Foliar application of humic acid decreased hazard of cadmium toxicity on the growth of Hybrid Pennisetum

Abstract

The positive effects of humic acid (HA) in alleviating the toxicity of cadmium (Cd) were demonstrated in pot grown Hybrid Pennisetum. Twelve treatments consisting of six concentrations of Cd (0, 10, 20, 50, 70 and 100 mg kg−1) combined with two levels of HA (0 and 1.0 g L−1) were applied. Our results showed that although Cd toxicity significantly inhibited plant growth, decreased the dry weights of leaves, stems and roots and substantially increased the leaf malondialdehyde (MDA) and Cd concentrations in different plant tissues, Hybrid Pennisetum had a strong ability in the uptake and transfer of Cd and could be used for phytoremediation in Cd-contaminated soils. The chlorophyll content and photosynthetic capacity of the leaves also decreased under Cd stress. The foliar application of HA alleviated the symptoms of Cd toxicity by increasing the net photosynthetic rate (Pn) and the content of photosynthetic pigments, although HA did not significantly decrease the concentration of Cd in the leaf, stem and root tissues. Compared with the zero-HA treatment, 1.0 g L−1 HA increased the root, stem and leaf biomasses by approximately 2.7–99.4%, 3.9–11.2% and 18.2–87.1%, respectively. MDA had a significant negative relationship with Pn. The MDA content of the HA-treated Hybrid Pennisetum plants decreased by approximately 4.4–12.2% compared with that of the Cd-only plants, indicating the high Cd tolerance of Hybrid Pennisetum. It is concluded that foliar application of HA (1.0 g L−1) can alleviate Cd toxicity and increase Cd stress tolerance in Hybrid Pennisetum, thereby lowering the potential risk of soil Cd contamination.