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Pteroceltis tatarinowii Maxim.(Ulmaceae),a tertiary relict plant of a temperate deciduous tree species endemic to China,is widely distributed in bare limestone mountains across the mainland in China.The bark(phloem fiber) of this plant has long since been used as the sole raw material for manufacturing Chinese traditional Xuan Paper.However,the species are subject to many threats due to its distribution pattern characterized by small patches;its decreasing population size resulted from overexploitation,and reduction of the original forest ecosystem.Thus,it has now been listed as a rare and endangered plant(National Grand III) in China. Using inter-simple sequence repeat markers,the genetic diversity and structure of 628 individuals from 27 populations of P.tatarinowii were detected.A total of 66 bands,of which 63 were polymorphic,were presented from the 8 selected primers screening across all samples,with the percentage of polymorphic bands up to 95.45%.The result of POPGENE revealed quite high level genetic diversity for the plant at the species level(PPB=95.45%,Ao=1.9545,Ae=1.5729,He=0.3335,I=0.4980).At the population level,TS population from Gansu harbored the highest genetic diversity(PPB=84.85%,Ao=1.8485,Ae=1.5217,He=0.3033,I=0.4516),whereas NL population from Guangdong with the lowest genetic variation(PPB=54.55%,Ao=1.5455,Ae=1.3135,He=0.1841,I=0.2756).The mean population genetic level(PPB=69.98%,Ao=1.6998,Ae=1.4449,He=0.2561,I=0.3793) and population genetic differentiation of P.tatarinowii(Gst=0.23,ФST=0.25) were both at the middle level compared to other species.Gene flow(Nm) was estimated to be 1.65.In addition,different genetic variation and patterns were found between North China and South China.Populations of North China presented higher genetic diversity(PPB=95.45%,He=0.3332,I=0.4964) and lower genetic differentiation(ФST=0.22) than those of South China(PPB=93.94%,He=0.3220,I=0.4814 and ФST=0.25).It would seem the extant genetic pattern of P.tatarinowii was might mainly attributed to its long evolutionary history,wide-ranging distribution,outcross mating system,long life cycle and complex differences of terrain and vegetation between North China and South China. According to our aforementioned results and the evidence from our cpDNA data,in situ conservation was the preferred way to maintain the species′ high level genetic diversity.Especially,much more attention should be paid to populations with higher genetic diversity(TS,JX and QY) and populations harboring peculiar cpDNA haplotypes(XA,SD,ML,NXS,LP,YF,WYS,GL,AL).In the condition of establishment of artificial plantation and germplasm bank,the above peculiar populations should been given prior consideration.Regarding the genetic pattern difference between South China and North China,more populations in South China and fewer representative populations with more individuals in North China should been sampled to obtain the utmost genetic diversity of P.tatarinowii.

In order to investigate high densities and planting methods of summer maize in north China,in 2010—2011,two varieties(Ludan 981,LD981,and Ludan 818,LD818) were selected and planted in two densities(6 0000 and 90000 plants/hm2) at two planting methods(single and double plants at one place).In this experiment,eight treatments were designed to study the effect of planting methods on root-shoot development and grain yield of summer maize under high densities.The results showed that vertical distribution of canopy resulted in a trend that dry matter weight increased with planting density increased;however,root vertical distribution resulted in a opposite trend.Under high density conditions,there were different competitive ability of root and shoot between different maize varieties.The canopy of LD981 was more sensitive to space,and its space competition of root was stronger than that of the canopy,hence,the yield limiting factor of LD981 was the kernel storage capacity;however,the root of LD818 was more sensitive to space,and its space competition of canopy was stronger than that of the root,hence,the yield limiting factor of LD818 was the spike number per unit.Under the condition of 60000 plants/hm2,the quality of canopy structure and function of LD981 was better,double plants at one place reduced competitive stress in the canopy,as a result,achieved root and shoot equilibrium.Hence,the population yield enhanced;under the condition of 90000 plants/hm2,the quality of canopy structure and function of LD818 was better,double plants at one place alleviated root competition,the quality of canopy structure and function improved partly,therefore,the root and shoot coordinated,which let to the yield increased.