How Wine Grapes Respond to Water Stress---Part 1

During this extended heat of summer, we can see in our vineyard that some of our vines are faring okay, while others are not. I looked at a few references to learn how wine grapes respond to water stress.
The importance of water in the grapevine:

• Greater than 95% of water absorbed by grapevine root systems is transported directly to the canopy and lost to the atmosphere via transpiration
• Water exits the leaves through the stomata, where it is exchanged for CO₂ needed for photosynthesis
• Water that remains within grapevines is used for maintaining cell turgor, building and expanding new cells, translocating nutrients and sugars, providing evaporative cooling, and facilitating gas exchange

Vitis vinifera grape is considered a ‘drought-avoiding’ species, as it contains a large, deep root system. No matter how drought tolerant a vine is, extended drought conditions like we are currently experiencing in southeastern Connecticut can be detrimental to vine health. Just how stressed a vine is, can be measured using a pressure bomb. A pressure bomb is a device that is used to measure leaf water potential, to quantify the water pressure inside a leaf and ascertain how the plants dropped that leaf water potential to compensate for increasingly severe drought conditions.^1,2

If you don't have access to a pressure bomb, we don't, these are signs to look for in drought stressed grapevines:³

• One of the earliest signals of drought stress is reduced shoot tip and tendril growth
• As grapevines approach moderate water stress, shoot growth and leaf expansion cease completely
• Under moderate to more severe water stress shoot tips will dry up and fall off and reduce the apical dominance within the shoot
• Under moderate to severe water stress leaves change angle and orient themselves parallel to the sun’s rays, thus reducing incident radiation and heat load as evaporative cooling associated with transpiration is lost
• Moderate to severe water stress limits photosynthesis via damage to various components integral to light harvesting, electron transport, and carbon fixation by photosynthetic enzymes
• Severe water stress can result in delays in ripening, reduced bud fruitfulness, reduced winter hardiness, and even sudden vine collapse can eventually occur at this stage

References:
1.GoodFruit Grower, Wine grapes show a spectrum of stress.
2. Einat Gerzon, Iris Biton, Yossi Yaniv, Hanita Zemach, Yishai Netzer, Amnon Schwartz, Aaron Fait, and Giora Ben-Ari1, Grapevine Anatomy as a Possible Determinant of Isohydric or Anisohydric Behavior, Am. J. Enol. Vitic., 66:3 (2015).
3. Silvina Dayer, Idan Reingwirtz, Andrew J. McElrone and Gregory A. Gambetta, Response and Recovery of Grapevine to Water Deficit: From Genes to Physiology, Chapter in The Grape Genome, Springer Nature Switzerland AG 2019.
Illustration from Tools of the Trade: Pressure “Bomb” for Measuring Plant Stress; Text from Using a Pressure Chamber in Winegrapes