THE LIGHT ENVIRONMENT WITHIN GRAPEVINE CANOPIES .2. INFLUENCE OF LEAF-AREA DENSITY ON FRUIT ZONE LIGHT ENVIRONMENT AND SOME CANOPY ASSESSMENT PARAMETERS

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 (PPF D), 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 zon e at berry set, veraison, and harvest. Leaf area per vine was also det ermined for each above-mentioned stage of fruit development at each si te. PPFD and R:FR fell below 1% and near 10% of their ambient values, respectively, when leaf area density exceeded 8 m(2) m(-1) canopy leng th. Sunflecks illuminated <10% of the fruit zone when leaf area densit y surpassed 5 m(2) m(-1) canopy length, and were absent when leaf area density approached 10 m(2) m(-1) canopy length. A close, positive lin ear 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 a lso closely related to several indirect indices of leaf area density. Point quadrant leaf layer number (LLN), atmometer evaporative potentia l, and pruning weight (kg prunings m(-1) canopy length) provided high correlations with PPFD in the fruit zone. Canopy leaf areas greater th an or equal to 4 m(2) m(-1) canopy length resulted in low density cano pies (fruit zone PPFD greater than or equal to 5% of ambient), while c anopy leaf areas greater than or equal to 8 m(2) m(-1) canopy length r esulted in high density canopies (fruit zone PPFD less than or equal t o 1% of ambient), for the two-wire vertical trellis system evaluated i n this study.