Objective  To study the sampling proportion and to determine the sampling strategy on the basis of corresponding genetic analysis, analyzing the genetic diversity and constructing the core germplasm of P. tomentosa can provide molecular foundation for the introduction, genetic breeding, collection and preservation of Leuce germplasm resources. It also can provide reference for the core germplasm construction of other tree species.

  Method  Based on 16 pairs of fluorescent SSR primers, the genetic diversity of 272 accessions of P. tomentosa and poplar hybrid was analyzed by capillary electrophoresis. Based on expected heterozygosity data, all samples were ranked according to their contribution to the overall genetic diversity. The sampling ratios from top to low were 50%, 45%, 40%, 35%, 30%, 25%, 20% and 15%. The representativeness of core germplasm was analyzed by comparing the average effective allele number Ne, the average Shannon information index I and the average expected heterozygosity He, et al. Then the appropriate sampling proportion was determined.

  Result  With the decrease of sampling proportion, the values of Ne, I and He were all increasing, which were all greater than the corresponding values of the original germplasm. The values of He were all greater than 0.5, indicating abundant genetic diversity, while the values of the original germplasms were less than 0.5. According to 25% sampling ratio, the first 68 germplasms were obtained, including 18 hybrid germplasms and some excellent germplasms selected by all provinces. The Ne, I and He values were 2.761, 1.094 and 0.539, respectively, which were all higher than the corresponding values of the original germplasm 2.075, 0.825 and 0.432. It was showed by the t-test that there was no significant difference between the genetic diversity of the core germplasm and the original germplasm resources, indicating that these 68 germplasms have a very reliable representation in terms of genetic diversity and can be used as the core germplasm. The genetic consistency between Beijing germplasm and Hebei germplasm was 0.997, followed by Shanxi of 0.990.

  Conclusion  The best sampling proportion of core germplasm of P. tomentosa is 25%, and the best sampling range is 20%−40%. If the number of germplasm resources is large, it can be reduced to 15%; if the base is small, it can be increased to 45%. He, Ne, I, etc. all indicate that the above core germplasm has abundant genetic diversity. The genetic variation of hybrid germplasm is rich, which aggregates the good alleles of parents. It is proved at the molecular level that hybrid germplasm is an important breeding resource for genetic improvement of P. tomentosa. We suggest that relevant departments or breeders should attach great importance to the collection, preservation and reuse of poplar hybrid germplasm.