Heat and heatwaves

Heat is intuitively treated as the thing that ripens grapes, and above a threshold the relationship reverses.

This has moved from a marginal concern to a central one across southern Europe, Australia and California.

Net photosynthesis peaks in a moderate temperature band and falls away above it, as respiration rises and stomata close to limit water loss. Ripening slows or stops.

Anthocyanin synthesis has its own optimum and is inhibited at high temperatures, so extreme heat is associated with reduced colour in red varieties — a direct contradiction of the assumption that hotter means darker.

Malic acid respiration accelerates with heat, particularly at night, so acid falls faster than sugar rises.

Sunburn is direct radiation damage to exposed berries. It is not the same as heat damage and is managed differently — by leaving leaf cover on the exposed side rather than removing it.

Sugar concentration can continue to rise during a heat event purely through berry dehydration, producing high potential alcohol with no corresponding flavour or phenolic development.

Responses include shifting to later-ripening varieties, canopy strategies that shade rather than expose fruit, higher training to lift fruit away from reflected ground heat, and in some cases rethinking site selection entirely.