Effect of Heat Wave on Grape Leaf Stomata

We are in for a heat wave in the Northeast and may be setting some record temperatures. I wondered what effect this will have on the grapevines. I learned in the UConn Master Gardener course that when the temperatures get too high, this adversely affects sugar accumulation. When I first heard that I was really puzzled. In order to understand this conundrum, I needed to learn about the functions of the grape leaf stomata. The image on the right shows the stomata which are located on the underside of the grape leaf.¹ The stomata functions in the evaporation of water as well as in gas exchange. Carbon dioxide entering through the stomata is converted to carbohydrates via photosynthesis. When temperatures approach 100^o F, the stomata on the leaves close in order to conserve water and the translocation of carbohydrates to the grapes stops. We may actually be okay because I believe that the grapes are currently in the lag phase of their growth. (See Vine Growth) However, if the grapes are in veraison, even 3-4 days of excessively high temperatures (>105^o F) may retard ripening. Once temperatures have returned to normal, sugar accumulation can resume.²
1. Regents Biology Chapter 19
2. Nick Dokoozlian Grape Berry Browth and Development