Grapevines are drought resistant enough that California’s wineries may be able to dial back the irrigation and keep their plants alive, new research says. That’s good news as the state heads into what looks to be another dry year. There is a catch, however: while the study suggests vines might survive a thirsty spell with minimal watering, it doesn’t say if they’ll still make enough grapes to keep wine glasses full.
Researchers investigated how different varieties of grapes reacted to drought conditions: they spun the stems in a centrifuge and left the vines without water to find out when they start dying of dehydration. All of the varieties tested, including cabernet sauvignon and merlot, were found to be more or less equally resistant to drought, contrary to previous research that said some were more tolerant than others. The researchers also looked at years of data collected by wineries in Bordeaux, France and Napa, California. The grapes in these regions didn’t reach lethal levels of thirst even during California’s recent drought, according to a study published today in Science Advances. Pinpointing just how dehydrated vines can get before they die could be key to keeping wineries afloat when water supplies dry out.
Thirsty grapevines can run into trouble when they try to suck up water from dry ground. When they suck too hard, air bubbles can form in the delicate pipes that transport sap, blocking the sap from getting where it needs to go and killing the plant. The researchers wanted to test how easily one of those dangerous air bubbles could form in five varieties of grapes — grenache, syrah, cabernet sauvignon, regent, and merlot — to find out just how readily a drought could kill them.
The team spun grapevine branches in a centrifuge. The idea is that the centrifuge’s fast spin lets gas inside the sap expand into air bubbles just like the ones that would occur if a thirsty vine tried to suck up water too hard. Then they measured how much those bubbles blocked the flow of sap. They found that as the vines grew from soft and green in May to tough and woody in October, their vessels also got tougher and more resistant to those air bubbles forming. All of the varieties they tested were equally resistant to those air bubbles, meaning they’re all equally resistant to drought.
Then the researchers tried to kill their grapes by cutting off their water supply. To track how dehydrated the vines got, they measured how much pressure it took to wring water from their leaves (relatively little in a well-watered leaf and a lot in a dry one). They found that when the vines dried out so much that half the sap stopped flowing, they lost their leaves and didn’t fully recover even after being watered again.
The grapes didn’t get to that point in the fields of Bordeaux or Napa that the researchers analyzed; both underwent recent dry periods, according to the paper. Of course, most of those vineyards in Napa were irrigated, and the researchers don’t know how much water the wineries used, so they can’t tell the wineries how much they can afford to cut back.
These new findings could give wine growers a way to carefully titrate water levels: if the vines dry out by more than 50 percent, then they might not survive. If it’s less than that, they might just make it. “Now you know the thresholds where it can kill the plant,” says Guillaume Charrier, a plant physiologist who led the study while working at Bordeaux Sciences Agro. That could help cut down on over-watering when water supplies dry out. “I think in California you have quite a lot of troubles in the summer with that, and that would help — at least a bit,” Charrier says.
The findings are a rosier take on the usually dire predictions about how climate change could kill California’s wine industry, says Markus Keller, a professor of viticulture at Washington State University who was not involved in the research. “It takes a step back and says, ‘Look, guys, everything is not doom and gloom. Grape vines will still be able to grow,’” he says. But even though the science itself is sound, he wishes that the study authors had made it clearer that the Napa wineries were irrigated — so the findings don’t mean the grapes will survive a dry spell without being watered. “Would they be able to grow those vines without irrigation and still not leave your glass empty?” he says. “I’m convinced that glass would go empty very quickly.”
That’s true. Grapevines can die if you stop watering them, says study author Gregory Gambetta, a professor of viticulture at Bordeaux Sciences Agro. Instead, the point was to find out just how much dehydration the grapevines can withstand. That’s key if wineries have to cut back on their water use during droughts. If you knew how little water you could get away with using, he says, “You would totally decimate your productivity for a couple years, but you wouldn’t have to rip out your vineyard.”