Safety And Phytotoxicity Mechanism Of Halosulfuron-methyl For Sugarcane And Intercrops

Halosulfuron-methyl（HSM）is a high efficiency,selective sulfonylurea herbicide（SU）,and lowtoxicity to mamals,for the control of sedge in sugarcane（Saccharum officinarum L.）,corn and other crops.Long-term and heavy application cause potential soil residues,even a small amount of SU residues may cause severe damage and potential production decrease to sensitive nontarget crops.Sugarcane usually interplants with soybean[Glycine max（L.）Merr.],peanut（Arachis hypogaea L.）,or maize（Zea mays L.）.Thus,it is very important to assess the safety of sugarcane and intercrops and study the phytotoxicity mechanism.In the present study,comparative growth inhibition and peroxidization injury among the most common 7 varieties of sugarcane in Guangxi province,intercrops peanut,tolerance C4 species maize and sensitive C3 species soybean were observed at subcellular,biochemical and physiological levels.This information is to provide a theoretical basis for SU herbicide phytotoxity safety assessment and guidelines for herbicide use.The main results were as follow:1.A HPLC-UV analysis method for HSM residues was developed,and dissipation dynamics,final residues and leaching characteristics of HSM in soil,sugarcane plant and stem were studied.The datas of dissipation dynamics and final residues of HSM in soil,sugarcane plant and stem showed HSM applied for pre-and post-emergence at recommended dose are safe for sugarcane,and the final residues were all below 0.01mg/L（Maximum Residue Limit）;2.Safety assessment of HSM for 7 main varieties of sugarcane,intercrops of soybean and peanut were studied.Compared to the control,there were no significant growth inhibition on sugarcane with post-emergence application of HSM at 3～4 leaves stages of sugarcane with recommended dose of 67.5 g a.i./ha HSM;While sugarcane stem height was inhibited after pre-emergence application of 67.5 g a.i./ha HSM.The order of 7 varieties of sugarcane for HSM sensitivity is Yuetang 60>Guitang 29>Guitang 31>Guitang 42>Guitang 40>Taiyou>Xintaitang 22（ROC22）,and ROC22 was the most tolerance variety;HSM was safe for sugarcane,especially ROC22.In the harvest time,HSM had no significant inhibition effect on the ROC22 sugarcane stem height and yield.3.Safety assessment of HSM for intercrops（soybean and peanut）were completed.（1）HSM had no effect on seed germination,while the growth and yield of soybean and peanut were significantly inhibitied by HSM.When the concentration of HSM residues in soil higher than 1 mg/kg,growth of peanut was totally suppressed by HSM,and the growth inhibition rate of soubean plant was 30%.So peanut was more sensitive to HSM than soybean.4.Physiological and biochemical responses of sensitive plants and the injury of growth inhibition,chlorisis under HSM stress were studied,and enzymatic and morphological methods for HSM phytotoxicity diagnosis were developed.（1）Acetolactate synthase（ALS）is a key enzyme in the biosynthesis of branched amino acids（BCAAs）.ALS activity in maize leaves was inhibited by HSM.Accordingly,BCAAs and free amino acid content were lower than the control;However,BCAAs and the total free amino acid content in soybean leaves unexpectedly increase with increasing concentrations of HSM;（2）Transiently interveinal yellow green showed in maize leaf,but reversed to the level of control by 14 DAT（days after treatment）,however,leaves of soybean were observed with ever-increasing chlorosis and vein reddening,which became heavy injury and even died by 14 DAT;（3）H₂O₂ accumulation in maize leaves did not result in peroxidation damage,whose MDA level was lower than that of the control;while high levels of H₂O₂ accumulation in leaves of soybean lead to high levels of MDA,ethylene and accumulation;（4）Starch granule accumulation in chloroplast and decrease of soluble protein content were observed in both maize and soybean leaves;（5）No significant change in SOD activities was observed in comparison to the untreated control for maize;HSM had two-phase effects on CAT and GST activities:low doses promote while at high doses it had negative effect on CAT and GST;HSM stimulated SOD,POD and GST activities and suppresses CAT activity for soybean;To sum up,HSM caused H₂O₂ accumulation,ethylene release increase and lipid peroxidation,while H₂O₂ at a low level stimulated the detoxification and antioxidase enzyme glutathione S-transferase activitiesin in the tolerant maize,which contributes to HSM tolerance.5.Identify the injury mechanism of growth inhibition and chlorsis.HSM induced BCAAs deficiency,lipid peroxidation and ethylene release contribute to growth inhibition and chlorisis of sugarcane.BCAAs deficiency directly caused growth inhibition of maize,while lipid peroxidation and ethylene release were the main reasons of soybean growth inhibition and chlorsis.6.Identify the injury mechanism of withered and die.Sensitive soybean HSM induced photorespiration,then salicylic acid tilt the dynamic balance between H₂O₂ and antioxidant enzymes,which contribute to irreversible peroxidation damage to sensitive C3 soybean,eventually wither and die.HSM induced BCAAs deficiencies,oxidation damage and ethylene release increase directly contribute to growth inhibition,which determine the tolerance threshold of soybean.When the irreversible BCAAs deficiencies and peroxidization injury at high concentrations of HSM exceeding tolerance threshold,injury could not be reversed and ultimately lead to growth inhibition,chlorosis,withered and die.The results above showed a complex series of physiological and biochemical responses associated with HSM stress,which provided basis for further studies on phytotoxicity mechanism.