Objective   Chromosome karyotype information is an important basis for plant genetics and breeding research. The inter-subsectional distant hybridization is a significant direction in modern tree peony breeding. Because of the high morphological similarity of chromosomes in Paeonia sect. Moutan, the karyotype analysis of distant hybrids requires a large number of high-quality chromosome preparations. Pretreatment is a key link affecting the quality of chromosome preparation. It is of great significance to obtain the accurate karyotype and study the genetic law of distant hybridization of tree peony by improving the quality and efficiency of chromosome preparation with the promoted pretreatment method.

  Method  In this study, we used the pistils from the young flower buds of Paeonia × lemoinei ‘High Noon’ to quantify the influence of different pretreatment solutions and pretreatment time on the degree of chromosome contraction and dispersion at 4 ℃, and to analyze the differences of karyotype under diverse pretreatment effects.

  Result   Pretreatment with p-dichlorobenzene or p-dichlorobenzene-α-bromonaphthalene could get excellent chromosome preparation. After being pretreated with p-dichlorobenzene for 36−72 h or p-dichlorobenzene-α-bromonaphthalene for 24−48 h at 4 °C, a large number of chromosomes with moderate contraction and good dispersion could be stably obtained. More karyotype information of P. × lemoinei ‘High Noon’ was obtained by a large number of high-quality chromosome preparations. The karyotype formula was 2n = 2x = 10 = 8m (2SAT) + 2st (2SAT), and the satellites were located on the short arm of chromosome 4, 8, 9 and 10, among which the satellites on the chromosome 9 and 10 were more easily observed than those on the chromosome 4 and 8.

  Conclusion   Improving the pretreatment effect could effectively promote the quality and efficiency of chromosome preparation of P. × lemoinei ‘High Noon’, and obtain more abundant and accurate karyotype information. The quantitative method used in this study could effectively evaluate the quality of chromosome preparation from the two aspects of chromosome contraction and dispersion. The selected pretreatment method is stable and efficient, which lays a favorable foundation for karyotype analysis and further genetic research of distant hybrid of tree peony.