Objective In poplar breeding, especially in Populus tomentosa, triploid breeding has emerged as an effective strategy for improving growth performance and forestry traits. This study aimed to investigate the technique of female gamete chromosome doubling in P. tomentosa using parental combinations with different combining abilities, and to select superior triploid offspring. The research provides important references and germplasm resources for hybridization and triploid breeding in P. tomentosa.

Method Based on previous studies, 11 hybrid parental combinations with high, intermediate, and low combining abilities were predicted and configured according to molecular genetic distances. The meiotic process of megaspore mother cells was identified in real-time based on correlations between morphological traits and female gametophyte development. The effective treatment period for embryo sac chromosome doubling was determined based on the optimal pollination time. Superior triploid germplasms were identified through clonal seedling comparative trials based on the Breckin multi-trait comprehensive evaluation method and t-test significance analysis.

Result (1) When the female inflorescences had basically emerged from the bud scales, the florets appeared pale yellow, the stigmas changed from light pink to pink, and the bifurcation angle reached 30°–40°, corresponding to the stage when most megaspore mother cells were at the terminal stage of meiosis, this period was identified as the optimal treatment stage for megaspore chromosome doubling. The effective treatment period for embryo sac chromosome doubling was approximately 24 hours after pollination. (2) A total of 1 765 triploids were obtained, including 1 517 from megaspore doubling and 248 from embryo sac doubling. (3) Hybrid triploids showed significantly greater height and diameter than diploids, with those from high combining ability combinations performing best. Specifically, seedling height and stem diameter of triploids from high combining ability combinations were approximately 15% and 5% higher than those from medium combining ability combinations, and 78% and 41% higher than those from low combining ability combinations, respectively. Thirty-eight superior triploid germplasms were selected from two high combining ability combinations.

Conclusion Morphological changes in female inflorescences and the optimal pollination period can serve as important indicators for determining the effective treatment stage for female gamete chromosome doubling in P. tomentosa. Based on this, combined with parental combinations showing high combining ability, a total of 38 triploid germplasms with outstanding growth performance were obtained through high-temperature-induced female gamete chromosome doubling. These results provide a theoretical basis and germplasm resources for the breeding and promotion of elite triploid varieties of P. tomentosa.