Shade

shade, the absence of sunlight in a vine canopy. This is due to leaves blocking out sunlight, as the transmission of light through one leaf is less than 10% of full sunlight. In most vineyards, the blocking leaves are other vine leaves, but occasionally they may be the leaves of weeds or even adjacent trees, as occurs in the rías baixas region of north-west Spain. For most vineyards in the world, shade is due to vigorous vines being trained to restrictive vine-training systems.

Sunlight levels in the centre of dense canopies with many leaf layers can be as little as 1% of the levels above the canopy. At this very low level of light, photosynthesis is almost zero, and in time the leaves turn yellow and then fall off. Leaves deep in the canopy also experience filtered sunlight with altered spectral composition, in that red light is reduced and far red light relatively enriched. The ratio of red to far red light can act as a signal system for the vine and other plants, and may play a role in the vine’s response to shade. Similarly, ultraviolet radiation (UV) is filtered in the canopy, which can have significant impact on phenolics. Since the vine evolved in forests, it has mechanisms like tendrils for avoiding shade by climbing towards the sunlight.