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Abstract
Hydrangea macrophylla (Thunb.) Ser. and H. serrata (Thunb.) Ser. are popular and commercially important landscape and floriculture crops. Although both species are typically diploid, induced polyploids often exhibit horticulturally valuable traits. However, procedures for inducing polyploidy vary by species and often have low or inconsistent efficacy.

In this study, oryzalin and nitrotyrosine were investigated as in vitro mitotic inhibitors for inducing polyploidy in H. macrophylla ‘Robert’ and H. serrata ‘MAK20’. First, shoot apices of ‘MAK20’ were treated with 15 µm oryzalin for several treatment durations and the ploidy of shoots was determined after 8 weeks. Regression analysis showed that the proportion of polyploids (tetraploid + mixoploid shoots) increased with exposure duration. In a follow-up experiment, ‘MAK20’ and ‘Robert’ were treated with oryzalin and several concentrations of nitrotyrosine in a factorial treatment arrangement. The combination of 15 µm oryzalin and 50 µm nitrotyrosine resulted in significantly more polyploid induction for ‘Robert’ than with oryzalin alone.

Morphology and pollen germination of autotetraploid ‘Robert’, ‘MAK20’, and H. macrophylla ‘David Ramsey’ plants were compared to their diploid counterparts a year after plants were moved ex vitro. Compared to diploids, tetraploid hydrangeas had larger leaves, thicker stems, lower leaf area/fresh weight ratios, and longer internodes. Though all tetraploids exhibited significantly fewer inflorescences per plant, both H. macrophylla cultivars had larger inflorescences and ‘David Ramsey’ had a greater number of showy florets per inflorescence. Sepal colors were compared using International Commission on Illumination L*a*b* color space. Tetraploid ‘MAK20’ had lower L* values (darker sepals) while tetraploid ‘Robert’ and ‘MAK20’ both had higher a* values (redder sepals). Pollen germination rates were greatly reduced in all tetraploid lines but retained some viability.

These results provide an effective protocol for in vitro polyploid induction of Hydrangea sp. and documented certain desirable traits associated with tetraploid phenotypes.