Back to the future: not all grape varieties succumbed to the extreme heat wave of 2019

46°C in the shade. On 28 June 2019, France’s highest temperature ever was recorded in Vérargues, in the southwest Hérault region, caused by a mass of burning-hot air from the Sahara Desert. Just a few kilometres away, scientists from Institut Agro and INRAE were conducting a study involving more than 250 grape varieties grown in pots in the Pierre Galet experimental vineyard in Montpellier. These conditions enabled the team to conduct a full-scale assessment of how different grape varieties respond to extreme temperatures.

A few hours after the temperature peak, part of the canopy had literally burned. What was the surface temperature of the leaves most exposed to the sun? According to the researchers’ estimates, nearly 54°C — a temperature above the viable thermal limit for many plants.

However, not all grape varieties suffered the same fate: some suffered severe damage while others remained unscathed. The scientists then used association genetics to compare the observed symptoms with the genetic information available on the diversity of the grape cultivars. By doing so, they identified the parts of the genome involved in the responses.

Six regions of the genome were found to play a role in heat tolerance. Researchers identified clusters of genes in these regions but have yet to confirm whether only one is important, or whether they all work together. These genes are linked with the management of oxidative stress (whereby molecules are produced which destabilize plant cells) and high temperature signalling, but not transpiration — a surprising result given that transpiration could have lowered the surface temperature by almost 5°C.

This result could arise from a trade-off between the plant’s need for water to cool down and the availability of water in the soil. Heatwaves often coincide with drought episodes, and maintaining water in the soil and inside the plant is just as crucial as regulating its temperature.

The intensity and frequency of extreme climate events will increase in the future. The June 2019 heatwave allowed researchers to explore some of the genetic potential of grapevine under such conditions. The findings of this study could enable important progress in improving grape varieties, and potentially other species, as a solution to adapt crops to climate change.