2019 ASHS Annual Conference

Polyploidy – also known as whole genome duplication – has long been associated with changes to ornamental traits in plants. Interploidy hybridization which results in fertile progenies may generate new, desirable traits in established hydrangea cultivars or allow for the transfer of traits between hydrangea species. The objective of this study was to compare pollen tube development, seed set, and seed viability among intra- and interploidy pollinations of Hydrangea macrophylla. Three triploid (3n) Hydrangea macrophylla cultivars (Blaumeise, Kardinal, and Taube) and two diploid (2n) cultivars (Oakhill and Decatur Blue) were used to make a series of reciprocal crosses. There were significant associations between cross type and percentage of flowers with pollen tubes reaching the ovaries at 24 h (χ² = 30.6, p < 0.001) and 48 h (χ² = 26.5, p = 0.001) post-pollination. After 24 h, pollen tubes had reached the ovaries of 48.7% of 2n and 8.7% of 3n flowers, respectively. There was no difference in percentage of pollen tubes reaching the ovaries in diploid and triploid flowers at 72 h after pollination (χ² = 7.5, p = 0.60). Analysis of covariance showed that pollen tube length was significantly influenced by ploidy of the female parent and the flower length of the female parent. Crosses between two triploid parents produced an average of 24 seeds per fruit, while 2nx2n, 2nx3n, and 3nx2n crosses produced an average of 6.7, 5.2, and 6.7 seeds per fruit, respectively. Germination rate was highest in 2nx2n crosses (46%) followed by 3nx3n (6%), 3nx2n (1.7%), and 2nx3n (0%) crosses. All resultant progenies will be evaluated for genome size and ornamental characteristics. These variants produced via interploidy crossing will broaden the gene pool of cultivated Hydrangea macrophylla.