Five seedlings of Rhododendron occidentale SM189 X Rhododendron calendulaceum 'Colossus' have been determined to be a triploid using flow cytometry by
Dr. João Loureiro, Dr. Silvia Castro, José Cerca, and Mariana Castro
Plant Ecology and Evolution Group,
Centre for Functional Ecology,
Department of Life Sciences,
Faculty of Science and Technology,
University of Coimbra, Portugal.
Rhododendron occidentale is diploid species. Rhododendron calendulaceum is a tetraploid species.
In the case of these 5 triploids (39 chromosomes), a diploid crossed with a tetraploid produced a triploid indicating normal meiosis occurred where the diploid contributed a 1x gamete (13 chromosomes) and the tetraploid contributed a 2x gamete (26 chromosomes).
Summary of Ploidy Levels
Diploid Rhododendron occidentale,
Triploid 5 seedlings from Rhododendron occidentale SM189 X Rhododendron calendulaceum 'Colossus',
Tetraploid Rhododendron calendulaceum
Source: Bruce Clyburn
John and Sally Perkins