High-Temperature Induction of 2n Female Gametes to Produce Triploid Birches: Timing, Parameters, and Growth Outcomes

Triploids are typically formed through the fusion of a haploid gamete from a diploid organism and a diploid gamete from a tetraploid organism or through chromosome doubling in gametes by other means. To circumvent the multi-year flowering wait associated with tetraploid parents in conventional breeding, we developed a novel method for inducing triploid in birch through high-temperature treatment of female inflorescences. This approach integrates sexual hybridization with precise thermal treatment, with success hinging on the precise identification of the pollination window and the optimal treatment parameters. Our study systematically defines the optimal conditions for triploid production in birch via this high-temperature induction technique. The results demonstrate that the optimal period for stigma pollination was between day 5 to 6, immediately following the emergence of the stigma from the enclosing bracts. The most effective pollination was characterized by a bract dehiscence angle exceeding 60° on Day 15 after the pollination marks the phase of megaspore mother cell development. At this stage, the optimum treatment is either 40 °C for 2 h or alternately 42 °C for 1 h. These treatments result in the highest triploid induction rates of 33.82%, calculated with the total number of detected seedlings as the denominator. In addition, a logistic model was established between the ovary length-to-width ratio and the accumulated growing degree hours (GDH), providing a reliable quantitative indicator for determining the optimal timing of the high-temperature treatment. Compared with the conventional approach reliant on tetraploid parents, our method eliminates the lengthy phase of tetraploid induction and flowering wait (approximately 8 years), thereby reducing the triploid breeding cycle by about 6 years. The results substantiated the effectiveness of utilizing high temperatures to induce chromosome doubling in female gametes of birch species, providing a viable pathway for efficient polyploid breeding in this tree species.