Simulated heat waves to help vineyard owners mitigate heat stress

Researcher standing among grapevines in growth chamber.
WSU’s Ester Hernández-Montes can closely monitor changes to grapes and grape vines in specialized growth chambers.

By creating “heat waves” in environment-controlled growth chambers, WSU post-doctoral research associate Esther Hernández-Montes aims to help growers better mitigate the effects of heat-stress on wine grapes.

Because heat-stress and drought are becoming common concerns in arid grape-growing regions like Washington state, growers are looking for tools and methods to help protect their crops from these often-unpredictable conditions while simultaneously improving grape quality.

Hernández-Montes’ current project stems from previous work in which her team focused on the effects of heat waves on physiological processes like photosynthesis, plant growth and grape quality.

“In the previous study, we found that acidity and pH were affected by heat waves,” Hernández-Montes said.

This finding is important because both acidity and pH are key parameters that affect wine quality, along with chemical and microbial stability and sensory properties.

It now appears that temperature is the main driver affecting these parameters.

“We learned that excessive heat during grape ripening decreased acidity and increased pH,” she added.

Now they are focusing solely on grape acidity, studying the relationship between organic acids, pH and potassium, specifically in grapes grown for white wines.

“Consumers usually want a refreshing white wine,” she said, “and acidity is an important factor in white wines.”

Significantly, the team found that heat waves actually accelerate the degradation of the malic acid in the grapes, potentially affecting that acidic trait so sought after in white wines.

“We are studying in detail the effect of irrigation, different intensities of light exposure of the grape clusters and the temperature in the clusters on acidity, pH and potassium, looking for variations during the developmental stages,” Hernández-Montes said.

Identifying any variations or changes in acidity, pH, and sugar content during different growth periods of heat-stressed grapes is vital when it comes to recommending best irrigation and canopy management practices for growers in drought-prone climates.

The team, led by Markus Keller, found that plants will recover after one or two days of a heat wave, but prolonged periods may require a much more methodical and sustainable approach.

For their experiment, the team used the growth chambers to simulate temperature conditions that have occurred historically in the field. They then identified three stages to focus on: bloom, veraison (i.e. when grapes change color), and post-veraison.

“First, we download the temperature data from different meteorological stations in our area from the last 10 years,” Hernandez-Montes said.

Then the team sets the growth chamber temperatures to mimic the 10-year hourly averages for each phenological stage.

“For instance, in the bloom-stage experiment, we averaged the temperatures from the month when bloom occurs in the field and simulated those in the chamber. Then we did the same for the veraison and post-veraison stages,” she said.

From there, the team can adjust the chambers by both temperature and light intensity.

Thermal image of grape clusters.
Thermal image of grape clusters from Hernández-Montes’ research.

“Each growth chamber has a computer that allows us to change the conditions hour by hour. We also adjusted them for day and night, to simulate the real conditions in the field.”

Learning how the vines, fruit, and the acids in particular respond to simulated heat stress using different soil water contents will help give growers a nimbler set of management skills, especially when it comes to irrigation.

“In a heat wave or drought conditions, we want to know what the best irrigation practices are for each phenological stage of the grape’s maturity,” she added.

Although the team is currently focusing on grapes for white wines, the same studies can be applied to grapes for red wines as well, potentially helping all wine-grape growers across the state.

By utilizing the four growth chambers in Markus Keller’s laboratory, Hernández-Montes isn’t limited by seasons or environmental conditions to conduct her research, allowing the team to monitor their subjects year-round.

“Using this technology, we can simulate heat waves whenever we want and at whatever phenological state we want,” she said.

Hernández-Montes and the team hope their research will provide yet another critical tool growers can utilize not just for mitigating heat stress, but for unlocking new and innovative ways to think about growing high-quality wine grapes in the state of Washington and beyond.

This research was funded by Washington State University, Auction of Washington Wines, and all of Washington’s wine grape growers and wineries in partnership with the Washington State Wine Commission.