Understanding self-incompatibility and exploring the reproduction biology in Linnaeoideae – with a focus in Abelia – to accelerate cultivar development

Abstract

Despite being in cultivation for more than 135 years, relatively few cultivars of Abelia have been developed and are commercially available. Cultivar development has been limited by self-incompatibility and crossing barriers with other species. To help overcome these barriers, self-incompatibility systems were investigated in Abelia chinensis R. Brown, Abelia macrotera var. engleriana (Graebner & Buchwald) Rehder, Abelia xgrandiflora (Rovelli ex André) Rehder, and Vesalea floribunda M.Martens & Galeotti. A full diallel cross with self-pollinations was performed in greenhouse-grown plants on recently opened flowers. Styles were removed one and two days after pollination, fixed in a solution of ethanol and glacial acetic acid (3:1) for at least 24 hr, transferred to a solution of 4 N NaOH for 24 hrs to soften the tissues, and then stained with 0.1% decolorized aniline blue with 0.1 M K3PO4. Stained styles were analyzed for pollen tube growth under an inverted epifluorescent microscope. Pollen tubes were inhibited in the upper half of the style on self-pollinations and reached the base of the style on cross-pollinations suggesting the presence of gametophytic self-incompatibility in Abelia and Vesalea. In vitro pollen germination was evaluated in A. chinensis, A. macrotera var. engleriana and Diabelia serrata (Siebold & Zuccarini) Landrein. The most effective germination medium consisted of 10% sucrose, 0.01% H3BO3, 0.02% CaCl3 and 0.5% agar with a pH of 7. Pollen did not germinate in liquid medium and sucrose concentrations of 15, 20 or 25% were not effective. Three samples of pollen from different dehiscent anthers of recently opened flowers from greenhouse-grown plants were placed on top of a slide containing germination medium. Slides were placed on a petri dish with a wet paper towel to ensure 100% humidity and cultured in light at 28.5 ± 0.5 °C. Germination of 100 pollen grains was evaluated at 2 hrs and 24 hrs after culture. Overall, Abelia macrotera var. engleriana had the highest germination, with 80% of the grains germinating after 24 hrs of culture. Diabelia serrata had 49.8% germination, while A. chinensis exhibited very low germination in the three different pH levels tested. In vitro germinated pollen grains of A. chinensis, A. macrotera var. engleriana, and Diabelia serrata were examined for number of nuclei. Pollen was stained for one hour with 0.25 mg∙L−1 or 0.5mg∙L−1 of a 4′, 6-diamidino-2-phenylindole (DAPI) solution and evaluated under a fluorescent microscope. Observations indicate the presence of three nuclei in pollen in Abelia and Diabelia. This is the first report of a self-incompatibility system in Abelia and Vesalea, and pollen nuclei in Abelia and Diabelia.