Analysis of Ploidy Level and Its Effects on Guard Cell Length, Pollen Diameter, and Fertility in Hydrangea macrophylla

Abstract

Ploidy level was estimated in Hydrangea macrophylla (Thunb) Ser. using flow cytometry. For H. macrophylla ssp. macrophylla, 42 diploid and 19 triploid cultivars were identified. All 14 H. macrophylla ssp. serrata (Thunb.) Makino cultivars tested were diploids. Somatic chromosome counts confirmed the ploidy of three diploid (2 n = 2 x = 36) and three triploid (2 n = 3 x = 54) cultivars. Stomatal guard cell length and pollen diameter of H. macrophylla ssp. macrophylla diploid cultivars were smaller than those of triploid cultivars. However, because the range of measurements for the diploids overlapped that of the triploids, neither guard cell nor pollen measurements are recommended for determining ploidy of H. macrophylla cultivars. Fertility was estimated using pollen staining and controlled pollinations. Stainable pollen for triploid cultivars averaged 63% and ranged from 25% in ‘Masja’ to 85% in ‘Marechal Foch’. Viable seed was obtained when four triploid cultivars were used as pistillate or staminate parents in controlled pollinations to diploid H. macrophylla ssp. macrophylla cultivars. A bimodal distribution of pollen sizes, which is suggestive of unreduced gamete production, was observed in one cultivar; however, more detailed genetic and cytologic studies are needed to elucidate the mechanism behind triploid formation in H. macrophylla taxa.