| The Yarlung Zangbo River,also known as Yangchabu Zangbo,is a large plateau river and one of the most important international rivers in China,also is the main source of fresh water and water vapor in Tibet.In recent years,the desertification in the Yarlung Zangbo River basin has become more serious due to human activities and also climate change,affecting the ecological balance of the basin and the development of local animal husbandry.Pennisetum centrasiaticum is a perennial grass species widely distributed in the Yarlung Zangbo River basin,possessed high nutritional value and ecological value.In this study,by investigating the characteristics of P.centrasiaticum community in the main distribution areas of the middle and upper reaches of the Yarlung Zangbo River,carrying out phenotypic characteristics analysis and germplasm screening of P.centrasiaticum,studying on the biomass distribution characteristics of P.centrasiaticum in reproductive growth period,analyzing chloroplast genome sequence and phylogenetic research of the P.centrasiaticum,our results showed that:(1)119 plant species were recorded in the distribution area of P.centrasiaticum,which belonged to 35 families and 96 genera,mainly included Compositae,Gramineae and Leguminosae.The species richness increased with altitude,which ranged from 12(Sangri county)to 37(Gyantse county).The percentage of the underground biomass of26 P.centrasiaticum communities in the soil layers of 0-10 cm,10-20 cm and 20-30 cm in P.centrasiaticum communities were 60.38%,26.36% and 13.26%,decreased with the deepening of the soil layer.Soil nutrient factors and precipitation have an impact on the community structure of P.centrasiaticum.The soil water content and soil p H increased with the deepening of the soil layer,while the contents of soil organic matter,total nitrogen,total phosphorus,available nitrogen and available phosphorus decreased with the deepening of the soil layer.The proportion of underground biomass in the community increased with altitude,while the height of functional groups decreased with altitude.The functional group of Gramineae possessed the largest height,coverage and importance value at different altitude gradients.(2)The phenotypic traits of germplasms collected from P.centrasiaticum distribution area of the Yarlung Zangbo River showed a strong correlation with environmental factors.The ratio of the last internode,fresh to dry ratio,coverage,crude protein content and crude fat content of P.centrasiaticum increased with the altitude,while the rest of the traits decreased with altitude,indicating that the phenotypic traits of P.centrasiaticum were sensitive to environment,and P.centrasiaticum has madeadaptive changes to complex environmental factors.P.centrasiaticum possessed high crude protein content and high ash content,made the grass be a valuable forage.However,the grass also possessed high crude fiber content,made the stalk relatively rough and less palatable.The germplasms of P.centrasiaticum collected from Mainling County,Sakya County,Gonggar County,Danang County and Nyemo County screened by TOPSIS analysis method could be used in the next step of domestication and breeding.(3)The research on the biomass distribution in the reproductive growth period of P.centrasiaticum shows that: plant height,stem diameter,the ratio of litter,stem,leaf sheath,ear,reproductive branches and aboveground vegetative organ of P.centrasiaticum showed a "∩" type change with the growth process;the ratio of root decreased from 65.42% at the booting stage to 45.10% at the filling stage,and then significantly increased to 63.22% at the maturity stage,showing an "∪" type change.With the stem to leaf ratio was 2.16,and the ratio of root was only 45.1% in the filling stage,P.centrasiaticum possessed the highest value with the largest aboveground biomass percentage of 54.90%.(4)With the help of the high-throughput sequencing platform,26 complete chloroplast genome sequences of P.centrasiaticum in this study were obtained.In the sequencing results of 26 germplasms,the coverage was 193.53,the genome length was138295.81 bp,and the average GC content was 38.61%.135 genes(88 protein-coding genes,39 t RNA genes,and 8 r RNA genes)were annotated in the chloroplast genome of P.centrasiaticum in Nedong,136 genes(87 protein-coding genes,41 t RNA genes,and 8 r RNA genes)were annotated in P.centrasiaticum collected from the other 25 sample sites.The results of sequence alignment analysis showed that P.centrasiaticum chloroplast genomes collected from different distribution area of the Yarlung Zangbo River possessed high similarity,but there were also some highly differentiated regions and highly variable genes,which could be used for further development of candidate DNA barcodes and universal primers.The LSC/IR and SSC/IR boundaries of different P.centrasiaticum were highly similar,but there were also expansion and contraction phenomena.The codons of different P.centrasiaticum chloroplast genomes all prefer to end with A/T.There were 30 codons with RSCU value > 1 in the chloroplast genome of P.centrasiaticum collected in Nedong District.Except for the same 29 codons as those of P.centrasiaticum in the other 25 collection sites,there was an extra codon TCA encoding serine.P.centrasiaticum collected in Nedong District has a preference for using serine to encode proteins.Based on the complete chloroplast genome sequence and chloroplast gene sequence(mat K,rpl16),phylogenetic trees were constructed by bioinformatics technology to obtain the phylogenetic relationship of Trib.Paniceae species,Subtrib.Cenchrinae species and plants with different photosynthetic characteristics. |
