Assessing Drought, Heat, and Cold Tolerance in Hybrid Bunch Grapes in South Mississippi

Haley N. Williams and Eric T. Stafne, Mississippi State University

There is a clear interest from local growers and winemakers in expanding the production of bunch grapes in the Gulf South region. High quality production is low due to high disease and climate pressures. Hence, there is a need to explore which grape cultivars are best suited to the region.

Challenges in Cultivar Selection

One of the biggest obstacles to growing grapes in this region is Pierce’s disease (PD), which is especially detrimental to traditional European wine grapes (Vitis vinifera). Thus, the industry has relied on PD-tolerant cultivars that were bred in the South, but they don’t always offer the best wine quality or yields. Recently, new cultivars, such as ‘Ambulo Blanc’, have been released by breeding programs in California. These grapes show promise because they combine PD resistance with a high percentage of V. vinifera, offering better wine quality and productivity. However, we don’t yet know how these new cultivars will hold up to our subtropical climate.

Cultivar Climate Resilience

Southern grape cultivars may be better adapted to subtropical conditions. However, more frequent extreme weather events, such as drought, heat waves, and spring frosts, pose major challenges to grape production, affecting vine physiology and resultant fruit production. It should be noted that grapevines are generally more resilient to drought than other crops, as discussed by Gambetta et al. (2020). However, a great deal of variation in stress tolerance exists among cultivars, and it is unclear whether this variation is primarily due to genetic differences or environmental influences. Thus, our goal is to identify cultivars that can best tolerate drought, heat, and cold as well as the already difficult climate of the Gulf South.

Study Design

In 2023, a study took place at the Mississippi State University South Mississippi Branch Experiment Station in Poplarville, MS, to evaluate drought, heat, and cold stress responses in nine bunch grape cultivars: ‘Ambulo Blanc’, ‘Black Spanish’, ‘Blanc du Bois’, ‘Lomanto’, ‘Mamont Noir’, ‘MidSouth’, ‘Miss Blanc’, ‘Muench’, and ‘Victoria Red’. Many of these vines were planted in 2022 and are maintained under drip irrigation. From May to September, field measurements included environmental conditions (temperature, precipitation, humidity, solar radiation, growing degree days, and chill hours), which were collected from a weather station located within 700 feet of the majority of the vines included in the study, and vine physiological measurements (stomatal conductance, transpiration, vapor pressure deficit, and chlorophyll fluorescence), which were collected using a LI-600 porometer/fluorometer (Table 1).

Laboratory tests on 1 inch leaf disks simulated drought and heat to assess water loss, electrolyte leakage, and other physiological measurements (Table 1). Leaves were collected from the field, and then leaf disks were cut out prior to undergoing stress treatments. For drought simulation, disks were placed on a custom mesh platform suspended over a saltwater solution inside a foil container (Figure 1). The container was sealed and placed in a growth chamber set to 80.6°F, 85% relative humidity, and 200 µmol/m²s lighting for 4 hours, with disks being weighed every hour to determine water loss. For heat simulation, leaf disks were wrapped in damp paper towels, enclosed in foil, and incubated in a growth chamber programmed to increase from 77°F to 113°F over 4 hours at constant 85% relative humidity and 200 µmol/m²s lighting, with electrolyte leakage being measured every 2 hours with a conductivity meter (Figure 2).

To assess cold tolerance, differential thermal analysis was conducted on dormant buds from December 2023 to February 2024. The dormant buds were collected from the field, and then nine buds of one cultivar were placed in individuals wells of a tray (Figure 3). Each well was positioned on a thermoelectric module designed to detect low temperature exotherms, representing the lethal temperatures (LT) to the buds. The buds were then subjected to a freezing program in a chamber that gradually cooled from 39.2°F to -40°F. This process allowed for the determination of average LT, as well as the LT corresponding to 10%, 50%, and 90% bud mortality.

