The Light Environment Within Grapevine Canopies. II. Influence of Leaf Area Density on Fruit Zone Light Environment and Some Canopy Assessment Parameters

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Abstract
The influence of leaf area density on the fruit zone light environment and several canopy assessment parameters of grapevines was studied in eight commercial Caberent Sauvignon (Vitis vinifera L.) 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 the percent canopy area illuminated by sunflecks were determined in the fruit zone at berry set, veraison, and harvest. Leaf area per vine was also determined for each above-mentioned stage of fruit development at each site. PPFD and R:FR fell below 1% and near 10% of their ambient values, respectively, when leaf area density exceeded 8 m2 m-1 canopy length. Sunflecks illuminated 2 m-1 canopy length, and were absent when leaf area density approached 10 m2 m-1 canopy length. A close, positive linear relationship (r = 0.976) between PPFD (log % ambient) and R:FR in the fruit zone was found. Leaf area density and fruit zone PPFD were also closely related to several indirect indices of leaf area density. Point quadrant leaf layer number (LLN), atmometer evaporative potential, and pruning weight (kg prunings m-1 canopy length) provided high correlations with PPFD in the fruit zone. Canopy leaf areas ≤ 4 m2 m-1 canopy length resulted in low density canopies (fruit zone PPFD ≥ 5% of ambient), while canopy leaf areas ≥ 8 m2 m-1 canopy length resulted in high density canopies (fruit zone PPFD ≤ 1% of ambient), for the two-wire vertical trellis system evaluated in this study.

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