Sensitivity Of Main Broadleaf Weeds To Glyphosate In Maize Fields And The Mechanism Of Tolerance To Glyphosate

Maize?Zea mays L.?is an important food,feed and industrial crop,which plays an important role in agricultural production and national economy in our country.The common broad-leaved weeds including Xanthium sibiricum,Amaranthus retroflexus,Portulaca oleracea,Chenopodium album and Abutilon theophrasti,are the five main weed species in our two major maize producing areas of Northern China and Huanghuaihai.And their occurrence seriously affected maize growth and reduced yield in maize fields.Glyphosate is a broad-spectrum,non-selective systemic herbicide that can control a variety of annual weeds.In our study the whole plant bioassay was used to determine the sensitivity of these five common broadleaf weeds to glyphosate.Each of these weed species were separately collected from two major corn producing areas of Northern China and Huang Huaihai.In our paper,three populations of A.theophrasti with different glyphosate levels were selected to study the tolerant mechanism to glyphosate.The physiological and biochemical changes were determined by examining the accumulation of shikimaic acid and chlorophll fluorescence parameters.And the differences in EPSPS gene expression among three abutilon populations were researched at the molecular level.The main results were presented as following:The GR₅₀ values of all populations of A.retroflexus,X.sibiricum and P.oleracea were lower than900 g a.i.ha^-1?recommended dose of glyphosate application?,and their GR₅₀0 average values of these three weed species were lower than 450 g a.i.ha^-1?1/2X recommended dose of glyphosate application?,respectively,indicating that these three weed species were sensitive to glyphosate.94%of C.album populations that their GR₅₀0 values were less than 450 g a.i.ha^-1,showed that the vast majority of C.album populations were sensitive to glyphosate.However,only one population of C.album found that its GR₅₀value was more than 900 g a.i.ha^-1,indicated that it had a certain tolerance to glyphosate.GR₅₀ values of72%populations'GR₅₀ values that were higher than 900 g a.i.ha^-1,indicating that the vast majority of velvetleaf populations had tolerances to glyphosate.Three velvetleaf populations?17-3,18-16 and 18-22?with different glyphosate tolerances were selected to research the tolerant mechanism to glyphosate,of which GR₅₀ values were 1730.45,965.01and 506.16 g a.i.ha^-1,respectively.they had separately strong tolerance,tolerance and intolerance to glyphosate.After treated with 900.00 g a.i.ha^-1?X dosage?glyphosate,the accumulation of shikimic acid in 18-22 population increased rapidly after glyphosate application,and reaching the maximum value of 222.01?g/g on the 5 d,indicating that shikimic acid pathyway was greatly affected,and its Pm,Y?II?and Y?I?values were significantly lower than those of the control groups,indicating that the photosynthesis was inhibited.The content of shikimic acid in the populations of 18-16 was stable after X dosage of glyphosate treatment,which remained at 36.80?53.53?g/g,slightly higher than their control groups,indicating shikimic acid pathyway was slightly inhibited.In 18-16,the Pm value increased first,and reached the highest value of 2.03 on the 8 d,which was significantly higher than that of the control group.The Fv/Fm,Y?II?and Y?I?values were merely higher than those of the control group.The content of shikimic acid in the populations of 17-3 was stable after X dosage of glyphosate treatment,which remained at 30.97?86.07?g/g respectively,slightly higher than their control groups,indicating shikimic acid pathyway was slightly inhibited.In 17-3,the Pm value firstly increased,then reached the maximum value of 2.13 on the 8 d,which was significantly higher than that of the control group,the values of Y?II?and Y?I?in 17-3 increased first and then tended to stable at 4 d,the maximum values of Y?II?and Y?I?was 0.13 and 0.33,respectively at 4 d,which were significantly higher than these of the control groups.The Fv/Fm values were sightly higher than those of the control group,indicating that the photosynthesis was obviously promoted.After spraying 4X dosage of glyphosate,the accumulation of shikimic acid in 17-3,18-16 and 18-22 increased rapidly,the chlorophyll fluorescence parameters of velvetleaf populations 17-3,18-16 and 18-22,all decreased rapidly,indicating that the shikimic acid pathyway and photosynthesis were obviously inhibited.The cDNA sequence and expression level analysis of EPSPS gene were performed using alignment and the qPCR.The 695 bp cDNA sequence of EPSPS gene among three velvetleaf populations,which included two conserved regions of EPSPS amino acids?90?110 and 175?200 sites?,there were no differences in cDNA sequence and amino acid sequence found in these two conserved regions.That result suggested that the differences in glyphosate tolerance among the three velvetleaf populations may not be caused by the differences in gene sequence.After treated with X dose of glyphosate,there was no proportional between their GR₅₀ and the maximum value of EPSPE expression level.After treated with4X dosage of glyphosate,the peak expression levels of EPSPS in 17-3,18-16 and 18-22 appeared at 6 h was 3.30,4.24 and 8.71,respectively,which was significantly different.And the highest expression levels of EPSPS gene among three velvetleaf were negatively correlated with their GR₅₀ value.