The Light Environment Within Grapevine Canopies. I. Description and Seasonal Changes During Fruit Development

Abstract

Seasonal changes in the light environment within Cabernet Sauvignon (Vitis vinifera L.) grapevine canopies was examined in eight commercial vineyards in the North Coast of California. Photosynthetic photon flux density (PPFD), the ratio of red (R) to far-red (FR) light (R:FR) and percentage of canopy area receiving sunflecks were determined along a vertical transect in the center of the vine row and along a horizontal transect in the fruit zone at berry set, veraison and harvest. Leaf area density (m² leaf area per meter canopy or row length) was also determined at each site at each of the above mentioned stages of fruit development. PPFD, R:FR and sunflecks decreased markedly beneath the canopy surface, and continued to decline along the vertical transect from the top of the canopy to the fruit zone. PPFD, R:FR and sunflecks reached their lowest values near the fruit zone, then increased from the fruit zone to the ground. A close, positive relationship between PPFD (log % ambient) and R:FR within the canopy was found. PPFD in the fruit zone at harvest ranged from 7.7 (Site 1, leaf area density = 2.2 m² m^-1 canopy length) to 0.3% (Site 8, leaf area density = 12.2 m² m^-1 canopy length) of ambient, while R:FR in the fruit zone at harvest ranged from 0.49 (Site 1) to 0.05 (Site 6, leaf area density = 11.3 m² m^-1 canopy length). The canopy surface in the fruit zone illuminated by sunflecks at harvest ranged from 30% (Site 1) to nil (Sites 6 and 8). Despite a near doubling of leaf area between berry set and harvest at some sites, only minimal changes in canopy light environment were observed during fruit development. The results indicated that PPFD in the canopy interior was reduced to 1% or less of ambient, while R:FR and sunflecks were reduced to 10% or less of ambient, when leaf area exceeded 8 m² m^-1 canopy length. In contrast, PPFD, R:FR and sunflecks were 10% or more of their ambient values when leaf area was ≤ 4 m² m^-1 canopy length.