Loss of Winter Hardiness in R. 'Supernova', An Artificial Polyploid
Stephen L. Krebs
David G. Leach Research Station
The Holden Arboretum
Polyploids are organisms that have extra sets of chromosomes, beyond the diploid pair common to most. This condition occurs naturally in over 30% of plant species and is evidently an important adaptive aspect of plant evolution. In genus Rhododendron, naturally occurring polyploids have been documented in subgenera Rhododendron (Jamaki Ammal et al., 1950), Pentanthera (Sax, 1930), and, more recently, Tsutsusi (Eeckhaur et al., 2004). The number of chromosome sets (ploidy level) in these species ranges from four (tetraploid) to twelve (dodecaploid).
Polyploidy can also be induced artificially, by treating diploid cells with chemicals such as colchicine or oryzalin that prevent separation of paired chromosomes during mitosis - the process of cell division and multiplication - and result in cells with double the normal number. This technique has been used successfully in floriculture to improve ornamental characteristics, because more chromosomes per cell mean larger cell size and subsequently larger tissues and organs. In recent years, application of this technique to rhododendrons has been done with the objectives of larger flowers, longer flower retention, thicker leaves and stems, and a more compact growth habit (Pryor and Frazier, 1968; Kehr, 1996; Väinölä, 2000).
The increase of chromosome number by artificial means often has a negative impact on plant adaptations (Levin, 1983). A relevant example of this is provided by the study of Väinölä and Repo (1999), which compared the cold hardiness of two synthesized rhododendron tetraploids to their diploid progenitors - R. PJM Group (diploid) versus R. 'Northern Starburst' (tetraploid), and R. 'Cunningham's White' (diploid) versus an unnamed tetraploid form of it derived from colchicine treatment. Their approach was to expose leaves, stems, and floral buds from cold acclimated plants to stressful freezing conditions in the laboratory, and to estimate subsequent freezing damage. The results showed that leaves from both diploid cultivars were 12-13°C hardier than the corresponding tetraploids, and that R. PJM Group flower buds were about 3°C hardier than R. 'Northern Starburst'.
At the Leach Research Station in northern Ohio, we have been able to corroborate these findings with an additional comparison from field grown plants. The diploid cultivar R. 'Nova Zembla' and its artificially derived tetraploid counterpart R. 'Supernova' were adjacently planted (2 gal. containers) in one of our testing fields five years ago. During the past two years, winter daytime temperatures were subfreezing for sustained periods of time and maximum lows reached -10°F (-23.3°C) in 2003 and –9°F (-22.7°C) in 2004. Spring bloom in both years revealed that flower buds on R. 'Supernova' were winter damaged - the estimate of injury (% of flower buds killed) was 95% in 2003 and 73% in 2004. In contrast, bloom on R. 'Nova Zembla' was unaffected by winter conditions in both years (Figure 1). No signs of freezing injury appeared on leaves or stems of either cultivar.
Flower bud hardiness in R. 'Nova Zembla' is reported at -25°F (-32°C) (Salley and Greer, 1992), which is one reason why it is the most widely grown "red" rhododendron in the US. Our field data indicate that R. 'Supernova' is about 15°F (9°C) less hardy under test conditions provided during those two winters. The difference in flower bud hardiness observed here is much greater than the 3°C loss of hardiness reported in tetraploid R. 'Northern Starburst', compared to R. PJM Group (Väinölä and Repo, 1999), but the authors of that study concluded that the R. PJM Group plants were probably not fully cold acclimated.
|Figure 1. Images of R. 'Nova Zembla' (A) bloom in 2004, and R. 'Supernova' during the 2003 (B) and 2004 (C) season.|
The physiological relationship between chromosome doubling and reduced freezing tolerance in artificial polyploids is mostly a matter of speculation at this point. Increased cell size results in increased water volume, decreased osmotic pressure, and a reduction in the ratio of cell surface area (membrane) to cell volume (Hancock, 1997; Väinölä and Repo, 1999). Because freezing tolerance requires reduced cell water content, these physical changes in polyploids could impair the process by increasing the amount of water to be removed or slowing the rate at which water is moved out.
Some of the natural polyploids in genus Rhododendron, such as R. lapponicum and R. canadense, are native to very cold areas. These adaptations reflect genetic backgrounds that are very different from synthetic polyploids, in addition to a long history of natural selection that is absent in newly derived "colchiploids." Chemical doubling of chromosomes is genetically equivalent to inbreeding (selfing), because it creates an individual that has more homozygous gene interactions than the source plant. For cross-pollinated plants such as rhododendrons, inbreeding can cause a significant depression of vigor and adaptability.
In contrast, most natural polyploids arise from matings between genetically different individuals, via fusion of unreduced gametes (pollen and eggs with full, rather than halved, chromosome sets). Such matings occur repeatedly in some plant populations, resulting in multiple offspring that are less inbred than artificial polyploids and genetically distinct from each other. This diversity among polyploids is important, because it allows natural selection to favor the best-adapted genotypes. As horticulturists, practicing both cultural and natural selection, we will determine over time whether artificially derived rhododendron polyploids have a future in our gardens.
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Stephen Krebs is a member of the Great Lakes Chapter.