Water stress and drought

This is the clearest example in viticulture of a stress being useful in the right amount, and it is why "the vine must struggle" contains a real idea alongside a great deal of romanticism.

It is also becoming the central constraint on viticulture in several major regions, which changes it from a quality question to a viability one.

Stomata are the pores through which a leaf exchanges gases. As soil water declines, the vine closes them to conserve water, which also restricts the carbon dioxide uptake that photosynthesis requires.

Mild stress after fruit set slows shoot growth before it slows photosynthesis meaningfully. The vine stops building canopy and puts resources into the crop, and berries stay smaller — which raises the skin-to-juice ratio and therefore the concentration of skin-derived compounds.

Severe stress closes stomata far enough that photosynthesis effectively stops. Sugar accumulation halts, and the sugar concentration that continues to rise does so only because the berries are dehydrating.

That distinction matters enormously in practice: a wine made from dehydrated fruit has high potential alcohol and undeveloped flavour, which is precisely the modern over-alcoholic, hollow style.

Prolonged severe stress causes leaf loss and can cause permanent hydraulic damage to the vine, reducing capacity in subsequent years.

Regulated deficit irrigation is the technique built on this: applying a controlled shortage at a chosen stage, rather than either full irrigation or none.