Influence of plant growth regulators and humic acid on the phytoremediation of lead by maize in a Pb-polluted calcareous soil

Phytoremediation is a cleanup technology for the removal of contaminants from polluted soils. The influences of humic acid (HA) and four plant growth regulators, i.e. gibberellic acid, salicylic acid, 3-indoleacetic acid, and 6-benzylaminopurine on the phytoremediation of Pb by maize were investigated in a pot trial on a Pb-contaminated soil. Foliar spray of 6-benzylaminopurine and salicylic acid significantly increased mean root dry matter but none of growth regulators had a significant effect on that of shoot. Addition of HA decreased root dry matter and had no significant effect on that of shoots. Application of all growth regulators except gibberellic acid significantly increased root Pb uptake. Application of salicylic acid significantly increased mean uptake efficiency and shoot Pb concentration and uptake. Although addition of HA significantly increased mean phytoextraction and translocation efficiencies, these responses were mainly attributed to the role of HA in the reduction of both root dry weight and Pb uptake, meaning that HA is not promising in Pb phytoremediation by maize. Among the four plant growth regulators evaluated, salicylic acid was the most effective in enhancing phytoremediation through phytostabilization of Pb.