Cultivar‐Specific Production of Unreduced Pollen in the Ornamental Bigleaf Hydrangea ( Hydrangea macrophylla ssp. macrophylla )

Unreduced gametes naturally promote polyploidization and are valuable in breeding polyploid offspring. Bigleaf hydrangea ( Hydrangea macrophylla ssp. macrophylla ), a major ornamental crop, has shown spontaneous polyploidization rates ranging from 0% to 94% in previous crosses. These rates depended on the male parent and indicated unreduced pollen formation. To identify cultivars producing unreduced pollen and assess the variation and stability of this trait, we determined the 2C and pollen DNA content from 81 cultivars using flow cytometry. Diploid and triploid cultivars were identified that produced exclusively unreduced pollen, whereas others produced a mixture of unreduced and reduced pollen in varying proportions or exclusively reduced pollen. Flow cytometry of pollen samples collected over multiple years, combined with offspring analyses from repeated crosses, confirmed the consistent production of cultivar‐specific pollen qualities, indicating strong genetic control. Thus, cultivars producing pollen with one, two or—in the case of triploids—three or aneuploid chromosome sets were identified, providing a basis for targeted polyploid breeding. Additionally, we provide pollen data of five H. macrophylla ssp. serrata and four H. paniculata cultivars.