Kenneth Buck – Cornell University, School of Integrative Plant Science, Horticulture Section
Carmen Johns – University of Arkansas System Division of Agriculture, Department of Horticulture
Jason Londo – Cornell University, School of Integrative Plant Science, Horticulture Section
Margaret Worthington – University of Arkansas System Division of Agriculture, Department of Horticulture
Introduction
One significant factor limiting the commercial production of muscadine grapes (Muscadinia rotundifolia) is their lack of cold hardiness compared to other grape species. Familiar bunch grape species such as Vitis vinifera, the source of common wine and table grape cultivars, are cold hardy enough for production in the Northeast U.S. and Canada, while many muscadine cultivars struggle even in the upland South. This vulnerability has been highlighted by extreme winter events in February 2021, December 2022, and January 2024, when temperatures dropped below 10 °F across the southern U.S., resulting in widespread muscadine vine mortality.
As research efforts continue to address this issue in muscadine physiology through the development of novel, hardier muscadine cultivars, it is essential to narrow down what exactly the weak link in muscadine hardiness is. In grapes, cold hardiness is generally limited by vegetative buds, but field observations following these extreme cold events suggest that muscadines may differ. In some vines, buds broke and shoots grew for a time before suddenly collapsing, often when temperatures and water requirements were at their midsummer peak. Some vines also sprouted large masses of aerial roots from the cordons, presumably due to a disconnect somewhere in the root-to-shoot water transportation pathway. Based on these observations, we hypothesize that, contrary to other grape species, the weak link in muscadine vines during the dormant season is not vegetative buds but rather stem tissues.
Methods
We measured the cold hardiness of vegetative buds and stem tissue in five muscadine cultivars (Mighty FineTM, ‘Carlos’, ‘Supreme’, ‘Paulk’, and Oh My!®) and two bunch grape cultivars (‘Jupiter’ and ‘Reliance’) grown at the University of Arkansas System Division of Agriculture in Clarksville, AR.
Vegetative buds, which survive winter by supercooling (keeping the water inside living cells in liquid form well below the usual freezing point), were tested using differential thermal analysis (DTA). We removed buds from the canes and placed them in a programmable freezer that slowly cooled from room temperature to -40 °C (-40 °F). When the supercooled water inside the buds finally froze, the buds died, and this produced a small burst of heat (an exotherm) that the DTA equipment could detect. That signal told us the exact temperature at which buds were killed. We used DTA to measure the cold hardiness of 30 buds collected from each cultivar every two weeks from the beginning of November 2023 to early April 2024.
Because stems cannot supercool the way buds do, a different approach was needed. To test stem hardiness, we cut uniform dormant cane pieces about one inch long, placed them in tubes of distilled water, and froze them at 5 °C intervals from –10 °C (14 °F) down to –50 °C (-58 °F). As cells in the stem tissue froze and burst, their contents leaked into the water, increasing its electrical conductivity. By measuring conductivity across the temperature range, we built a regression curve showing the temperature at which stem tissue was damaged. We focused on the temperature causing about 20% damage (LT20), which is commonly used as the point where plant tissues begin to fail. In addition, we visually scored the amount of oxidative browning (OB) in frozen stem tissue as another indicator of injury. Stem tests (electrolyte leakage and OB) were carried out once a month from November 2023 through April 2024.
Results
Throughout the trial, very few significant differences were observed between the bud hardiness of the hardiest muscadine cultivars (Mighty FineTM and ‘Carlos’) and the bunch grape cultivars (‘Jupiter’ and ‘Reliance’), which supports our hypothesis that buds are not the weak point separating muscadine and bunch grape cold tolerance. In fact, preliminary results suggest that bunch grape buds may lose hardiness slightly earlier in spring than muscadine buds. The least hardy muscadines (‘Supreme’ and ‘Paulk’) were generally a few degrees less cold tolerant than the bunch grapes, though in midwinter these differences did not exceed about 5 °F.
Stem hardiness, however, told a very different story. In January 2024, when bud hardiness showed little distinction between hardy muscadines and bunch grapes, stem LT20 values (the temperature at which ~20% of stem tissue is killed) differed by as much as 20 °F. In other words, while buds looked similar in their ability to survive the cold, muscadine stems were far more vulnerable. Due to individual variation, mean hardiness of buds reflects about half of the buds dying, but the LT20 value for stems points directly to the temperature at which canes, cordons, and trunks begin to fail. This makes stem LT20 a much more practical indicator of whole-vine survival. Visual scoring of oxidative browning (OB) is still underway, but we expect these results to reinforce that LT20 is the best parameter for assessing stem hardiness in grapes.
Taken together, these results suggest that the real weak link in muscadine cold tolerance lies in the stems, not the buds. With a second year of data nearly complete, we expect to confirm this pattern and provide growers with clearer guidance on why some muscadine vines fail after extreme cold events.