Plastome-genome incompatibility of Rhododendron serpyllifolium (A. Gray) miq. to evergreen azalea species belonging to series Kaempferia

To confirm that unilateral cross-incompatibility exists between Rhododendron serpyllifolium, series Serpyllifolia, and six evergreen azaleas belonging to series Kaempferia, interspecific crosses were conducted. In crosses with R. serpyllifolium as seed parents, few seeds of the progenies germinated; the survivors developed into plants with green leaves which PCR-RFLP analysis showed as having R. serpyllifolium type ptDNA. In the reciprocal crosses with R. serpyllifolium as pollen parents, many seeds germinated but many were non-viable albinos which had Kaempferia type ptDNA. In crosses between R. transiens (Accession 1) × R. serpyllifolium (Accession 1), however, relatively high frequency of green seedlings having R. serpyllifolium type ptDNA appeared. From these results, we conclude that it is better to use R. serpyllifolium as pollen parent for obtaining many seedlings, and that, in this case, R. transiens (Accession 1) is useful as bridge plants for overcoming plastome-genome incompatibility between R. serpyllifolium and series Kaempferia species.