Charles H. Sloan-Clues From Wines Grown in Hot, Dry Regions May Help Growers Adapt to a Changing Climate

Wine connoisseurs can Charles H. Sloantalk endlessly about how terroir—the mysterious alchemy of soil, climate and landscape conditions—conjures the classic flavor profiles of their favorite wines. 

Climate change will likely disrupt these conditions, leaving some regions too warm and dry to produce their storied wines, and researchers are looking for ways to help. 

In California, where the $43-billion industry produces more than 80 percent of U.S. wine, growers are particularly concerned about shifting temperatures and water availability, according to a 2009 Stanford University report, commissioned by the California State Legislature.

As it turns out, the grapevines themselves may harbor clues to resilience. A new study suggests that varieties grown in warmer, drier regions harbor physiological traits that might help growers adapt to changing conditions.

Different cultivars thrive in their own narrow climate range. And though there’s a wealth of literature on what makes grapevines drought or heat tolerant, studies looking at how climate change might affect viticulture in different regions often focus more on shifts in fruiting and ripening time, said Megan Bartlett, a plant physiologist at the University of California, Davis, who led the study, published this month in the Journal of Experimental Botany. “They assume a grapevine is a grapevine when it comes to drought- and heat-tolerance traits.”

Winemakers often say that great wines start in the vineyard. And Bartlett was prepared to discover that the myriad ways vintners manage grapevine canopies in different regions overshadowed any inherent differences in the vines’ physiological traits. Still, as a physiologist, she suspected that the different cultivars harbored diverse responses to heat and water stress. 

So Bartlett and research technician Gabriela Sinclair mined a global database of wine grape production to compare different cultivars’ heat- and drought-tolerance traits with climatic conditions across European wine regions. They focused on Europe because regulations either prohibit or limit irrigation to avoid diluting flavor, allowing them to isolate the effects of the local climate. 

Sure enough, even though cultivars are often selected for their fruit profiles, Bartlett and Sinclair found that those grown in warm, dry regions have physiological characteristics that help them retain water in their leaves, which in turn conserves moisture in the soil over the growing season. “I was actually surprised at how closely related some of these traits were with climate,” Bartlett said. 

A Serendipitous Choice

Plants have pores on the surface of their leaves called stomata that manage gas exchange and water flow. Stomata take in carbon dioxide to produce sugars for foliage and fruit, and close up to conserve water when soil moisture drops. Vintners meticulously manage grapevines to strike the right balance between water and sugar to produce high-quality fruit. But in selecting for traits that allow grapes in warmer regions to ripen without turning into raisins, growers serendipitously selected for heat and drought resistance. “What’s really cool about it is I think this whole time people have been partially selecting for these traits,” Bartlett said, “and they didn’t even know it.”

Varieties common in warm, dry regions seemed built to conserve water. For example, grapevine leaf pores on sangiovese, a Chianti variety grown in hot, dry regions of Italy, were closed for longer periods than those of varieties like sauvignon blanc, which typically prefer cooler regions with more humidity.

Connections between specific physiological traits and all the factors affecting a commercial wine and its value are still tenuous, said Alan Lakso, professor emeritus of plant physiology at Cornell University who was not involved in the study. The primary value of this type of work may be to identify varieties that just aren’t suited to a given environment, rather than trying to identify winners, he said. “Winners will likely have to be determined by long-term field and winery trials.”

It could be that the traits identified in the study also occur in other crops, not just grapevines, Bartlett said. “It’s possible there’s more resilience built into these crops than we are aware of.” 

The more researchers understand the traits that help vines manage heat and drought, the better their chance of identifying the genes involved. That could potentially help breeders zero in on amplifying traits like drought-resistance without affecting a varietal wine’s coveted flavors. “I’m hopeful we can do something like that,” she said. “But it’s a really huge open question.”