Jason P. Londo, Cornell University; Erica Casagrande Biasuz, Cornell University; Michael Basedow, CCE ENYCHP
The goal of the Londo research program is to help adapt New York fruit crop production to climate change. As a part of that effort, we are working to understand how winter temperatures impact apple rootstocks and scions, specifically how the ability to survive freezing temperatures changes throughout winter. We hope to identify elite rootstocks that start out winter very cold hardy, retain that ability through midwinter, and resist losing cold hardiness (deacclimation) in late winter and early spring.
Methods
To gain a deeper understanding of cold hardiness in apple scions and rootstocks, we initiated a screening study that began in November of 2021 for 21 different rootstocks and 4 scion cultivars. In that first year, we focused on developing our capacity to test cold hardiness using a technique called electrolyte leakage. The method itself is pretty basic: 1–2-year-old apple stem tissue is collected from the field and chopped up into 1-inch pieces. These pieces are placed in a tube of distilled water and then frozen in batches at different temperatures. We used 9 different temperature levels and a control “no damage” temperature of 39°F. The freeze temperatures ranged from 14°F to -67°F. We put the tubes of water and cuttings into this programmable freezer and then after 1 hour at each temperature exposure, we pull out a set of tubes and let them thaw. The concept is that freeze damage results in cellular damage in the cutting. When that damage occurs, it causes leakage of sugars, proteins, and other electrolytes into the water solution. After all the tubes are frozen, thawed, and shaken, we measure the electrolyte levels with a conductivity meter. In my lab we use a robot to help us with this, since there are so many samples to do each month. (Figure 1). After we check the electrolyte levels from the first freeze, we freeze all the tubes a second time in a -112°F freezer. This causes a complete kill and maximal damage. After this freeze, we measure electrolytes a second time. The data is then scaled against the control treatment (zero damage) and second -112° freeze (100% damage). This gives us an estimate of the percent damage that occurs at the different freeze temperatures. We then plot this data and perform some logistic regression on the data to determine the relative cold hardiness of each rootstock or scion, in each month of the winter. In addition, we look at cross sections of the stem tissue and evaluate how much of the tissue is damaged based on how much turns brown following freezing. This method is called oxidative browning and while it can work for evaluating cold hardiness, it can be hard to judge how much of the tissue is brown, and if specific tissues are damaged. So, in our research, we use it to help us decide on what level of electrolyte leakage is most appropriate for reporting freeze damage.
This current winter 2022-2023 is our second winter of testing, and we have expanded our efforts to examine 22 scions (including 6 cider varieties) and 21 rootstocks. We are also testing to see if there are measurable differences in cold hardiness in a single scion that is grafted to multiple rootstocks (Gala on 10 rootstocks) and if there are differences in deacclimation (loss of cold hardiness in warm temperatures) between 15 different rootstocks.
Figure 1. Experimental setup for doing electrolyte leakage. Stem segments are placed in water and frozen at different temperatures. Electrolyte conductivity is then measured to determine cellular leakage with a conductivity meter. “Stanley” the conductivity robot. “Stanley” can process up to 224 samples at a time.
Figure 2. Differing levels of freeze damage, expressed as increasing browning of tissues. Evidence of cambial browning evident at -13°F. This type of damage tends to occur in correlation with electrolyte leakage data that indicates 25% of maximal damage.
Results
The results of these studies are relative damage assessments and typically people report the temperature that results in 50% of maximal damage, known as the lethal temperature 50, or LT50. However, in apple, the LT50 temperatures determined with electrolyte leakage are far below what we typically experience in a New York winter. When we compare the browning patterns across our samples, we see that the phloem and cambium tissue are the first to show damage. This damage appears to correlate closely with the LT25 temperatures we measure with electrolyte leakage.
Cold hardiness of the rootstocks changes between the months sampled, with stems collected from December and January typically more freeze resistant than in November. The least cold hardy rootstocks sampled in November were M9, CG.4004, and G.202, while the most cold hardy were G.210, G.41, CG.5257, and CG.6589. G.210 and G.41 retained its deep cold hardiness throughout the sampled months. When comparing the change in cold hardiness between the months, early indications of deacclimation are evident for G.222, G.890, and G.935, which all lost substantial hardiness between December and January. This could be due to our recent mild winter conditions as deacclimation processes accelerate when warm weather occurs in late winter. We will continue to sample for February and March, and April if conditions allow it.
Cold hardiness in scions seems to be a little less variable compared with the rootstocks we are testing. Most start out the winter with good cold hardiness, around -15 to -20 °F. The least cold hardy in early winter were three cider varieties, Dabinett, Porter’s Perfection, and Goldrush. In contrast, the cider variety Ellis Bitter had deep cold hardiness early in the season and maintained that deep cold hardiness into December. Jonagold, Golden Delicious, Evercrisp, and Sunrise Fuji all seem to have lost much of their cold hardiness between the two sampled months. We are currently running the January samples for our scion collection, so updates on the whole season will have to wait for future updates.
Current Takeaways
Apple rootstocks and scions both have excellent baseline cold hardiness and most winter conditions will be tolerated without significant injury. We are seeing some interesting differences between rootstocks and scions, with much higher variation in the rootstocks. This has important implications for rootstock-scion interaction. Our ongoing work looking into the effect of rootstocks on scions is too early to report on but may suggest that specific pairs are more optimized for winter hardiness.
Future Work
These sorts of studies require multiple years of data to build up the framework needed to make recommendations to growers on which rootstocks, scions, and combinations are the most appropriate. Given the variety of climates in New York, we want to make sure we capture as much year-to-year variation as possible in these responses. We will continue this work for at least the next three years. In addition, we are working to characterize the deacclimation response of rootstocks, to determine which are the most resistant to late winter false spring events.
Acknowledgements:
None of this work is possible without a research team. The Londo and Robinson programs collaborate on these projects and the work would not be possible without the help from Hanna Martens, Maria Mott, and Erik Verdehem in the Londo program.