The Absorption And Distribution Of Systemic Pesticides Used In The Corn Seed Treatment And Its Effects On The Physiological Responses Of Corn

Corn is the food crop with the widest planting area and the highest total yield in my country.In order to prevent and control corn diseases and insect pests,the method of coating treatment before sowing is mainly adopted at present.However,recent studies have found that the residues of commonly used corn seed coating agents such as neonicotinoid insecticides and triazole fungicides can cause potential harm to non-target organisms and plants themselves.In addition,current research shows that pesticides will affect the growth and development of the plant itself,but most of the research on corn seed coating agents stays at the physical and chemical reactions of the corn seedling stage,and the research on the effect of the entire growth and development period of corn is still unclear.To this end,this study adopted the field test method to select common seed coating agents for corn,Thiamethoxam and Difenoconazole,and analyzed the effects of these two systemic pesticide seed coating agents on corn plants.The residues in the growth and development of the parts,and at the same time clarify the impact of the different use of these two systemic pesticides(single application,compound application)on the corn pesticide residues.It also analyzes the effects of these pesticides on leaf protein,chlorophyll and antioxidant defense mechanisms and changes in the expression of some genes during the growth of corn plants,hoping to provide a theoretical basis for the safe and reasonable use of systemic pesticides in seed coating agents.The main findings are as follows:(1)In this study,corn was used as the research object,and 3 treatments were set according to the recommended seed coating concentration(6ml/kg),including no seed coating(CK),thiamethoxam seed coating(T)and thiamethoxam alone.The mixed application of promethazine and difenoconazole(TD)was conducted to determine the pesticide residues in various parts of corn plants at different stages through field experiments.To explore the residual distribution and persistence of thiamethoxam during the entire growth period of corn plants when thiamethoxam is applied alone and in combination with difenoconazole.The main results are as follows.The residual amount of the two systemic pesticides is 18.4%-46.4% lower than the theoretical value,and the pesticides are degraded during storage and seed dressing.The residual distribution of thiamethoxam and difenoconazole in corn plants is not uniform,and their distribution in plants is related to the growth and development of corn.No difenoconazole residues were detected in the stems and leaves of corn,and the residual time of difenoconazole was 47-64 days.It shows that difenoconazole has poor mobility in corn plants and has a long residual time.The distribution of thiamethoxam residues in various parts of the corn plant is different at different stages of maize.The migration ability of thiamethoxam from root to leaf in seedling stage is stronger than jointing stage to milk maturity stage.This shows that the ability of plants to absorb thiamethoxam is related to the concentration of thiamethoxam and plant growth.In addition,the experiment found that the residual time of thiamethoxam in each part of the corn is different.The residual time is as long as the leaf ≈ the root> the stem.The residual time of clothianidin is longer than that of thiamethoxam,and thiamethoxam is in each part of the corn plant.The remaining time of thiamethoxam is related to the concentration of thiamethoxam and the absorption capacity of thiamethoxam in various parts of the plant.During the growth of corn,clothianidin,a metabolite of thiamethoxam,was detected in both seed coating treatments,and clothianidin remained the longest in corn plants.Therefore,the degradation of thiamethoxam is the main factor in the detection of clothianidin in plants.No residues of the two systemic pesticides were detected in the ears.This shows that the seed coating agent is safer for corn ears at this test dose.(2)In order to explore the effect of seed coating agent application on the growth of corn plants,the growth indexes of corn plants during the whole growth period were monitored.The results showed that the effect of seed coating agent on the weight of corn plant roots,stems and leaves gradually decreased with the decrease of the pesticide content in each part of the plant,which indicated that the concentration of pesticide was the key factor affecting plant growth.In addition,it was found that the treatment of thiamethoxam had an inhibitory effect on the growth indicators of the corn seedling stage,and the mixed application of thiamethoxam and difenoconazole promoted the increase in plant height and the thinning of the stem thickness.Both treatments reduced the corn leaves.The trend of weight,stem weight,root weight,plant fresh weight and root-to-shoot ratio.However,the mixed application of thiamethoxam and difenoconazole compared with thiamethoxam alone,the fresh weight of the ears of the two groups of seed coating agents increased significantly,which promoted plant growth.Therefore,the application of seed coating agents has a negative impact on plant growth,but can increase ear weight.Because the application of seed coating agent may create a stress environment for plants.It shows that there is no significant difference in yield per mu between the treatment with seed coating agent and the treatment without seed coating agent.In summary,the seed coating agent treatment has a certain negative effect on plant growth,and the seed coating agent treatment has no significant effect on the corn yield.This result can be used as a basic information for the safe and reasonable application of the seed coating agent.(3)In order to further explore the effects of seed coating agents on the physiological indicators of maize plants,the content of antioxidant defense enzymes,chlorophyll,and soluble protein in maize leaves were continuously measured during the growth and development of maize.The results showed that the seed coating agent treatment had an impact on the content of these substances during the growth and reproductive growth of corn plants.Seed coating agent treatment increased the chlorophyll content of plants,and the mixed application of thiamethoxam and difenoconazole treatment increased the chlorophyll content more significantly than the single application of thiamethoxam,indicating that the seed coating agent may promote the photosynthesis of corn plants.The activities of CAT,SOD,and POD in the treatment of the two seed coating agents were higher than those in the control,and compared with the single application of thiamethoxam,the mixed application of pesticides increased the CAT and SOD during the reproductive growth of corn,but SOD,CAT and POD during the vegetative growth of the corn plant The activity of single thiamethoxam treatment is higher than that of mixed pesticide treatment.It shows that SOD,POD,CAT,GSH,soluble protein and chlorophyll all respond to the seed coating agent and regulate the metabolic process of cells under the stress of the seed coating agent.In summary,the mixed application of thiamethoxam and thiamethoxam all caused stress responses in corn plants,and the response mechanisms of the plants to the two application methods were different.(4)The relative expression of SOD3,SOD9,CAT1 and GST4 genes were determined by selecting maize leaves at the big bell mouth stage and milk maturity stage.It was found that the expression of SOD9 gene was down-regulated,and the SOD3 gene was up-regulated in the big bell-mouth stage when thiamethoxam was applied alone.In summary,thiamethoxam in corn may activate the expression of Cu/Zn-SOD related genes;CAT1 and GST4 genes may be involved in the antioxidant response mechanism of plants under pesticide stress.