Physiological and biochemical responses of Kandelia obovata leaves under stress of diethyl phthalate

Diethyl phthalates (DEP) widely use as plasticizers are ubiquitous in the environment, particularly in coastal areas such as mangrove wetlands. DEP are likely to undergo biodegradation and produce products that can potentially harm living organisms. However, little is known about the toxicity of DEP and their metabolites, especially in higher plants. The present study aims to evaluate the effects of DEP on Kandelia obovata, a dominant plant species in mangrove wetlands in South China. Healthy propagule of K. obovata were exposed to six concentrations of DEP, that is, 0.02, 0.2, 2, 5 and 10 mg/L, under hydroponic culture for six months. The same set-up but without any DEP was also prepared as the control. At the end of the exposure, the contents of chlorophyll (chl) and malondialdehyde (MDA), and activities of three anti-oxidative enzymes in leaves were determined. Results revealed that chl content, including chl a, chl b and chl a+chl b, increased with DEP concentrations, implying DEP would pose toxicity (or stress) to K. obovata seedlings, while the increasing chl content in leaf could counter-balance the toxic oxidative stress. The content of MDA and activities of catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) in low (0.02 and 0.2 mg/L) and medium DEP (2 and 5 mg/L) were also significantly higher than that in the control. However, these values decreased at high DEP concentration (10 mg/L). The maximum activity of the three anti-oxidative enzymes was found in treatments with medium DEP concentrations (2 and 5 mg/L). These findings indicated that DEP posed toxicity to K. obovata but seedlings could tolerate DEP stress through enhancing their chlorophyll content and anti-oxidative enzyme activities. Nevertheless, such defence mechanism may be collapsed if the concentration of DEP is too high. The present study is first time reporting the effects of DEP on mangrove plants and how leaves respond to DEP stress.