Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2007
Publication Date: 6/23/2007
Citation: Jones, K.D., Reed, S.M. and Rinehart, T.A. 2007. Analysis of ploidy level and its effects on guard cell length, pollen diameter and fertility in Hydrangea macrophylla. HortScience. 42:483-488. Interpretive Summary: Polyploid cultivars of ornamental plants are often superior to diploids because of their larger leaves and flowers. The purpose of this study was to investigate ploidy differences among H. macrophylla, or bigleaf hydrangea, cultivars. Of the 75 cultivars examined, 19 were found to be triploids and the rest diploids. Ploidy level was determined using flow cytometry, which is fast but requires sophisticated and expensive equipment, and confirmed by chromosome counts, which are labor-intensive and often difficult to perform. Other quick and easy methods were investigated as alternatives to flow cytometry and chromosome counts for determining ploidy of H. macrophylla cultivars. As with many other plant species, pollen grain diameter and stomatal guard cell length were determined to be larger in triploid than in diploid cultivars. However, since the range of pollen grain and guard cell sizes for the diploids overlapped that of the triploids, these measurements do not appear to be reliable methods for determining ploidy level of H. macrophylla cultivars. While triploids of many plant species have very low fertility, many of the H. macrophylla triploid cultivars appeared to have a moderate to high level of fertility. This work provides the basis for future efforts to utilize polyploidy in breeding superior cultivars of H. macrophylla.
Technical Abstract: Ploidy level was estimated in Hydrangea macrophylla (Thunb. Ex J.A. Murr.) Ser. using flow cytometry. For H. macrophylla ssp. macrophylla, 42 diploid and 19 triploid cultivars were identified. All 14 H. macrophylla ssp. serrata cultivars tested were diploids. Somatic chromosome counts confirmed the ploidy of three diploid (2n = 36) and three triploid (2n = 54) cultivars. Stomatal guard cell length and pollen diameter of ssp. macrophylla diploid cultivars were significantly smaller than those of triploid cultivars. However, since 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 the triploid cultivars averaged 63% and ranged from 25% in ‘Masja’ to 85% in ‘Marechal Foch’. Viable seed were obtained when four triploid cultivars were used as pistillate or staminate parents in controlled pollinations to diploid 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 cytological studies are needed to elucidate the mechanism behind triploid formation in H. macrophylla.