Michael Cook, a viticulturist with Texas A&M AgriLife Extension Service, jokes about the summertime weather in Dallas-Fort Worth: “What’s the difference between day and night? Nothing. The lights go out.” North Texas, like the Texas Hill Country and the Gulf Coast, is a viticultural region that lacks a diurnal shift, a climatic feature that gives grapevines and other plants a chance to cool down at night. However, despite such challenges, viticulture is not only present in North Texas, but in a state of maturation and refinement.
Grapevines—and all horticultural crops—are contending not only with more heat stress during the growing season, but unpredictable and extreme changes in temperature. This is why Amit Dhingra, Ph.D., the head of the department of horticultural sciences at Texas A&M University, emphasizes the term “climate resilience,” rather than simply climate change. The issue of climate resilience transcends Texas.
For example, Frédéric Panaïotis, the chef de caves at Ruinart in Champagne, recalls harvesting at his grandparents’ vineyard in the 1980s, “hands frozen and eating warm sausages at 10 in the morning.” Three years ago, Ruinart began tracking temperature, sunshine hours, rainfall, and growing cycles, establishing that today’s annual temperature is elevated from historic norms by roughly 1.3 degrees Celsius, which means a loss of 10 to 12 days in the growing season.
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Dr. Dhingra explains that plants already have some ability to acclimate to higher than normal temperature conditions by revisiting the image of a frog in boiling water; in 19th-century experiments, if the temperature increased slowly, the frog did not attempt to escape the water. “When you have persistent heat, the plant becomes accustomed and can survive that quite a bit,” he explains. “It’s extreme events that cause damage quickly, like the burning of leaves and complete drying, because the plant doesn’t get to engage mechanisms that can conquer some of the challenges.”
Given that extreme changes in temperature can damage grapevines and other plants before they can prepare themselves for survival, plants have no choice but to become more resilient, but how much heat and unpredictability can they tolerate?
Here, we look at three lifelines grapevines have as they contend with both persistent and intermittent heat stress: their intrinsic capacity for self-protection; progressive, intentional farming practices; and ongoing research that aims to identify cultivars that can withstand the heat.
What is Heat Stress?
Brianna Crowley, a technical winegrowing analyst for Apical Texas and former viticulture program specialist at Texas A&M, defines heat stress as an “increase in temperature above a threshold level that persists long enough to cause irreversible damage to plant growth and development.” Even an increase of 10 to 15 degrees “above ambient temperature” can cause heat stress, she explains.
Photosynthesis—when light and oxygen are converted to sugar that is later metabolized for energy—is optimal for grape leaves between 77 and 95 degrees Fahrenheit. “The challenge is when it’s hot early in the morning, and it gets hot fast and stays hot all day, the plants start to lose ground after a certain point,” says Greg Pennyroyal, a professor of viticulture at Mt. San Jacinto College and the vineyard manager for Wilson Creek Winery & Vineyards in Temecula, California. As the plant enters stasis, where the movement of water, nutrients, and sugars ceases, it turns to stored sugars, starches, and proteins to survive.
The Effects of Heat Stress
Heat stress and dehydration go hand in hand. “When the soil is too dry, the stomates—the organs that regulate gas exchange in and out of the leaf—begin closing to reduce transpiration [the exhalation of water vapor] and water loss in the vine,” says Justine Vanden Heuvel, Ph.D., a professor at the Horticulture Section School of Integrative Plant Science at Cornell University. “Root growth and photosynthesis are subsequently reduced, and the vine isn’t producing sugars for growth or other metabolites for the fruit.”
“We have traditionally thought of these concepts as individual, but in nature, when heat happens, water limitation also happens,” explains Dhingra. In a recent study, Dhingra and colleagues from Washington State University examined the effects of individual and combined water and heat stress in Cabernet Sauvignon and Riesling grape berries. Water stress had a greater impact on the leaves as “that’s where the water exchange [transpiration] is happening,” says Dhingra. “A grape berry is not transpiring too much, which is why heat has a major influence on grape berries.”
The timing of heat stress produces different effects in berries. At bloom, says Crowley, heat stress can cause berries to shatter, reducing the number of berries per cluster. After fruit set, growth can be inhibited by heat, and scarring or cracking can occur.
This is why diurnal shift can make a critical difference in warm wine regions, as stomates can open for a short period at night, allowing for gas exchange. “With a cool morning and full sun, plants can meet most of their photosynthetic requirements in the first hours of the day,” says Pennyroyal.
“At an altitude of 3,500 feet and in a semi-arid desert region with lower nighttime temperatures and low humidity, the plant doesn’t have to work as hard to cool itself off,” says Andy Timmons, the owner of Lost Draw Vineyards in the Texas High Plains. “Rain and irrigation help, but that’s a real balancing act because you don’t want to overwater and see plants grow and grow.”
However, adds Pennyroyal, “If at dawn it’s 95 degrees Fahrenheit and it was 95 degrees Fahrenheit all night, they never really get a chance to recover.”
