Objective   Based on phenotypic and physiological characters, the diversity and population structure of Populus deltoides germplasm resources were studied, which provided a theoretical basis for the scientific management, efficient use and effective protection of the resources.

  Method   The phenotypic and physiological characters of one-year-old plants of 258 clones from 27 sampling sites of 6 provenances of P. deltoides were measured. The differences among provenances were studied by variance analysis and multiple comparisons. By correlation analysis and principal component analysis, the correlation between characters was studied and the phenotypic and physiological characters of clones were comprehensively evaluated. The genetic structure of P. deltoides population was studied by cluster analysis.

  Result   The variation coefficients of 22 phenotypic and physiological traits of P. deltoides germplasm resources were between 0.56% and 53.48%, and the variation of stem and root biomass traits was large, the variation of chlorophyll fluorescence parameters (F_v/F_m) was small. The Shannon-Wiener index ranged from 1.844 to 2.097. The results of variance analysis showed that, except for the chlorophyll fluorescence parameters, there were significant differences in other 21 traits among individuals and between provenances (P < 0.01), and the coefficient of phenotypic differentiation (V_st) was between 1.37% and 31.40%. It was found that the phenotypic traits between provenances showed greater genetic variation than those of physiological traits. The results of correlation analysis showed that there were strong positive correlations among the plant height, ground diameter, root biomass, stem biomass, leaf biomass, net photosynthetic rate (P_n), leaf shape and leaf carbon and nitrogen content of P. deltoides, and the intercellular CO₂ concentration (C_i) and transpiration rate (T_r) were negatively correlated with plant growth traits. Five principal component factors were extracted by principal component analysis, and the cumulative contribution rate reached 80.51%. The evaluation model of phenotypic physiological characters was constructed, and the clones were divided into four grades: excellent, good, moderate and poor. Based on the square Euclidean distance between provenances, six provenances were divided into three categories: the clones of Missouri (Mis), Tennessee (Ten), and Louisiana (Lou) provenances located in the middle and lower reaches of the Mississippi River belong to the same group; the clones of the Iowa (Iow) provenance distributed in the upper Mississippi River and the Quebec (Que) provenances in the St. Lawrence River Basin were the same; the Washington (Was) provenance clones located in the Columbia River Basin in the northwest USA were divided into a separate class.

  Conclusion   The phenotypic and physiological characters of P. deltoides were rich in diversity, and the traits of plants between provenances and within provenances had different degrees of genetic variation, and the expression of phenotypic and physiological traits of P. deltoides clones was related to the distribution and climate type of provenances. The results of this study provide a scientific basis for the protection, management and utilization of P. deltoides resources, as well as the breeding and evaluation of excellent germplasm resources.