Weather Overview

Monthly weather data revealed August as the hottest and driest month, with peak temperatures, solar radiation, and growing degree days, along with minimal rainfall, humidity, and chill hours. January, on the other hand, had the lowest temperatures and growing degree days and highest rainfall and chill hours (Table 2).

Field Evaluation Results

During field evaluations, cultivar responses to environmental conditions varied most in August. ‘Ambulo Blanc’ showed high rates of stomatal conductance and transpiration (T) combined with low vapor pressure deficit (VPD), indicating efficient self-cooling, but likely at the cost of water use. In contrast, ‘Mamont Noir’ and ‘Lomanto’ had low T, favoring drought tolerance. ‘Muench’ exhibited the highest VPD, suggesting limited evaporative cooling but higher water conservation. Chlorophyll fluorescence was highest in ‘Lomanto’ and lowest in ‘Miss Blanc’, reflecting differences in resilience and indicating that ‘Miss Blanc’ was less photosynthetically active than ‘Lomanto’ in response to their environmental stressors (Table 3).

Drought Tolerance Results

Under drought simulation conditions, water loss increased linearly for all cultivars, but the overall rate of water loss declined in the later months that were hotter and drier, suggesting some form of vine adaptation. Stomatal conductance (SC) and transpiration (T) patterns closely aligned, with ‘Lomanto’ and ‘Black Spanish’ showing the highest rates, while ‘Victoria Red’ and ‘Mamont Noir’ generally maintained lower rates, indicating less water use. Chlorophyll fluoresence (CF) declined across all cultivars, but ‘Mamont Noir’ maintained the highest rates overall. The combination of low SC and T with high CF highlights ‘Mamont Noir’ as the most drought-resilient cultivar evaluated, while high SC and T in ‘Lomanto’ and ‘Black Spanish’ suggest greater susceptibility (Table 3).

Heat Tolerance Results

Under heat simulation conditions, electrolyte leakage remained stable across all months except August, when ‘Muench’ displayed significantly higher rates of leakage, indicating greater cell membrane damage. Transpiration (T) was highest in ‘Lomanto’ and lowest in ‘Mamont Noir’. Chlorophyll fluorescence (CF) declined sharply at the end of each simulation, with ‘Ambulo Blanc’ consistently showing the lowest CF while ‘Black Spanish’ maintained the highest. Based on high T and CF, ‘Lomanto’ and ‘Black Spanish’ appeared most heat tolerant, while ‘Muench’, ‘Mamont Noir’, and ‘Ambulo Blanc’ showed the greatest sensitivity to heat stress among our tested grapevine cultivars (Table 3).

Cold Tolerance Results

Lethal temperatures (LT) at which 10%, 50%, and 90% bud mortality occurred varied by month. Buds collected in December were the least cold-hardy, with mean LT values of 9.3°F, while increased tolerance was observed in January at -1.8°F and February at 3.2°F (Table 4). Additionally, the lowest LT, at which 90% bud mortality occurred, was -9.6°F in January when the average temperature and cumulative chill hours were 49.3°F and 294, respectively (Table 2 and 4), with the lowest recorded temperature of 14°F. This pattern reflects increasing bud cold hardiness as more chill hours accumulate, indicating seasonal adaptation. Average LT values revealed no differences among the nine cultivars tested, suggesting similar baseline cold hardiness.

Conclusion

These results show how grape cultivars differ from each other in their responses to drought, heat, and cold, with signs of seasonal adaptation, such as slower water loss and improved cold tolerance. ‘Mamont Noir’ showed strong drought tolerance, while ‘Lomanto’ and ‘Black Spanish’ showed strong heat tolerance. By combining the data from this trial with a repeated 2024 trial, we aim to develop predictive models that connect controlled simulations with actual vine performance. Overall, these findings offer guidance for selecting resilient grape cultivars in the increasingly variable climate and support sustainable vineyard management and future breeding efforts in the region.

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