Water stress is a physiological state that grapevines experience when there is insufficient water supply. Some of the physiological responses of grapevines include effected cell development, closing of leaf stomata, reduced photosynthesis and, in the worst case, cell dehydration and death of the vine.

Most of these responses are dynamic (as the level of water stress increases so does the response). For example, leaf stomata (which influence photosynthesis and hence potential sugars) do not completely close at the first signs of water stress, but slowly close as water stress increases. The physiological reaction of a vine to water stress will affect the growth and development of the shoots, leaves and fruit depending on the timing and level of water stress during the growing season.

Water stress may also have less obvious or indirect effects on fruit yield and quality. For example, reducing berry size increases the skin to juice ratio, which may increase the concentration of anthocyanins and phenolics of red grapes.

While climate and humidity play important roles, a typical grape vine needs approx 650-900mm of water a year, occurring during the spring and summer months of the growing season, to avoid stress. The presence of water is essential for the survival of all plants. In a grapevine, water acts as a universal solvent for nutrients and minerals needed to carry out important physiological functions-which the vine receives by absorbing the nutrient-containing water from the soil. In the absence of water in the soil, the root system of the vine may have difficulties absorbing these nutrients. During the process of photosynthesis, water molecules combine with carbon derived from carbon dioxide to form glucose which is the primary energy source of the vine as well as oxygen being a by-product.

While there is disagreement over exactly how much water stress is beneficial in developing grapes for quality wine production, most viticulturists agree that some water stress can be beneficial.