Cross compatibility of intersubgeneric hybrids of azaleas on backcross with several evergreen species

For the reduction of several undesirable traits, such as deciduousness and weak heat tolerance, of intersubgeneric hybrids between interspecific F ₁ hybrids of evergreen azaleas and a deciduous species (Rhododendron japonicum f. flavum), back cross with evergreen azaleas was conducted. Though pollen of the intersubgeneric hybrids was highly sterile, fertility was restored in a tetraploid plant (9027-5). Unilateral cross incompatibility was observed between the intersubgeneric tetraploid hybrid 9027-5 and evergreen azaleas, i.e., progenies obtained only in the cross with the tetraploid plant 9027-5 used as a pollen parent. Leaf color of most of the progenies was green, and hybridity was confirmed by isozyme analysis. PtDNA of the green progenies inherited from the seed parent (evergreen azaleas), and plastome-genome incompatibility between the plastid genome from evergreen azaleas and nuclear genome from R. japonicum f. flavum was not observed in the back cross.