Abstract:

Pollutions of agricultural land by heavy metals impose a more and more 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 on phytoremediation, a safe and inexpensive approach to remove contaminants from soil and water using plants, in the past decade. However, in most cases, agricultural land in China can not afford to grow phytoremediator plants instead of growing crops to be economically sustainable. 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) which sequester heavy metals in their edible parts, such as fruits of vegetables or grain of cereals, at low enough level 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 sequester 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 contents in edible and inedible parts of 25 varieties of Zea mays grown under Pb contaminated soils. The soil concentrations of Pb were 595.55 mg·kg^-1 in the high Pb-stress group and 195.55 mg·kg^-1 in the control group.
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 highly Pb- stressed condition, while the increments of Pb concentration in grain were relatively lower. Under the high Pb-stress, the grain Pb concentrations of 12 varieties exceeded the National Sanitation Standard of China (NSSC) and were inedible. This indicates that there is a high Pb pollution risk forZea mays grown on Pb polluted sites. Although 22 of the 25 tested varieties had harvest loss under the highly stressed condition, ranging from 0.86%-38.7% of the grain biomass acquired at the control, the average harvest loss of all tested varieties was only 12.6%, which is usually imperceptible in normal farming practices. Therefore the risk of Pb pollution in Zea mays products can not be promptly notified and prevented based on the outcome of the harvest. However, we did find 13 varieties of 25 tested varieties had grain Pb concentration lower than the NSSC. It is, therefore, possible to reduce the pollution risk if these favorable varieties are used for Zea mays production in Pb polluted or potentially polluted agricultural lands.
Pb concentrations in vegetative tissues (root, stem and leaf) were significantly correlated with each other, while Pb concentrations of each vegetative tissues 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 NSSC level, 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-stress, and the highest Pb sequestration in vegetative tissues (51.69 mg·plant^-1, 12 times as much as in the control). The similar features were also observed in varieties No.2, No.3 and No.6, which sequestered 36-42 mg·Pb plant^-1 under high Pb-stress. We recommend these varieties of Zea mays to be used for bioremediation of Pb contaminated soil and crop production at the same time.

Key words: Zea mays, Pb sequestration, Variation among crop varieties, Phytoremediation, Heavy metal pollutions in agricultural soil