Grapevines / Heat Damage

A couple of days of 40C deg plus and some fierce drying winds have taken their toll on the grapevines.
When you take into consideration that the official temperatures are taken in the shade it's not hard to imagine what the exposed temperatures would be. Some estimates are 10-20 degrees higher.
Plants draw moisture up from the soil via their roots and expel it mainly through their leaves in what is known as transpiration.
When the rate of transpiration exceeds the rate of absorption the plant comes under stress.
The plant tries to compensate by closing the stomata in its leaves to prevent water loss but under extreme conditions of heat, air movement and low humidity this is not always possible.
This is when the leaves start to wilt or droop. If a plant reaches permanent wilting point (PWP) it will more than likely die.

The lack of moisture in the leaves makes them susceptible to burn. We noticed this happening with the grapevines very early in the heatwave as we have had very little rain and the soil moisture content is very low.
A couple of days later the leaf damage was more than obvious.
These damaged leaves will eventually die and fall.
A reduction in the number of leaves will affect the ability of the vine to ripen the fruit.

But the leaves weren't the only plant part affected. The fruit was also dehydrated by the heat with considerable raisining of the bunches.

All this will probably mean a considerable reduction in yield.
This is very disappointing as the vintage this year had been progressing well and harvest is or, maybe now, was only a few weeks away.
The worst affected is the Pinot Noir followed by the Tempranillo.
The Cabernet seems to have weathered the storm a little better as the vines are much older and the roots a lot deeper.
Only time will tell.