Garden impatiens (I. walleriana, Balsaminaceae) has been a top-selling bedding plant in the United States, generating an estimated wholesale value of 150 million dollars in 2005 (USDA/NASS, 2006). This plant’s popularity in the floriculture and landscape industries is primarily due to its wide range of flower colors, long flowering season, and excellent shade tolerance (Morgan, 2007).
Impatiens downy mildew, incited by the oomycete P. obducens, is a devastating disease of impatiens. Outbreak of this disease in the United States in recent years has caused the loss of hundreds of million dollars to the U.S. floriculture industry. Largely because of this disease, the wholesale value of impatiens in the United States in 2015 dropped to ≈60 million dollars (USDA/NASS, 2016). All cultivars of impatiens (I. walleriana) and interspecific hybrids with I. walleriana are highly susceptible to IDM (Warfield, 2011). Although IDM resistance is available in New Guinea impatiens (Impatiens hawkeri), the resistance has not been transferred into garden impatiens through conventional breeding. The two species are unable to produce viable seeds and offspring because of differences between the species in chromosome number, morphology, and size (Uchneat, 2007). Impatiens walleriana is a diploid with 2n = 2x = 16, whereas New Guinea impatiens is a diploid with 2n = 2x = 32 (Arisumi, 1973a) or 48 (Jones and Smith, 1966).
Chromosome doubling is a powerful ploidy manipulation tool for plant breeding and genetic improvement. Following chromosome doubling, autopolyploids may produce larger organs and larger amounts of biomass, have higher biological yields, show changes in anatomical structures (primarily in stomata size and density), and/or have improved ornamental values. Induced tetraploids may also show improved disease resistance. For example, tetraploid Glycine tabacina became more resistant to the leaf-rust fungus Phakopsora pachyrhizi (Burdon and Marshall, 1981), and tetraploid Arabidopsis thaliana accession C24 became more tolerant of the necrotrophic pathogen Botrytis cinerea (Fort, 2013). Autopolyploidization has already been proposed as a means to improve disease resistance in bananas (Hamill et al., 1992). Based on mathematical models, Oswald and Nuismer (2007) predicted that autopolyploids could be more resistant to diseases than their corresponding diploids.
Arisumi (1973b) studied the morphology and breeding behaviors of colchicine-induced polyploids in several other Impatiens species, but few reports in the literature are available regarding the effects of induced tetraploidy on the morphology of I. walleriana, and there is no information on the effects of induced tetraploidy on disease resistance in I. walleriana. In this study, tetraploids were induced and their changes in morphology and IDM resistance were investigated by comparing them with diploids.
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