Title: “Three-year screening for cold hardiness of garden roses”
See:
“2.1. Plant material
One species and 16 hybrids belonging to different USDA plant
hardiness zones were selected for this study (Table 1).”
“5. Conclusions
Index of injury at a defined target temperature is a valuable tool for
screening a larger number of genotypes in roses. Results of relative cold
hardiness were reproducible during the three years for the most cold
hardy genotypes and most susceptible genotypes. Both sucrose and
oligosaccharides contribute to cold hardiness in roses. This is a fast and
practical approach to screen new rose genotypes and establish their
relative ranking using a limited set of known genotypes without the
need for long-term field screening”
Thanks Henry-
I’m attempting a low-tech version of this experiment this winter. Rather than conditioning the sections of stem with sugars, I’m waiting for them to shut down for the winter (which seems to take place around January 1 most years where I live.)
I will freeze sections of stem to -20c for six days, while leaving a control group of the same cultivar above zero. From there I will try to stimulate growth on the sections- maybe though the burrito method, maybe with hardwood cuttings outdoors, and see what the mortality rate is for each cultivar: does freezing increase mortality on tender cultivars vs hardy ones? Do hardy ones grow after freezing, as I predict?
If I get anything consistent I will try to use this method to predict whether my seedlings will be hardy. (I’m trying to breed hardy seedlings, but I live in zone 8, so any system that helps me evaluate their cold-hardiness from here is helpful.)
Thank you Henry. I could not access the first article in Sci Hort from home. But I was referred to a similar article by Science Direct, and it is available without paying access fees. It focuses on 4 roses representing 4 hardiness zones, out of the 16 screened.
Journal of Plant Physiology
Volume 232, January 2019, Pages 188-199
Seasonal changes in cold hardiness and carbohydrate metabolism in four garden rose cultivars
LinOuyangaLeenLeusbEllenDe KeyserbMarie-ChristineVan Labekea https://doi.org/10.1016/j.jplph.2018.12.001
Abstract
We studied metabolic adaptations to cold stress in roses and identified genes in the carbohydrate pathway during acclimation and deacclimation. A field experiment with four rose cultivars belonging to different USDA plant hardiness zones was set up in Melle, Belgium (51° 0′ N, 3° 48′ E). The more cold-hardy cultivars (‘Dagmar Hastrup’ and ‘John Cabot’) reached their lowest LT50 value in December, indicating a rapid acclimation after the first occurrence of frost. Less cold-hardy cultivars (‘Abraham Darby’ and ‘Chandos Beauty’) reached their lowest LT50 in January/February when exposed to prolonged freezing temperatures. A cell dehydration pattern was found in the less cold-hardy cultivars ‘Abraham Darby’ and ‘Chandos Beauty’. The expression of dehydrins (RhDHN5 and RhDNH6) was up-regulated during November-January. Carbohydrate metabolism is highly involved in cold acclimation in roses. Starch decreased from November towards January in all four cultivars and the hydrolysis of starch by the β-amylolytic pathway (BAM, DPE2) was identified in ‘Dagmar Hastrup’ from November to January. Oligosaccharides correlated with cold hardiness in three cultivars although no significant upregulation in RhMIPS and RhRS6, key genes in their biosynthesis, was found. Higher sucrose levels were found during acclimation in hardy cultivars, although transcript levels of RhINV2 was more prominent in ‘Chandos Beauty’.
The actual hardiness testing described in the paper is rather elaborate and I think Donald has the right idea. But somehow you have to control the rate of freezing after storing the stems in a refrigerator for a couple weeks. I fairly confident that refrigerator acclimation is abslutely essential, and slow freezing is also the only way to get a reasonable estimate of hardiness. I recall a paper from several decades ago which discussed supercooling of water down to some relatively low temp when the cooling rate is some thing like a degree C per hour. Hardy plants can go lower by having high levels of osmolytes (sugars, glycerin, whatever). You can get to 0 F without a refrigerator easily enough by starting with ice as in an old fashioned ice cream freezer, with your cuttings in the ice cream place. Gradually (very gradually) add salt and stir it into the crushed ice. When the water is fully saturated with salt you have 0 F or a couple degrees below, The total salt needed is about 2 lb per gallon (8 lb by wt) of ice slush. This may all lead to freezing point depression if it takes you too many days, but the idea is to take all day (or night) to get to the low temp. Allow thawing to be very slow, just letting the whole freezer sit at room temp for several days. That will better simulate how things happen in protected plants. Snap cooling and rapid thawing are sure to do damage to almost all, even hardy roses.
Larry- my acclimation will be outdoors: the roses will have already been at somewhere between 4c and -2c for days or weeks. That’s why I’m doing this now, and not in July. And yes the shock in the freezer might kill them- but roses in, say, Alberta need to survive sudden drops from 0 to -20c all the time, given the climate. It does take its toll, but it’s also a measure of their hardiness. Snap cooling and rapid thawing are definitely hard on plants- but it’s the reality for many in the north. So we’ll see.
My point was just that you could cool things at a rate emulating what happens outside. I know that there are times when it falls 50 F in 2 hr but mostly it is a cold clear night where the temp drops about 30 F from where it starts, then the next day rises partway up and drops a step further. so I was thinking to run it over say 10 hr from icewater (34 F), down to 0 F during a day, then put into freezer overnight and let thaw slowly over another 8 hr or so by simply returning to the refrigerator wrapped up a bit. this would be brutal enough to sort the lambs from the rams.