Effects of Arabisalpina L. var. parviflora Franch and Zea mays L. intercropping system on root-exudated organic acids under lead stress

Cultivated soils contamination by heavy metals have become increasingly contentious to decision makers, farmers, consumers and health professionals around the globe. Phytoremediation is a key strategy for decontaminating cultivated soils polluted by heavy metals. Hyperaccumulator plants are limited by their soil occupation rather than agricultural production in China. Intercropping system of hyperaccumulator plants and crops have been recommended for both of remediation and production in the same time. The accumulation of heavy metal in plants is due to root growth and root exudates. However, plant root morphology and exudates vary, which is a key issue in intercropping systems. In order to investigate the effects of lead (Pb) stress on the exudates of organic acids by hyperaccumulator plant and crop roots in intercropping and monocropping systems, a hydroponic aeration experiment was conducted in a greenhouse. A crop (maize) or hyperaccumulator (Arabisalpina L. var. parviflora Franch) monocropping and intercropping systems were set up as the control and treatment plots, respectively. The effects of 400 mgL^-1 Pb stress on root morphology, organic acids (oxalic acid, tartaric acid, citric acid, malic acid, lactic acid and acetic acid) exudation and lead accumulation in the intercropping and monocropping systems were determined. The results showed that compared with monocropped maize, lactic acid was obtained from intercropped maize root exudates. The numbers of split root, root surface area and root density of intercropped maize increased by 60%, 15% and 42%, respectively. Root and shoot biomass under intercropped maize increased by 108% and 75%, respectively, whereas root Pb content of intercropped maize decreased by 44%. Compared with monocropped A. alpina, acetic acid and lactic acid determined from root exudates of intercropped A. alpine, showing 103%1 700% increase in root exudates amount. Also Pb accumulation in underground and aboveground plant parts of intercropped A. alpina increased respectively by 49% and 75% with 22% increase in transfer coefficient of Pb. Furthermore, for monocropped A. alpine, Pb content in shoots was only significantly positively correlated with oxalic acid content; but for intercropped A. alpine, it was significantly positive correlated with contents of oxalic acid, oxalic acid and malic acid both in shoot and root of A. alpine. The results suggested that root exudation of organic acids was critical in changing Pb accumulation characteristics in maize and A. alpina intercropping system. The number and components of organic acids in root exudates changed under intercropping system, which affected Pb content and accumulation characteristics of A. alpine and maize. Pb content increased in A. alpine and decreased in maize. In short, hyperaccumulator A. alpine and maize were recommended for remediation of cultivated soil contaminated with Pb.