Grapevine leaf area index evaluation by gap fraction inversion

Controlling the ratio of grapevine vegetative growth to yield is an important management objective, since this ratio has a consistent relationship to grape quality. In this study, the relationships between several vegetative growth parameters and yield parameters were compared for three grapevine cultivars. The trials were part of irrigation trials for evaluating three class A Pan irrigation coefficients for optimizing yield and wine quality in the semi-arid Arad Plateau, near the Dead Sea, Israel. Leaf-Area-Index (LAI) was evaluated by Gap Fraction Inversion (GFL) at three zenith angles using two linear PAR probes (Ceptometer and SunLink). PAR transmittance of 12 vines in each of the cultivars Sauvignon Blanc, Cabernet Sauvignon and Merlot were measured 7 to 8 times after bud break, at 7-14 days intervals. Shoot elongation, number of leaves and leaf area (evaluated form vein measurements) were measured simultaneously with GFI, on the same vines. After full canopy development, destructive sampling of the vines was made to determine total leaf number and area, shoot number and length, and vine dry weight. Pruning weights of 10 adjacent vines, receiving identical irrigation treatments, were recorded during winter. LAI (1.50-5.45), evaluated by GFI, was highly correlated with leaf area of tagged shoots (200-3000 cm(2)/shoot) on the vine and with leaf area (5.61 15.85 m(2)/vine) measured by destructive sampling. Weekly interval measurements of shoot elongation acid leaf area, and total shoot length, were found to be reliable and useful tools for estimating vineyard canopy development and size. Similar leaf areas of Merlot, Cabernet Sauvignon and Sauvignon Blanc were associated with a wide range of winter pruning weights. Estimating actual canopy coverage (LAI) from GFI, for characterizing fruit load in vineyards in relation to fruit quality, was found to be equally reliable to measurements of winter pruning weights, offering the advantage of measurements in real time.