Differences on Pb accumulation among plant tissues of 25 varieties of maize (Zea mays)

Abstract

Pollution of agricultural land by heavy metals has imposed an increasingly serious risk to environmental and human health in recent years. Heavy metal pollutants may enter the human food chain through agricultural products and groundwater from the polluted soils. Progress has been made in the past decade on phytoremediation, a safe and inexpensive approach to remove contaminants from soil and water using plants. However, in most cases, agricultural land in China cannot afford to grow phytoremediator plants instead of growing crops due to food supply for the great population. Therefore, new and effective methods to decrease the risk of heavy metal pollution in crops and to clean the contaminated soils are urgently needed. If we can find crop germplasms (including species and varieties) that accumulate heavy metals in their edible parts, such as the leaves of vegetables or grains of cereals, at a level low enough for safe consumption, then we can grow these selected species or varieties in the lands contaminated or potentially contaminated by heavy metals. If we can find crop germplasms that take in low concentrations of heavy metals in their edible parts and high content of the metals in their inedible parts, then we can use these selected species or varieties for soil remediation. In this study, the feasibility of the method is assessed by analyzing Pb concentrations in edible and inedible parts of 25 varieties of maize (Zea mays) grown in Pb-contaminated soils. The soil concentrations of Pb were 595.55 mg/kg in the high Pb exposed treatment and 195.55 mg/kg in the control. The results showed that the Pb concentrations in different tissues were in the order of root>shoot ≈ leaf>grain. Compared with the control, the Pb concentrations in root, shoot and leaf were greatly increased under the high Pb exposed condition, while the increments of Pb concentration in grain were relatively lower. Under the high Pb exposure, the grain Pb concentrations of 12 varieties exceeded the maximal Pb limitation of the National Food Hygiene Standard of China (NFHSC) and were inedible. This indicates that there is a high Pb pollution risk for maize grown on Pb polluted sites. Although 22 of the 25 tested varieties had harvest loss under the highly Pb stressed condition, ranging from 0.86%–38.7% of the grain biomass acquired at the control, the average harvest loss of all the tested varieties was only 12.6%, which is usually imperceptible in normal farming practices. Therefore the risk of Pb pollution in maize products cannot be promptly noticed and prevented based only on the outcome of the harvest. However, we did find that 13 of the 25 tested varieties had grain Pb concentrations lower than the limitation of the NFHSC. It is, therefore, possible to reduce the pollution risk if these favorable varieties are used for maize production in Pb-contaminated or potentially contaminated agricultural lands. Pb concentrations in vegetative tissues (root, stem and leaf) were significantly correlated with each other, while Pb concentrations of each vegetative tissue were not significantly correlated with that of grain. Among the 25 tested varieties, some varieties had Pb concentrations in grain lower than (No.1 3 and No.6) or slightly above (No.4) the limitation of the NFHSC, while their Pb concentrations in the vegetative tissues were among the highest. When excluding these varieties, correlations between the Pb concentrations of grain and those of vegetative tissues of the rest of the tested varieties became highly significant. In addition, variety No.1 had the lowest harvest loss under high Pb exposed, and the highest Pb accumulation in vegetative tissues (51.69 mg/plant, 12 times as much as in the control). Similar features were also observed in varieties No.2, No.3 and No.6, which absorbed Pb for 36–42 mg/plant under high Pb exposed. We recommend these varieties of maize to be used for bioremediation of Pb contaminated soil and crop production at the same time.