Grape Health During a Heat Wave

We have been having an unusally hot summer, punctuated by heavy rainfall. We have been seeing some signs of berry stress in our Chenin Blanc where the berries in some clusters have been raisining. Here is a minor example of what we are seeing. I am bringing my tweezers to the vineyard so that I can clean these sunburnt grapes from the cluster.

I was telling my husband about what I was seeing and then I came across an article in Wine Industry Network, written by Mark Greenspan called "Growing High Quality White Wine Grapes".¹ It was a timely article to be reading because we are now in the lag phase of berry growth and experiencing high heat bordering on drought conditions and his article was focussed on this phase of berry growth.
White wine grape growing is more difficult than red wine grape growing. A white grape is inherently more vulnerable to degradation since it doesn’t have an arsenal of polyphenols protecting it which is the case in red grapes because they have anthocyanin compounds for protection.
However, a white grape does have some means of protection which can generate aromatic compounds if the stress event occurs during the grape lag phase.
Benefits of heat stress during grape lag phase:

• aromatic compounds are produced in abundance during periods of abiotic stress, such as a heat wave, because they help protect the plant from oxidation
• water stress imposed during lag phase upregulates the synthesis of many secondary metabolites associated with better color and mouthfeel
• other compounds associated with water stress imposed pre-veraison include thiols or mercaptans, organic compounds containing sulfur that are associated with passion fruit, guava, and grapefruit aromas and flavors
• Glutathione is an important compound in helping white grapes with stress
  
• glutathione is synthesized in many plants in response to abiotic stress such as drought, this compound breaks down into thiols, which accounts for the many desirable aromatic compounds in wine
• glutathione is integral in protecting plant tissue from oxidative stress, which is more important in white grapes given that the antioxidant anthocyanin is lacking
• biochemical pathways responsible for terpenes and 13-norisprenoids (derived from carotenoids) are upregulated by water stress between lag phase and veraison, these are the compounds associated with floral and earthy aromas

The link between phenolics and water stress is well-established. In places where irrigation is practiced, such as California, farming practices can be manipulated to increase the tannins and color by planning the appropriate time to irrigate after hot weather.
This article recommended that excessive leaf removal be avoided. Where severe heat events are on the rise, shade cloth and microsprinklers are being used to cool the fruiting zone.
My husband and I are quite conservative when it comes to leaf pulling. My husband is of the mindset that the leaves on the west side of the canopy should be left in place in order to protect the grapes from the heat of the afternoon sun. Currently, we are working in our Chenin Blanc, hedging, shoot thinning, shoot tucking, shoot positioning, and leaf pulling. This is what the canopy looks like on the left side and how the leaf pulled grapes look like on the right side of the photo.

We are trying to strike a balance between air flow, which is critical for maintaiining a clean canopy and grapes and preventing the grapes from getting too much sun so that they do not raisin before veraison.
The weather for the first few days in August shows no rain, but also thankfully, no temperature hitting 90 degrees!
References:
1. Mark Greenspan, Wine Industry Network, "Growing High Quality White Wine Grapes.