for analyzing their rates of radial oxygen loss (ROL).The results showed that the wetland plants with different ROL rates had significant effects on the mobility and chemical forms of Pb and Zn in rhizosphere under flooded conditions.These effects were varied with different metal elements and metal concentrations in the soils.Lead mobility in rhizosphere of the four plants both in the "clean" and contaminated soils was decreased,while Zn mobility was increased in the rhizosphere of the "clean" soil,but decreased in the contaminated soil.Among the four plants,V.serpyllifolia,with the highest ROL,formed the highest degree of Fe plaque on the root surface,immobilized more Zn in Fe plaque,and has the highest effects on the changes of Zn form (EXC-Zn) in rhizosphere under both "clean" and contaminated soil conditions.These results suggested that ROL of wetland plants could play an important role in Fe plaque formation and mobility and chemical changes of metals in rhizosphere soil under flood conditions.
Paddy fields in mining areas are usually co-contaminated by a cocktail of mixed toxic heavy metals(e.g.,Cd and Pb in Pb/Zn mines).However,previous studies on rice cultivars screened for effective metal exclusion have mostly focused on individual metals,and have been conducted under pot-trial or hydroponic solution conditions.This study identified rice cultivars with both low Cd and Pb accumulation under Cd-and Pb-contaminated field conditions,and the interactions of the toxic elements Cd and Pb with the micronutrient elements Fe,Zn,Mn and Ni were also studied.Among 32 rice cultivars tested,there were significant differences in Cd(0.06-0.59 mg/kg) and Pb(0.25-3.15 mg/kg) levels in their brown rice,and similar results were also found for the micronutrient elements.Significant decreases in concentrations of Fe and Mn were detected with increasing Cd concentrations and a significant elevation in Fe,Mn and Ni with increasing Pb concentrations.A similar result was also shown by Cd and Ni.Three cultivars were identified with a combination of low brown rice Cd and Pb,high micronutrient and grain yield(Wufengyou 2168,Tianyou 196 and Guinongzhan).Present results suggest that it is possible to breed rice cultivars with low mixed toxic element(Cd,Pb) and high micronutrient contents along with high grain yields,thus ensuring food safety and quality.
