heat stress, affects vines when air temperatures are high. Very high daytime temperatures, of more than 40 °C/104 °F, cause the vine to ‘shut down’, or virtually cease photosynthesis, as the enzymes responsible can no longer work. High temperatures also lead to water stress, especially when accompanied by bright sunshine, low humidity, and strong, dry winds. High temperatures cause fast respiration in vines and this leads to, for example, low levels of malic acid in mature fruit in hot regions. varietal character and red colour in grapes are also depressed by high temperatures. Australian research into the physiological impact of climate change has shown that, in a biochemical sense, sugar and anthocyanin accumulation in ripening berries is ‘decoupled’ by elevated temperatures. Some researchers claim that more moderate daytime temperatures, in excess of only 25 °C, can depress colour formation and varietal flavour expression.
