Antagonism or synergism? Responses of Hydrocharis dubia (Bl.) Backer to linear alkylbenzene sulfonate, naphthalene and their joint exposure.

The presence of surfactants may affect the bioavailability of polycyclic aromatic hydrocarbons. A hydroponic experiment was conducted to investigate the response of Hydrocharis dubia (Bl.) Backer to different concentrations of linear alkylbenzene sulfonate (LAS), naphthalene (NAP) and their mixture (0.5, 5, 10, and 20 mg/L) for 14 days and 28 days. The results showed that LAS had a greater toxic effect on H. dubia growth than NAP at treatment concentrations of 0.5-20 mg/L. The combined effect of LAS and NAP was damaging to H. dubia at concentrations of LAS + NAP ≥5 + 5 mg/L. When LAS + NAP ≥10 + 10 mg/L, the underground parts of H. dubia suffered more significant damage than the aboveground parts. Under the treatments with LAS, NAP and their mixture, H. dubia experienced oxidative stress. Soluble proteins and antioxidant enzymes were the main substances protecting H. dubia from LAS stress, and superoxide dismutase (SOD) and peroxidase (POD) were the main protective enzymes. When exposed to NAP, H. dubia growth was stimulated and promoted at the same time. In the short-term treatment (14 d), catalase (CAT) activity was sensitive to NAP stimulation, and soluble proteins and SOD were the main protective substances produced. Soluble sugars, SOD and ascorbate peroxidase (APX) played important protective roles during the longer exposure time (28 d). The physiological response of H. dubia exposed to the combined toxicants was weaker than the response to exposure to individual toxicants. The responses of SOD and CAT activity were positive in the short term (14 d), and these were the main protective enzymes. As the exposure time increased (28 d), the plant antioxidant system responded negatively.