Intergeneric hybridization and unilateral compatibility caused by pollen tube growth in Epidendrum (Epdendrideae)

Haruka Kondo is a researcher of the Laboratory of Floriculture & Ornamentals, Chiba University, Chiba, Japan. His research focuses on orchid breeding using intergeneric hybridization. Many intergeneric hybrids of Epidendrum between other Epidendroideae, such as genus Cattleya, Barkeria and Brassavola, have been artificially bred. However, intergeneric hybrids or later generations are usually partially or completely sterile, which makes further breeding programs difficult. This study will obtain knowledge of the success or failure of intergeneric crosses between Epidendrum and related genera with the aim of making new cultivars. Furthermore, Haruka focused on the unique orchid reproductive system, namely ovule development and fruit formation occurring during pollen tube growth. In the cross using Epidendrum as a pollen parent, high percentage of fruit set was observed in all cross combinations and plantlets were obtained from some. In the cross using Epidendrum as an ovule parent, the flower abscission occurred within 2-3 weeks in all cross combinations. It was found to be a unilateral incompatibility since Epidendrum does not form fruit in the crosses using Epidendrum as ovule parent. There are three possible causes of unilateral cross incompatibility: 1) inhibition of pollen germination or pollen tube stop in ovary, 2) pollen tube growth of related genera is slower than that of Epidendrum, 3) failure of fruit set, and ovule development caused by hormonal amount (e.g., insufficient amount of auxin, ethylene). Pollen germination and pollen tube growth of related genera was inhibited in the ovary of Epidendrum although the pollen tube growth in the self-pollination of related genera was equal to that of self-pollination of Epidendrum. Experiment of auxin treatment showed that all flowers aborted by 30 days after pollination although fruits swelled and days to flower abortion was extended. Therefore, Haruka concluded non-reciprocal intergeneric hybridization barrier is caused by pollen germination and pollen tube growth in the present study. A possible solution is stigmatic and cut-style pollination, to overcome these barriers.

Haruka Kondo won the ISHS Young Minds Award for the best poster presentation at the IV International Orchid Symposium in China in December 2022.

Haruka Kondo, Chiba University, Graduate School of Horticulture, 648 Matsudo, Matsudo city, 271-8510 Chiba, Japan, e-mail: harunohangetsu@gmail.com

The article is available in Chronica Horticulturae