In a new study, INRAE (the National Research Institute for Agriculture, Food and the Environment) and the University of Bordeaux have established a ranking of 30 grape varieties used around the world according to their tolerance to drought. The vines were grown in local greenhouses under the same environmental conditions in order to evaluate the resistance of their vascular system to water stress.
This was achieved through the use of a mega-cavitron—a device capable of characterizing the cavitation vulnerability of plants by measuring the water flow in twigs.
"This is a first," said Sylvain Delzon, a specialist in ecophysiology at INRAE and spokesperson for the research team. "We were unable to measure the resistance of vines to drought up until now."
According to the data, Ugni Blanc and Chardonnay counted among the most vulnerable traditional grape varieties, while Pinot Noir, Merlot and Cabernet Sauvignon ranked among the most resilient. Syrah, popular in the Rhone Valley, and Alsatian Sylvaner were also in the top third of the ranking.
"For the Bordeaux region, there is no urgency to change grape varieties, regarding this criterion. Its driest years have been the best vintages. The harvest yields have been lower than average, but the wines’ standard of quality is excellent," stressed Sylvain Delzon.
The winegrowing regions potentially most at risk do not necessarily have arid climates, but rather carry a significant proportion of vulnerable varieties, the study showed, citing the example of Cognac, France and Marlborough, New Zealand. As a result, Sylvain Delzon recommends diversifying the use of grape varieties. In the event of drought, they will react differently, thereby limiting losses.
Biological diversity also leads to improved resistance to diseases. In the United States, there are spots where Chardonnay has been widely planted, but Pinot Noir is also present, so the risk is less important than in the aforementioned locations. "Lassen County, comprising Chardonnay (67%) and Pinot Noir (33%) is a Californian winegrowing area, which could potentially suffer from dry-spells," pointed out Delzon.
How the Mega-Cavitron Works
Cavitation is the formation of an air bubble in the vessels of plants (the xylem), which damages the water column and thus makes the vascular system unsuitable for transporting sap. This happens during severe dry-spells and ends up killing the plant when it reaches significant levels.
The working principle of the mega-cavitron is based on the use of a centrifugal force to generate negative pressure in the xylem to simulate drought. By taking measurements at different speeds, the machine provides a vulnerability curve in less than 20 minutes, thereby offering the possibility of rapidly comparing species or genotypes with regard to cavitation vulnerability and therefore tolerance to water stress.
Implications
The study’s results could discourage the introduction of mildew-resistant varieties, of which around 40 are currently authorized for vine-growing in Europe. In France, around 20 Protected Geographical Indications can now make wine from some of these varieties, largely to reduce the use of fungicides.
However, and paradoxically, some hybrids—varieties genetically modified to deal with diseases—appear to be the least resistant to water stress. According to the study’s results, three new varieties from the ResDur program (sustainable resistance) added to the official French winegrowing catalogue in 2018 were the most sensitive to drought.
Floréal and Voltis, two white grape varieties known for their average productivity and second-period maturity and Vidoc, a high yielding late ripening black grape variety were ranked among the least resistant.
"Fifty days without any water would be enough to kill Floréal compared to 150 for Pinot Noir," stressed Delzon. As a result, "The choice of grape varieties must be multifactorial, rather than limited to resistance to global warming or diseases."
In view better monitoring the profiles of vineyards worldwide, Sylvain Delzon is currently creating a map of winegrowing areas in the Northern Hemisphere to identity which ones are most concerned by drought.
Source: WineBusiness Monthly