How Vines Respond to Persistent Heat Stress and Climate Extremes
“Plants are incredibly resilient,” says Pennyroyal. Their intrinsic mechanisms for self-protection include the initial closing of the stomata, the upregulation of heat shock proteins (HSPs), the activation of hormones and signaling pathways, and the production of reactive oxygen species and antioxidant mechanisms. These mechanisms activate in response to environmental threats like cold and drought as well.
HSPs are produced in response to stress, and are ubiquitous among living things, not just plants. Crowley estimates that 13 unique HSPs have been identified in plants, and play a critical part of the communication system that signals a cascade of concurrent mechanisms and signaling pathways that work to cool plants down. Pennyroyal describes HSPs as the plant “sending in the EMTs before the fire department arrives.”
At a certain point, “Heat shock proteins can’t be produced quickly enough,” says Crowley. “It’s like that phrase, ‘I put out one fire and another pops up.’ With stress upon stress, heat shock proteins are one more thing the vine has to do in order to exist and put out fruit, which is the biological imperative.” This is one reason that Dan Gatlin, the owner of Inwood Estates Vineyards in Fredericksburg, Texas, believes lightening the “physiological load” can also make a difference. “The most important thing about dealing with heat is making sure you are carrying less fruit,” he says.
Different grape varieties also respond differently to heat stress. “Part of that is adaptation, part of it is genetics,” says Crowley. “For instance, Pinot Noir can handle cold a bit better. Some varieties have a different transport system vessel size and better move water and nutrients throughout their canopy. The same is true for rootstocks.”
In terms of adaptation, “The relationship between the plant and the entire microbiome is important,” says Pennyroyal, adding that grapevines evolved in greener conditions, before much of the planet was deforested, and regional water cycles were more secure. “This is why many viticulturalists are bringing silviculture (tree agriculture), and animals like sheep into vineyards, as well as perennial cover crops, which cool the ground so the microbiome can feed the plant the nutrients it needs. Regenerative farming, which includes some biodynamic practices, is probably our greatest hope for turning the pattern around.”
Nikhila Narra Davis, the co-owner of Narra Vineyards in the Texas High Plains and Kalasi Cellars in Fredericksburg, agrees with Pennyroyal. “As we work on getting certified organic, we run sheep and use weed-removing technology, without the need for harmful sprays,” she says. Crowley adds that cover crops have been shown to cool the ground through infrared work, where vineyards with cover crops reflect a 30 degree Fahrenheit difference in ground temperature.
“We’re looking back to how nature dealt with this in the first place,” says Pennyroyal. “What have we done that disrupted that?”
Achieving Climate Resilience
Heat stress is topical universally, beyond warm wine regions in places such as Texas, Mexico, the Middle East, and southern California. “Extreme heat and then normalization used to be slow,” says Dhingra. “We use the term ‘resilience’ now because it can be the middle of winter, and a plant has accustomed itself to complete cold, and we have a heat spell of 60 degrees Fahrenheit or 80 degrees Fahrenheit for two days, and then the freeze returns.” One path forward is to further specialize in cultivars that have consistently shown resilience and tolerance to persistent heat and quick changes in temperature.
“My optimism around viticulture and climate change is for the discovery or development of cultivars that can thrive under the really variable conditions that we’re being faced with,” says Vanden Heuvel. “Some may already exist and perform well in high heat situations, given their anatomy and habits of sugar accumulation and water use. For example, we can identify genes associated with slow sugar accumulation first, so we don’t get decoupling of brix from everything else.” Hybrids also confer more disease resistance. “They’re robust. They have fruitful secondary shoots and large productive vines,” adds Vanden Heuvel.
“Some varieties have resilience in them,” says Dhingra. “For example, Tempranillo and Tannat can withstand persistent heat.” Crowley also notes Syrah, Albariño, Blanc du Bois, and Mourvèdre as noteworthy performers.
Pennyroyal again encourages expansive thinking. “One response could be linear—shade cloth or clay for UV protection, or the induction of heat shock proteins—but the thing about a crisis is it gets us to recognize how everything is interrelated, and there’s no such thing as something happening in isolation,” he says.
“We currently grow over 80 different varieties in Texas, and trial those in different regions,” says Dhingra. By focusing plant breeding on climate resilience, secondary metabolism in grapes (and other crops) that support adaptation to the growing environment, and advanced physiology—followed by statewide growing trials—Dhingra predicts the identification of “cultivars and growing practices that can withstand these extreme conditions and produce high-quality products despite the challenges.”
Dispatch
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Amy Beth Wright is a creative nonfiction writer and journalist covering wine, food, and travel. She contributes to Wine Enthusiast, Fodors, and The Alcohol Professor, among other outlets. Keep in touch on Instagram @amyb1021 and Twitter @AmyBethWright. Visit amybethwrites.com to read more of her work.
