Breeding for polyploidy in Belgian azalea (Rhododendron simsii hybrids)

Evergreen azalea hybrids belong to the main ornamental crops produced in Belgium. Few attention has been dedicated to the creation of polyploid cultivars. As a result, nearly all commercial genotypes are diploid. However, polyploid azaleas can be induced using three different protocols, two of which have already been described in literature. Firstly, through the use of mitosis inhibitors meristematic cells of seedlings can be tetraploidized and regenerated into solid tetraploids, although ploidy chimeras frequently occur. A second method is based on the regeneration of tetraploid flower margins in picotee chimeras in vitro. The main drawback of this system is that the number of cultivars with tetraploid petal margins is rather low. A third solution would be the application of unreduced gametes. The production of 2n gametes in azalea has been demonstrated in this study. At this moment, induction and screening methods for unreduced azalea gametes are being developed. Mixoploidy does not occur within the offspring; moreover, the capacity of sexual polyploidization to generate variation is higher. This triple production of polyploids allows to evaluate either option, and to unravel their typical advantages and drawbacks. A thorough knowledge of both the induction process and the genetic consequences will allow to efficiently integrate polyploidization in breeding programs of pot azalea or other ornamentals.