By: Becky Garrison
During a panel at the Oregon Wine Symposium held virtually from February 16 to 19, 2021, Simone Castellarin, Ph.D., of the Wine Research Centre, University of British Columbia, presented research that spoke to the biological mechanisms that determine grape and wine quality. Also, Nazareth Torres, Ph.D., of the University of California, Davis, unpacked her findings in sustainable irrigation.
Alexander Levin, Ph.D., a viticulturist at the Southern Oregon Research and Extension Center and assistant professor in the Department of Horticulture at Oregon State University, opened this conversation by summarizing the Fifth Oregon Climate Assessment findings.
“Hot summer days are projected to become more frequent in Oregon under continued global emissions of greenhouse gases, and overnight lows will continue to become warmer. The frequency, duration and intensity of extreme heat events are expected to increase. Not only are summers expected to warm more than annual average temperatures, but the hottest days in summer are projected to warm more than the mean summer temperature over the Pacific Northwest. The hot summers of 2015 and 2018 are salient examples of summer temperatures that are expected to become relatively common by the middle of the 21st century.”
Given the ongoing rise in temperatures, Levin said a major challenge for vineyards is mitigating the impacts of these climate changes. In particular, how do grapevines respond to extreme heat and drought, and what can be done about this in the vineyard?
Biological Mechanisms that Determine Grape and Wine Quality
Castellarin opened his presentation by commenting on how studies show that drought events affect grapevine physiology by limiting transpiration, photosynthesis, canopy, berry growth and yield, as well as impacting fruit composition. In particular, the topic of managing drought in vineyards has been investigated for many years and has always been a hot topic. “We know we need water to manage grape production. But we also know that we have to treat water carefully because by applying water, we might affect the quality of the grapes and also the quality of the wine. So we want to optimize the use of water in vineyards to optimize the quality of fruit and wine,” he said.
The major compounds affecting the quality of grapes and wines are aromas and phenolics. As most of these compounds are synthesized in the berry’s skin, their concentration can be affected when the size of the berry is reduced or increased through irrigation management.
Improving Berry Quality by Reducing Berry Size
Smaller berries tend to have a higher concentration of aromas and phenolics because they are synthesized in the skin. Less flesh, and the water produces a higher concentration of these com-pounds. By increasing the amount of water in the flesh, the water dilutes these compounds. The addition of water can occur via irrigation or high precipitations.
Castellarin cited studies that show that by applying some level of deficit before variation, after operation, or prolonged levels of deficits during the season, growers decreased their yield but increased the concentration of total phenols.
In his research, Castellarin found that applying a water deficit in vineyards increased the concen-tration of pigments and tannins, not only by reducing the size of the berry but also by stimulating the biosynthesis of these compounds. He also observed that several aromas were affected along with the color of the wine. In addition, wines produced from vines exposed to this water deficit had a darker or higher intensity of color and aromas associated with red fruit.
The effect of drought or water deficit application on white grapes is less noticeable. Water deficits in the vineyard are not often applied to white grape varieties. In a 2012 study on white grapes grown under severe stress, researchers observed that the fruit harvested from those stressed grapevines had a higher concentration of terpenes.
Researchers also observed that the trapped water stimulated the metabolic pathways that synthesize terpenes. That means that it did not only affect berry size but significantly affected the biosynthesis of specific metabolic pathways that work during berry ripening. The study showed that what they saw on the grapes directly affected the wine, and that by applying a water deficit, they could increase the concentration of these aromas.
A three-year study was conducted in the Okanagan Valley in British Columbia, a significant Canadian wine region and an area called the infamy desert “Nk’Mip” by Canada’s First Nations. This semi-arid shrubland has very low precipitation, with only 100 to 130 millimeters of rain during the growing season. In this study, researchers sought to develop strategies that limit irrigation by applying moderate stress levels to the grapevines to improve aromas and save water.
They wanted to assess how providing suboptimal irrigation amounts could affect yield and the composition of the fruit at harvest. The study focused on regulated deficit irrigation, managing irrigation so the plant receives regulated stress. They decided on moderate stress conditions that they knew would not strongly affect the plant’s growth but that they hoped could strongly affect the quality of the grapes.
The study concentrated on Gewürztraminer, a variety known for its aroma. The study started with a control treatment: standard commercial irrigation, which would not put the plant under stress. They applied stress 30 days from blooming to harvest for a moderate level of prolonged water stress. They analyzed brief gas exchanges, photosynthesis, all the physiological parameters related to the grapevine and physiology, and then the compositional parameters of the fruit like sugars, acids and free and bound terpenes.
According to Castellarin, they managed irrigation by weekly measuring leaf water potential and then applying irrigation volumes accordingly. “The irrigation volume change obviously depending on treatment. With the early deficit (a deficit applied from 30 days after blooming to veraison), we could save 30% of the irrigation water. With late deficit (a deficit applied from veraison to harvest), we saved 38% of the irrigation water. With prolonged deficit (a deficit applied from 30 days after blooming to harvest), we saved 50% of the irrigation water,” he said.
When they applied some level of stress, one of the first responses was a reduction in leaf gas exchanges. Moderate stress levels reduced photosynthesis by 50%, a finding that was pretty consistent across the seasons.
While early and prolonged deficit treatments reduced yield, applying deficits later on during the ripening process did not affect the number of grapes grown or harvested. When they measured the grapes for composition, total soluble solids and acidity, researchers found that the irrigation treatment with the most significant effect was the prolonged deficit, which reduced sugars and increased the acidity.
Across three seasons, the late deficit treatment consistently improved the concentration of some of the free terpenes, particularly Geraniol, which is the primary terpene synthesized in Gewürz-traminer grapes. They did not observe an increase in the bound terpenes. Researchers learned that if they applied a moderate water deficit during ripening, they could save 38% of irrigation water and increase the aromas of the grapes.
Castellarin noted the tension between grape quality and commercial viability when factoring in how much growers can stress their grapes. For example, when conducting a study on Merlot grapes, researchers found that they limited the yields to an unsustainable point for commercial vineyards when they applied severe deficits.
Considerations of Sustainable Irrigation
According to the California Water Resources, irrigated agriculture in California is the largest consumer of water, accounting for about 80% of the state’s water supply. Also, California’s climate is changing to limited or no cloud cover and a warming trend with no increase in precipitation supply.
To evaluate if standard irrigation practices are economically sustainable, Torres summarized the results of a two year study conducted at UC Davis Oakville Experimental Vineyard in Napa County, California. This study sought to investigate the effect of different irrigation amounts on the plant physiology and berry quality of Cabernet Sauvignon grown in the Napa Valley and evaluate the impact of these irrigation practices on water resources and arbuscular mycorrhizal fungi associated with grapevines.
Similar to Castellarin’s findings, this study found that water deficit irrigation strategies reduced the amount of water applied to grapevines, decreasing the water footprint and maintaining or increasing grape quality at the cost of some reduction of potential yield.
This study also found that the ecosystem services provided by arbuscular mycorrhizal fungi increased plant resistance against biotic stresses while reducing photochemical input and plant resistance to abiotic stressors such as drought, salinity, metals and other mineral nutrient depletion. These fungi can reduce the fertilizer requirement by promoting plant growth and increasing plant quality for human health while improving soil structure, stability and water retention.