Does water stress exacerbate the impacts of heat stress on berry development of Vitis vinifera cv. Semillon vines grown in controlled environment conditions?

Anecdotal evidence suggested heat stress in grapevines can be reduced if vines are fully irrigated during a heat stress but only a few cultivars have been evaluated. Accordingly, Vitis vinifera L. cv. Semillon vines were grown in controlled environments and treated to a four-day 40 degrees C heat event with and without water stress. Berry size, sugar content, gas exchange and water potentials were measured. Results indicated water stress was relatively mild, but berry growth was reversed when exposed to the heat event and water stress added to this effect whereas control berries continued growing. Similar results occurred with the sugar accumulation process, except sugar immediately stopped accumulating but heat plus water stress reduced the amount of berry sugar further. The stress treatments also impaired photosynthesis and more so when combined with water stress. While reductions in photosynthesis were mostly a result of stomatal closure, but perhaps also non-stomatal limitations, the reduced photoassimilates, especially of the heat plus water stressed vines, contributed to the lowered sugar amounts in the berries. These data strongly supported the conclusion that water stress exacerbated the effects of heat stress. The corollary is heat stress effects on vines will be ameliorated if no water stress is present.