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The high heat and lack of rain continues to have an impact on vine productivity over the last several days. At the Fredonia lab, we are now 5.6 inches of rainfall behind and 145 GDD ahead of the long-term average. Those of you in that received 1.5 inches of rain in the Westfield area are not as far behind but are still having water concerns.
As per my last update, we continue to take pressure bomb and photosynthesis measurements to monitor vine response to the drying soil conditions.
This figure shows how the vines continue to decrease midday leaf photosynthesis from Aug. 1 to Aug 6. If maximum leaf photosynthesis values are between 18 and 20, Balanced un-irrigated vines were only down about 25% on Aug. 1. By Aug 6, photosynthesis on the same vines was down about 50% from the optimum. As of Aug. 8, pressure bomb readings for irrigated vines are right around 8, indicating mild water stress, while non-irrigated vines are in the mid-10's, indicating higher water stress.
Can the high heat impact leaf photosynthesis? Yes, excessively high leaf temperature can also reduce net photosynthesis. This is usually a bigger problem in the hot Central Valley of California - not on the shores of Lake Erie. There appears to be a broad leaf temperature range for max potential photosynthesis, from below 60 to around 90 degrees F. Leaf temperature tends to be a few degrees higher than air temperature. So if the air temperature in the vineyard is 90 then leaf temperature may be around 93 and net photosynthesis may start to be inhibited.
Water stress is clearly a bigger factor in controlling leaf photosynthesis than leaf temperature on own-rooted Concord in Western, NY. However, the combination of water stress and heat stress does not help our situation. As leaves close their stomata to conserve water, they reduce photosynthesis but they also lose some of their ability to cool themselves on a hot day in direct sunlight (water stressed leaves feel warmer to the touch). Leaf temperature can increase further and compound the photosynthesis problem.
We will continue to monitor vine water status and report our findings to you. I wish I could make it rain for you. Stay cool and think wet!