Study On Environmental Behavior Of Three Strobilurin Fungicides

Strobilurin fungicides are a milestone as another class of agricultural fungicides for its unique mechanism,good environmental compatibility and actual application to improve crop yield and quality after benzimidazole and triazole were used.As the market share of strobilurin fungicides gradually grows,the influence of the fungicides on environment becomes an inevitable problem.In this paper,azoxystrobin,kresoxim-methyl and phenamacril are the research objects,their environmental characteristics and influencing factors of water,soil and water sediment system were studied.Tetrahymena thermophila,was used as a model to evaluate toxic effects of phenamacril and explore molecular mechanisms.The results will provide theoretical basis of proper using and environmental safety assessment of strobilurin fungicides.The studies were as follows:Hydrolysis of three strobilurin fungicides was studied under different pH and temperature conditions.The results showed that kresoxim-methyl is a easy hydrolysed,azoxystrobin and phenamacril are relatively difficult hydrolysed pesticides.Hydrolysis rate of three strobilurin fungicides increased with the increasing of temperature.Alkaline condition and higher temperature were beneficial for their hydrolysis.Under neutral condition,the absolute value of the activation entropy of the hydrolysis of phenamacril increases as temperature rises,showing a significant correlation with temperature.The average activation entropy value of of the hydrolysis of phenamacril was-256.29 J·mol-1·K-1.Photolytic experiment results showed the degradation half-life of kresoxim-methyl and azoxystrobin were 1.04 h and 1.76 h,it indicated that they were easily photolytic pesticides.The photolysis half-life of phenamacril was 17.8 h,indicating that phenamacril is hardly photolyzable.The result showed that the photolysis rates was irrelevant to the polarity of solvent.In methanol,the photolysis was influenced by photo sensitization.However,the photolysis was affected by photo-quenching effect on acetone.Humic acid had photo-quenching effect on the photolysis of phenamacril,and the effect was positively correlated with the concentration of humic acid.The photolysis of phenamacril in water was promoted by H2O2.The photolysis rate of phenamacril was positively correlated with the concentration of H2O2.Especially when the concentration of the added H2O2 was 8.0 mmol·L-1,the photolysis rate of phenamacril was 1.31 times faster than under light without H2O2.Using Quadrupole-Time of Flight Mass Spectrometer(UPLC/Q-TOF-MS/MS)to analysis the photolysis products of three strobilurin fungicides under UV light.The degradation products of kresoxim-methyl are mainly KP1 and KP2.Azoxystrobin mainly has six hydrolysis products and twelve photolysis products.The degradation products of phenamacril are API,AP2 and KP3.The Pathways such as dealkylation,oxime ether cleavage,hydroxylation and photohydrolysis reaction.The degradation of three strobilurin fungicides in soil was investigated.The photolysis of three strobilurin fungicides on soil surface was degraded difficultly,the order of photolysis intensity for three strobilurin fungicides was kresoxim-methyl>azoxystrobin>phenamacril.The degradation rate of three pesticides was faster under mercury light source than that under xenon light source.The photolysis of pesticides in water is much faster than in soil surface,which shows that the intensity of light source and the penetration force directly influence the photolysis rate.The in order of degradation rate of kresoxim-methyl in soil was Jiangxi red soil>Taihu paddy soil>northeast black soil,the in order of azoxystrobin was Taihu paddy soil>Northeast black soil>Jiangxi red soil,the in order of phenamacril was northeast black soil>Taihu paddy soil>Jiangxi red soil.As a whole,the degradation of kresoxim-methyl in three kinds of soil was faster,the hydrolysis may be the main degradation way in soil.The degradation of azoxystrobin and phenamacril were mainly affected by soil physical properties,the high organic matter content,alkalinity of northeast black soil and Taihu paddy soil were more favorable for degradation.Using UPLC/Q-TOF-MS/MS to analysis the degradation products of three strobilurin fungicides in Jiangxi red soil.It is concluded that there are six products are belonged to kresoxim-methy.Azoxystrobin mainly has four degradation products,including AP3,AP4,AP7 and so on.Phenamacril has seven degradation products.The pathways such as dealkylation,hydrolysis and oxidation reaction.The degradation half-lives for strobilurin fungicides in water-sediment system are shorter than one month both under aerobic and anaerobic conditions at normal temperature,indicating that they are easily degradable pesticides.The degradation of strobilurin fungicides under anaerobic condition was more easier than under aerobic condition.Meanwhile,it was more beneficial to the degradation of strobilurin fungicides as there is a higher content of organic matter in sediment.The sorption/desorption and mobility of strobilurin fungicides in three Chinese soils(Jiangxi red soil,Taihu paddy soil,and Northeast China black soil)were conducted using comprehensively analytic approaches including batch experiment and soil thin-layer chromatography.The strobilurin fungicides were hard to be adsorbed in Jiangxi red soil but had medium adsorption capability inTanhu paddy soil and Northeast China black soil,while the desorption of three strobilurin fungicides ranked in the order of Jiangxi red soil>Taihu paddy soil>Northeast China black soil.Soil properties including soil organic matter(SOM),pH,and cationic exchange capacity(CEC)affected the adsorption/desorption of the fungicides.AG of three the strobilurin fungicides in different soils showed negative values,indicating that their adsorption belonged to physical adsorption.The GUS values of three strobilurin fungicides were less than 1.8 in Taihu paddy soil and Northeast China black soil.The results of soil thin layer test show that the mobility of kresoxim-methyl in soil is poor.In Jiangxi red soil,the GUS values of azoxystrobin and phenamacril were 3.13 and 2.63,respectively.Neither azoxystrobin nor phenamacril in the three soils is is not easy to move.Due to their weak mobility in soils,these strobilurin fungicides tended to remain in the soil phase but not to shift downward to underground water.As azoxystrobin and phenamacril had a long retention period in soil,there may become persistent residues in the soil environment.Desorption of three strobilurin fungicides in different soils was performed.The results indicated that three strobilurin fungicides were hard to be desorbed in Northeast China black soil and Taihu paddy soil,but desorbed relatively easy in Jiangxi red soil.Finally,tetrahymena thermophila,was used as a model to evaluate toxic effects of phenamacril and explore molecular mechanisms by integrating phenotypic observation,RNA-Seq and transmission electron microscopy(TEM)Imaging technologies.Exposure to 0.25,2.5 or 25 ?M phenamacril for 5 days no significantly decreased the relative biomass by reducing number of cells,size of cells and quantity of cilia in a dose-dependent manner.It was found that expression of 1571 genes was differed in Tetrahymena thermophila when treated by 25 ?M phenamacril,which were enriched in 87 GO term,and in which biometablism,bioregulation,biosynthesis of cella components and degradation.It is suggested that the toxicity of phenamacril tetrahymena thermophila results from the inhibition on protein biosynthesis activity,and synthesis of nitrogen compounds.In summary,when exposed to phenamacril,significant changes were found in gene expression of tetrahtmena thermophila,in which genomic regulation and metabolism pathway were involved.At the same time,the up-regulating of ABC-2 transporter protein family suggests that the harm caused by phenamacril could be reduced by up-regulating of efflux pump gene to increasing the effluxing of phenamacril in tetrahymena thermophila.