Inducing triploids and tetraploids with high temperatures in Populus sect. Tacamahaca.

KEY MESSAGE: This study is the first to report that triploids and tetraploids have been successfully produced through embryo sac and zygotic embryo chromosome doubling with high temperatures in P. simonii Carr. and its hybrid. A new synthetic polyploid induced by hybridization with unreduced gametes and heterozygotic embryo chromosome doubling can effectively combine polyploidy and heterosis, which can provide two major breeding advantages. In Populus, successfully creating and cultivating new polyploid varieties have economic and ecological production value. This was the first successful study in which embryo sac and zygotic embryo chromosome doubling was induced using high temperatures to produce triploids and tetraploids in Populus simonii Carr. and its hybrid, P. simonii × P. nigra var. Italica, of Populus sect. Tacamahaca. The relationship between flower bud morphological characteristics (time after pollination) and female meiotic stage (embryo sac and zygotic embryo development) was established to guide the induction treatment period. In the resulting progeny, 37 triploids and 12 tetraploids were obtained and identified using flow cytometry. The optimal temperatures for embryo sac and zygotic embryo chromosome doubling were 38 and 41 °C, respectively. Cytogenetic analysis revealed that 66-72 h after pollination (HAP), a period characterized by a high proportion of one-nucleate and two-nucleate embryo sacs, was the optimal period for embryo sac chromosome doubling. For zygotic embryo chromosome doubling, 168 HAP was the optimal induction period, as there was a high proportion of two-cell and four-cell proembryos. The results indicate that inducing embryo sac and zygotic embryo chromosome doubling is an ideal method for producing polyploids. The methods for inducing polyploids and for evaluating ploidy and offspring with different ploidies and heterozygosity in this study will be useful for genetic research and Populus breeding programmes.