Single And Combined Toxicological Effects Of Mesotrione And Copper On Two Aquatic Plants

With the rapid development of industry and agriculture,a large number of herbicides and fungicides have been put into farmland.These chemical reagents will flow into rivers and lakes along with agricultural water,which will pose a great threat to aquatic ecosystem.Mesotrione is a newly herbicide in recent years.Copper is widely used in agriculture as the main component of various fungicides.The coexistence of mesotrione and copper in water are easy to cause compound pollution.However,the individual and combined toxicity of mesotrione and copper to aquatic plants is not clear.The compound pollution types of mesotrione and copper in aquatic ecosystems remains unknown.In this paper,we explorered the ecological sensitivity of Vallisneria natans（Lour.）Hara and Myriophyllum aquaticum（Vell.）Verdc to mesotrione and copper.The growth and physiological responses of two plants under different mesotrione concentrations（0,0.01,1,10,20 and 50mg/L）,different copper concentrations（0,0.1,0.3,0.5,1 and 2mg/L）and mesotrione+copper（0+0,0.01+0.1,1+0.3,10+0.5,20+1 and 50+2mg/L）after 14 days（d）were measured,respectively.The combined pollution types of mesotrione and copper in water were investigated.This study provided data basis for ecotoxicological study of aquatic plants and suggested the aquatic ecological risk of mesotrione and copper.The results showed that:（1）Under single stress,mesotrione and Cu²⁺had strong inhibitory effects on the photosynthetic pigment of V.natans.Pigment contents significantly decreased with the increase of single mesotrione and Cu²⁺doses.The nutrient accumulation of V.natans was blocked,and the soluble protein content decreased significantly.Both mesotrione and Cu²⁺stress could induce oxidative stress.V.natans activated antioxidant defense system.The superoxide dismutase（SOD）and catalase（CAT）were significantly changed but peroxidase（POD）and polyphenol oxidase（PPO）showed no significant changes at mesotrione exposure.SOD and POD activity increased but CAT activity was significantly inhibited at Cu²⁺exposure.The antioxidant defenses of V.natans were limited.When high concentrations pollution exceeded the oxidative defense capabilities of plants,malondialdehyde（MDA）accumulated and the leaves became yellow and rot.The combined toxic effects on V.natans were closely related to concentrations.When V.natans were exposed to low concentration of mesotrione+copper（1+0.3mg/L）,the antagonistic effects were showed.Under highest dose of mesotrione+copper（50+2mg/L）,the synergistic effects were showed.（2）The relative growth rate（RGR）and pigment contents decreased significantly at 0.01mg/L and 20mg/L single mesotrione exposure.However RGR and chlorophyll restored to the control level under intermediate concentrations（1 and 10mg/L）.MDA accumulation could be induced by single mesotrione and Cu²⁺stress.The antioxidant enzyme system was activated to maintain a relatively stable H₂O₂content in M.aquaticum.CAT and POD were the key enzymes in response to mesotrione stress and CAT was the key enzyme in response to copper exposure.In experiment concentrations,synergistic effects were showed at lowest concentration（0.01+0.1mg/L）and higher concentrations（20+1mg/L and 50+2mg/L）.But obvious antagonistic effects were showed at intermediate concentrations（1+0.3mg/L and 10+0.5mg/L）.Soluble protein decreased obviously in joint stress.the combined toxicity on M.aquaticum was mainly reflected in the inhibition of metabolism rather than the inhibition of chlorophyll.POD and PPO activity increased while CAT activity decreased under combined stress.Under combined stress,antioxidant enzyme system of M.aquaticum changed more significantly.（3）The toxic effects of mesotrione and copper on V.natans and M.aquaticum were different.In conclusion,M.aquaticum was more resistant to the combined stress.Therefore,different concentrations have different toxic effects on aquatic plants.Even the coexistence of mesotrione and copper at low concentrations may have significant effects on plants.It is necessary to guard against the toxicity to aquatic ecosystem when mesotrione and copper coexist.