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

Heat Wave and Our Cabernet Sauvignon

The summer of 2020 has been one of a series of heat waves. According to our local news, we have had 30 days of above 90^o F temperatures. Even the weeds in our vineyard are dying and the grass under foot is crunchy.
What does this do to grapes? In July, I blogged about Leaf Pulling in the Cabernet Sauvignon, exposing both sides of our canopy. We began leaf pulling around 10 days after fruitset, but during the time when temperatures were in the 90s.
This is the result:

Fortunately, not all of our Cabernet Sauvignon look like raisins. A good deal of the grapes survived our well intentioned but ill timed leaf pulling and look like this:

My husband informed me that what we were supposed to do was to leaf pull from the east side before veraison and then leaf pull the west side of the canopy after veraison. Lesson learned!

Why Some Oak Aged Wines Can Taste Bitter

I saw this article in one of my wine feeds which appeared in Wine Spectator. It was called French Researchers Unlock a Secret to Wine Bitterness: Oak Barrels. This article peaked my interest so I decided to read the original article.
Historically, oak barrels have been used in the transport of wines. Now, oak barrels are used in the fermentation and/or aging of wine. It is well known that this contact can modify the sensory properties of the wine. Compounds in the oak are responsible for changes in color, aromas, tactile sensations and taste. For example, a gain in sweetness is frequently observed during aging and can be explained by the release of sweet triterpenoids from oak wood.
Occasionally, however, aging in barrels can sometimes increase the perception of bitterness. This bitterness was initially attributed to ellagitannins, but their low concentrations suggested their limited influence on wine bitterness.
The authors of this research set out to identify the compounds presumed to be responsible for the sensation of bitterness in white and red wines.
Oak wood contains (+)-lyoniresinol which is known to be very bitter as well as coumarins, which are secondary metabolites originating from the phenyl-propanoid pathway via t-cinnamic acid.
Coumarin is well known for its characteristic odor reminiscent of vanilla pod but also of almond, together with the scent of cut hay. The authors focussed their research on coumarin and coumarin derivatives. They noted the presence of esculetin, fraxetin, umbelliferone, and coumarin in the oaked red wine, whereas scopoletin and 4-methylumbelliferone were not detected in red wine.¹

Tasting evaluation of the six compounds, esculetin, fraxetin, umbelliferone, coumarin, scopoletin and 4-methylumbelliferone showed that five of the six targeted coumarins were described as bitter. 4-methylumbelliferone was the most intense, followed by esculetin, scopoletin, then coumarin and umbelliferone.
In addition, 90 commercial wines, 44 red wines (24 from Bordeaux, 6 from Burgundy, 2 from Languedoc-Roussillon, 2 from Rhône Valley, 1 from Loire Valley, 1 from Beaujolais, 7 from Spain, and 1 from Germany) and 46 white wines (13 from Burgundy, 11 from Alsace, 9 from Bordeaux, 5 from Loire Valley, 4 from Languedoc-Roussillon, and 4 from Rhône Valley) with vintages from 1995 to 2017 were assessed for the coumarin concentration. Using the LC-HRMS method previously validated, different concentrations of the various coumarins were observed depending on the sample. In general, higher concentrations of coumarins were observed in red wines, possibly because more new barrels are used in the aging of red wines.
Apart from their presence, the sensory contribution of coumarins to the taste of wines and spirits remained largely unclear.

References:
1. Delphine Winstel, Eric Gautier, and Axel Marchal, Role of Oak Coumarins in the Taste of Wines and Spirits: Identification, Quantitation, and Sensory Contribution through Perceptive Interactions,J. Agric. Food Chem., 2020, 68, 7434−7443.

Weird Surprise in the Vineyard

Yesterday, it rained in the morning and then cleared and ended up being a steamy kind of day. The continuation of the hot, humid days are really no inducement for us to go back to the vineyard for the second shift. But, the vines called and we answered.
We are still shoot tucking, but now in our Chenin Blanc. My husband and I were working from the north end and tucking the shoots until we reached our marker for the half way point. We were doing the second alley and I was working my way back, when he said, "You need to see this!" The "this" was this. A perfectly laid row of some kind of eggs and then something kind of spiky on a trellis wire.

We took several pictures and went home to interrogate the internet for what this weird juxtaposition of eggs and spikes could be. Even after a few hours of searching, neither my husband nor I could come up with what this was. I then searched on "insect eggs in a row" and the first image to pop up was exactly what we took a photo of! We found the photo on The LarvalBug Bytes Archives and the site identified what we had as owlfly eggs!

The owlfly belongs to the order Neuroptera which also includes Antlions, Lacewings, Mantidflies and Allies. They belong to the family Ascalaphidae. The adult form kind of looks like a dragonfly. What is so exciting about finding these eggs, aside from the fact that they are so unusual looking, is that the larvae as well as the adult owlfly are voracious predators. The adult cruises the air, snatching and eating insects as they fly! In our book, an insect that eats other insects is a keeper!
References:
1. The LarvalBug Bytes Archives.
2. Information about the aborted owlfly egg is from Bug Guide.
3. Missouri Department of Conservation: Owlflies.