Objective  Diversity and characteristics of Pyrus pyrifolia Nakai were studied according to the stone cell (SC) contents of the germplasms to establish a grading system to facilitate analyzing and classifying the resource.

  Methods   SC content of the sand pear varieties was determined after freezing. A grading system was established based on the genetic diversity of the germplasms on SC content.

  Results   The 480 germplasms under study showed a positive skewness statistic distribution on their SC contents, which ranged from 0.043% to 2.748% with an average of 0.706%, a variation coefficient (VC) of 61.11%, and a genetic diversity index (GDI) of 1.95. The average SC content of the local varieties was 0.886% with a VC of 48.82% and a GDI of 1.98, that of the foreign cultivars 0.440% with a VC of 48.79% and a GDI of 2.00, and that of the selected cultivars (lines) for breeding 0.400% with a VC of 47.02% and a GDI of 2.01. On genetic diversity, among the local cultivars the germplasms from Yunnan Province had the highest SC content of 1.239% and VC of 51.07%; those from Sichuan, the largest GDI of 1.90; those from Zhejiang, the lowest SC of 0.595%; and those from Guangdong, the smallest VC of 14.02% and GDI of 0.95. Of the selected cultivars for breeding, those from eastern China showed the lowest SC of 0.372%, while those from the central region the lowest VC of 43.41% and the highest GDI of 2.04.

  Conclusion  The sand pear varieties investigated under this study exhibited a rich genetic diversity on their SC contents. Of all 480 germplasms, the local varieties had the highest SC content of 0.886% and the selected cultivars for breeding the greatest GDI of 2.01. To streamline the resource classification, 7 grades based on the SC content in the germplasms were established as SC＜0.1%, 0.1%≤SC＜0.2%, 0.2%≤SC＜0.4%, 0.4%≤SC＜0.7%, 0.7%≤SC＜1.1%, 1.1%≤SC＜1.6%, and SC≥1.6%. Their respective descriptive terms were scarce, low, medium low, moderate, moderately high, high, and extremely high on SC content.