On Classification And Identification Of Xanthium Intercepted By Chinese Ports

Xanthium L.is a worldwide weed that pose a hazard to ecological environment and agricultural production in the world.The interspecific hybrid caused taxonomical problems among species or subspecies.Chinese ports have intercepted Xanthium fruits(bracts with the achenes)from different countries in recent years.Without clear classification,it is not easy to confirm identification of traits which affecting the work of the port inspection and quarantine departments,but is also disadvantageous to provide evidence for foreign trade negotiations and sanctions.In the present study,94 samples of Xanthium L.were collected to carry out their classification and identification.Six of them were from China and the others were intercepted from abroad by ports.In order to study the classification and identification of Xanthium L.,their morphological and DNA molecular data were obtained,and then they were classified by statistical methods.The main contents and results were as follows:1.Based on 17 morphological traits of 94 samples of Xanthium fruits,morphological variation characteristics and the correlations of those traits were analyzed.The largest variation coefficients was the distance between the top of the two beaks,which up to 63.59%,followed by spine number and density,their variation coefficients are 39.00% and 36.33%,respectively.And the coefficient of the smallest length of spine,the largest length of spine and the average length of spine varied from28.77% to 30.38%,while the variation of the length and diameter was the smallest,being 18.56%and 20.15%,respectively.2.The 94 samples belong to X.spinosum(three samles)and X.strumarium(91 samples).By using the clustering analysis and detrended correspondence analysis based on 17 morphological traits,91 samples of X.strumarium could be divided into two groups: Group 1 X.strumarium var.glabratum(DC.)Cronquist with smooth involucre including four samples(X.strumarium fo.Cylindricum,X.strumarium fo.Inflexum,X.strumarium fo.chinese and X.strumarium fo.occidentale)and Group 2 X.strumarium L.var.canadense(Mill.)Torr.& A.Gray with glabrous involucre including two samples(X.strumarium fo.pensylvanicum and X.strumarium fo.echinatum).The second group had obvious hairy surface on involucre.3.The mean genetic distance of ITS,trnH-psbA,matK and rbcL genes decreased successively in Xanthium L.The nuclear gene ITS was more suitable as the DNA barcode of the genus Xanthium L.than the chloroplast gene trnH-psbA.And the matK and the rbcL were without the ability to be the DNA barcode.It is indicating that the trnH-psbA sequence is not suitable for the taxonomy of Xanthium L.,because it combines the species of Ambrosia L.and the species of the genus Xanthium L.together,not effectively classifying the genus.The sequence rbcL and matK were good at the classification between genus,whereas the variations among species were not obvious.In order to recognize the different species of Xanthium L.,the two sequences must be combined with other DNA barcode sequence.4.Based on the molecular Neighbor-Joining trees built by ITS,trnH-psbA,matK and rbcL genes,X.strumarium and X.spinosum are two distinct species with good genetic background.X.strumarium were firstly divided into three groups,corresponding to X.strumarium var.glabratum(DC.)Cronquist,X.strumarium L.var.canadense(Mill.)Torr.& A.Gray and X.strumarium L.var.strumarium.5.According to morphological and molecular data of 94 Xanthium fruits,our studies confirmed X.strumarium and X.spinosum.We suggested that X.strumarium could be divided into three varieties corresponding to X.strumarium var.glabratum(DC.)Cronquist,X.strumarium L.var.canadense(Mill.)Torr.& A.Gray and X.strumarium L.var.strumarium.Whithin these three varieties exist some variation populations(forms),and many transitional individuals among these forms.The Xanthium materials intercepted by Chinese ports mainly belong to X.strumarium,a few belong to X.spinosum.Within X.strumarium existed three varieties including var.strumarium,var.glabratum and var.canadense,the latter two varieties account for most of the intercepted Xanthium material.We suggest that the materials of Xanthium strumarium intercepted in Chinese ports had better be divided three varieties.