Exogenous Salicylic Acid Alleviates Physiological Stress in Salix matsudana Seedlings and Increases 2,4-Dinitrophenol Removal

2,4-dinitrophenol (2,4-DNP) is a new kind of pollutant that is highly toxic and difficult to be biodegraded. In this study, the feasibility of using exogenous growth regulator salicylic acid (SA) to improve the purification ability of Salix matsudana (S. matsudana) seedlings to 2,4-DNP stress was investigated by a hydroponic simulation experiment. The main research results are as follows: (1) After adding exogenous SA, a high concentration of SA (1000 mg·L−1) inhibited the photosynthetic process and the normal physiological process of the chlorophyll fluorescence system of S. matsudana seedlings to a certain extent. When adding a low concentration of SA (10 mg·L−1) to treat S. matsudana seedlings, all exogenous treatment groups could alleviate the stress of 2,4-DNP on the photosynthetic system and chlorophyll fluorescence system of S. matsudana seedlings, and 10 mg·L−1 SA (DNP + S1) was the best. (2) The addition of exogenous SA could alleviate the damage of 2,4-DNP to S. matsudana seedlings by enhancing the activity of its antioxidant enzymes to remove excess reactive oxygen species (ROS) in the body and reducing the level of membrane lipid peroxidation and the size of membrane damage. The treatment with 10 mg·L−1 SA had the best effect. (3) Exogenous low concentration of SA (10 mg·L−1) could alleviate the decline of biomass index of S. matsudana seedlings under 2,4-DNP stress, but a high concentration of SA (1000 mg·L−1) could not alleviate the 2,4-DNP toxicity of S. matsudana seedlings leaves. Exogenous SA could effectively alleviate the growth damage caused by 2,4-DNP stress on S. matsudana seedlings and increase the tolerance threshold range of S. matsudana seedlings to 2,4-DNP (8.81–33.78 mg·L−1). (4) Exogenous addition of SA could increase the removal percentage of 2,4-DNP in Salix matsudana seedlings. Among them, the removal percentage of Salix matsudana was the highest at 10 mg·L−1 SA, which was 1.46 times (5 days) and 1.19 times (10 days) higher than that of the DNP treatment group, respectively. Overall, when SA reached 10 mg·L−1, the photosynthetic productivity of S. matsudana was the highest, and S. matsudana had the best purification effect on 2,4-DNP in wastewater.