Keywords: 
• siduron /
• heavy metal /
• wheat /
• cucumber /
• root elongation /
• toxic effect /
• combined pollution

Abstract: In order to reveal the joint toxicity of typical lawn herbicide siduron and heavy metals on terrestrial plants, wheat and cucumber were exposed to single and combined treatment of siduron and four heavy metals. The germination rates, biomass and the elongation of root and shoot of tested plants were detected. The predicted no effect concentration (PNEC_soil) for siduron in soil was calculated using assessment factors. Results showed that siduron and heavy metals had significant single and joint effects on root elongation of both plants and shoot elongation of wheat (P<0.01). Apparent does-effect relationships were demonstrated. Cucumber was more sensitive to siduron than wheat. RI₅₀ of cucumber in root elongation was 0.281 mg·L^-1. Wheat was more sensitive to Cu, Cd, Pb than cucumber. When we combined siduron and those four heavy metals, cucumber root elongation was the most sensitive parameter, which thus could be selected as biomarker. Siduron and those four heavy metals had a synergic effects on the inhibition of wheat and cucumber root elongation. Compared to single treatment of siduron, root elongation of both plants were more sensitive to siduron with the addition of heavy metals. Joint toxicity of siduron and those four heavy metals to wheat shoot depended more on individual heavy metal and exposing concentration. PNEC of siduron in the soil (PNEC_soil ) derived from the acute toxicity data of cucumber root elongation inhibition using equilibrium partitioning was about 1.90 μg·kg^-1, which was far below the recommended application dose of 1.5-9 mg·kg^-1 siduron in field. When combined with heavy metals, the PNEC_soil of siduron significantly decreased, which may lead to increased ecological risk. Results presented in this study can provide basic data for ecological risk assessment of combined pollution of siduron and heavy metals.