Physiological Effects Of Glyphosate On Aquatic Plants And Mitigation Effect Of Phosphate

Glyphosate,as a broad-spectrum herbicide,is frequently detected in water,and phosphorus widely enters the water due to the extensive use of phosphorus-containing substances in agriculture,industries and daily life.Thus,aquatic ecosystems are exposed to both glyphosate and phosphorus,which may affect aquatic organisms.At present,there are few studies about effect of glyphosate on aquatic plants,and there is no relevant research on the joint effects of phosphate and glyphosate on aquatic plants.We conducted an experiment to explore the ecological sensitivity of Hydrilla verticillata(L.f.)Royle and Vallisneria natans(Lour.)Hara to glyphosate.Our research focused on the physiological responses of H.verticillata and V.natans after exposure to various concentrations of glyphosate(0,1,10,20,30,40,50 and 80 mg/L)in hydroponic culture after one day(1D)and seven days(7D).We also studied the physiological responses of the floating aquatic plant species Hydrocharis dubia(Bl.)Backer to different concentrations of glyphosate(0,1,5,15 mg/L)with different levels of phosphate(0,50,100 mg/L)after 14 days(D)of treatment.We explored glyphosate toxicity in H.dubia and investigated whether phosphate addition mitigates glyphosate toxicity in this species,which will provide a theoretical basis for the ecotoxicological study of aquatic plants.Our results show that:(1)after 1D,the soluble protein content of H.verticillata was significantly stimulated under low herbicide concentrations.Other indices for H.verticillata and V.natans had no remarkable changes at 1D.After 7D of treatment,the soluble protein content of H.verticillata showed no significant differences,while the malondialdehyde(MDA),pigment contents and catalase(CAT)activity significantly increased at low glyphosate concentrations.Guaiacol peroxidase(POD)activity in H.verticillata significantly increased with increasing herbicide concentrations.The chlorophyll a/b ratio of H.verticillata sharply decreased above 10 mg/L.For V.natans,soluble protein,chlorophyll a,and carotenoid content;and CAT activity declined significantly after glyphosate application,while other indicators showed no significant changes.(2)After 14D of treatment,treatments of glyphosate doses>5 mg/L significantly reduced the leave number of H.dubia.However,the root length was only significantly inhibited under the treatment of 5 mg/L glyphosate and there was no significant inhibitory effect on fresh weight and petiole length.Treatments of glyphosate doses ? 5 mg/L could significantly reduce chlorophyll content,but it had no significant effect on carotenoids and chlorophyll a/b.It also significantly increased H2O2,shikimate,proline and soluble protein content and MDA increased significantly only at 5 mg/L glyphosate treatment.Superoxide dismutase(SOD)and ascorbate peroxidase(APX)were significantly higher than the control under all treatments.CAT was significantly higher than control under treatments of glyphosate doses<5 mg/L,and POD significantly increased under treatments of glyphosate doses>5 mg/L.Polyphenol oxidase(PPO)only significantly increased under 5mg/L glyphosate treatment.After phosphate(50-100 mg/L)was added,compared with the single glyphosate treatment,the phosphate had no significant effect on the growth index of H.dubia,and 50 mg/L phosphate significantly increased the chlorophyll a,b and total chlorophyll content of 1 mg/L glyphosate-treated plants.However,100 mg/L phosphate had no significant effect on pigment content.The addition of phosphate maintained the MDA,shikimate,soluble protein content and CAT activity in glyphosate-treated plants at the control level,while it significantly reduced the H2O2 and proline content in>5 mg/L glyphosate-treated H.dubia,and significantly reduced POD,APX and PPO in 5 mg/L glyphosate-treated H.dubia.Moreover,100 mg/L of phosphate significantly reduced the SOD activity of H.dubia under all glyphosate treatments.In sumnary,glyphosate concentrations from 0 to 80 mg/L can induce oxidative stress in H.verticillata and may impede metabolism processes for protein and pigments without causing oxidative stress in V.natans.Taken together,our results suggest that the sensitivity of H.verticillata to glyphosate exposure is higher than that of V.natans.Glyphosate(<15 mg/L)can cause oxidative stress in H.dubia,which has a harmful effect on the growth of H.dubia.Phosphate can alleviate the oxidative stress response induced by glyphosate in H.dubia(reduce membrane lipid peroxidation,decrease enzyme activity,H2O2,proline and soluble protein content).Phosphate can alleviate the toxicity effect caused by glyphosate in H.dubia.