To explore the effect of exogenous plant hormone spraying on the absorption of heavy metals by hyperaccumulated plants, Bidens pilosa L. was selected as the tested plant owing to the large biomass, short growth cycle, and high accumulation efficiency. Here, the effect of foliar spraying 6-benzylaminopurine (6-BA), salicylic acid (SA), and 24-epi-brassinosteroid (24-EBR) on the remediation of cadmium (Cd)-contaminated soil by B. pilosa L. was examined. The results showed:① the efficiency of the remediation in Cd-contaminated soil by B. pilosa L. was effectively enhanced after the spraying of all three kinds of exogenous plant hormones with appropriate concentrations. The spraying of the three exogenous plant hormones could promote the cadmium concentration in the leaves of B. pilosa L. to increase by 4.21%, 31.79%, and 14.89%; promote the translocation factor (TF) to increase by 9.67%, 18.83%, and 17.85%; promote the phytoextraction rates (PR) to increase by 15.36%, 32.33%, and 64.38%, respectively. ② The growth of B. pilosa L. was significantly promoted after the spraying of the three kinds of exogenous plant hormones with appropriate concentrations. The spraying of the three exogenous plant hormones could promote plant growth under cadmium stress, and the dry weight of the plant root, stem, and leaf was increased by 37.53%, 74.50%, and 104.02%, respectively. ③ The photosynthesis of B. pilosa L. was significantly enhanced after the spraying of the three kinds of exogenous plant hormones with appropriate concentrations. The chlorophyll concentration of the plant was significantly increased after foliar spraying with plant hormones, and the concentration of chlorophyll a was increased by 79.31%, 92.27%, and 51.12%; the photochemical quenching coefficient (qP) was increased by 11.32%, 89.16%, and 78.43%; and the non-photochemical quenching coefficient (NPQ) was increased by 51.71%, 241.12%, and 27.85%, respectively, after foliar spraying with appropriate concentrations of 6-BA, SA, and 24-EBR. ④ The antioxidant capacity of B. pilosa L. was significantly strengthened after the spraying of the three kinds of exogenous plant hormones with appropriate concentrations. The malondialdehyde (MDA) concentration of the plant was reduced by 62.41%, 68.67%, and 46.76% after the application of 6-BA, SA, and 24-EBR, respectively. Meanwhile, superoxide dismutase (SOD) was increased by 68.33%, 10.28%, and 6.17%, and catalase (CAT) was increased by 31.43%, 37.87%, and 37.31%, respectively. Generally, the spraying of exogenous 6-BA, SA, and 24-EBR with the appropriate concentration under Cd stress could significantly increase the biomass of B. pilosa L. and promote the accumulation of heavy metals in the plant, improve the photosynthetic ability of the plant, reduce the oxidative damage of the plant under heavy metal stress, enhance the antioxidant capacity, and improve the absorption and tolerance of plants to Cd. It also could promote the transfer of Cd from roots to shoots, improve the phytoextraction rates of Cd from the plant, and effectively strengthen the phytoremediation efficiency. Among them, 30 mg·L-1 SA foliar spraying had the best effect.
Keywords: exogenous plant hormones; foliage spraying; oxidative stress; phytoremediation; translocation factor.