The grapevine is more resistant to summer drought than had been thought, scientists reported Wednesday in the open-access journal Science Advances based on a 10-year study. Growers prone to prophylactically overirrigating vineyards lest their babies dry up and blow away can probably stop doing that, despite forecasts of intensifying aridity throughout the American Southwest, parts of Europe, the Sahel and the Levant under conditions of global warming.
“Previously, the scientific literature had reported confusing results about the vine’s resistance,” lead author Guillaume Charrier told Haaretz. “We believed that most of this confusion stemmed from inaccurate measurements.”
Setting out to reassess the plant’s behavior, the scientists found that vines studied in Napa, California and Bordeaux, France resist drought better than assumed, compared with other plants and compared with expectations of the grape itself.
The large team did find small intervarietal differences, between Syrah and Grenache, for instance. But the bottom line is that grapes are tough, period.
People have been making wine for at least 8,000 years, going by the sediment found in giant Neolithic wine jars found in the Caucasian hills. (The prehistoric Chinese were making a mixed drink involving fermented juice from grapes and other fruit even earlier but their tipple isn’t considered a “true wine”).
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Ergo, the grape has been an important crop going back to the dawn of modern civilization and it remains so, though admittedly, the total world area planted with grapevines has crept down from 7.8 million hectares to 7.5 million in the last 15 years.
The vine’s hardiness when times turn tough could be one reason for the industry’s persistence. Grapevines aren’t petunias: they’re perennial. You plant a vine and hope to reap fruit for decades. Combine that thought with the expectations of mounting freshwater stress in the world and you get frightened farmers fretting about what to plant in marginal zones that are, in some cases, already showing signs of drying out.
Moreover, despite the current flooding in France, much of Europe and all of the Levant and Mediterranean basin anticipate mounting aridity as climate change bites down. For Israel, where wine exports totaled $40 million in 2016, and for Europe, the hardiness of the vine is good news. But how hardy is it, exactly?
The grapevine and the embolism
When water-depleted, plants become “drought stressed:” They emit more water by breathing than they absorb through their roots.
In water-starved plants, normal biological processes including photosynthesis slow or stop. Growth is constrained. Leaves shrivel.
Somehow it hadn’t been scientifically noticed that under seasonal (i.e., summer) drought conditions that cause many other plants to flop over and die, grapevines do not. Note that there are thousands of varieties of Vitis vinifera growing in Europe, the Middle East and North Africa.
In drought, the viniferas sort of turn into temporary zombie vines, not doing much and barely breathing, but not going gentle into that good night, either. “The berry developmental stage is observed at the period of highest drought stress,” Charrier observes.
Plants regulate water loss by opening and closing their leaf pores (the stomata). When drought strikes, the stomata close. The idea is for the plant’s breathing to decrease before the internal conditions beckon embolism.
Embolism is the same in plants and people. A circulatory vessel (blood, water) gets blocked by air bubble or crud. It is deadly in both. Embolism is common in all plants, Charrier points out: it’s just that different species have different dehydration thresholds for embolism to develop.
Grapevines are perverse
Not being desert dwellers, grapevines had been assumed to be prone to embolism in summertime on a daily basis, Charrier explains. They are not.
The grapevine’s vulnerability to embolism actually decreases as drought stress increases, in contrast to expectations of other plants, the team found.
Embolism resistance under drought stress was found in all viniferas varieties they checked, he says. “I would not expect big differences at the intraspecies level,” says Charrier: differences would probably be greater at the intragenus level.
Crucially, under conditions of seasonal drought, the vinifera vines they observed never experienced hydraulic failure — namely, utter failure of their water transport systems, says the team. They didn’t die even under very dry conditions, though obviously, there are thresholds beyond which the climatic conditions would be fatal even to the hardy liana.
"With the dynamic and quantitative thresholds measured in this study, [growers] could control the level of stress experienced by the plant at any date, and adapt their management to the available water resource," says Charrier.
At the end of the day, the world could save fresh water on irrigating vineyards so people can have their Chardonnay and drink it too.