Influence of Row Orientation and Cluster Exposure to Sunlight on the Microclimate and Composition of Cabernet Sauvignon Fruits

Cluster exposure to sunlight is a critical parameter when evaluating grapevine canopy efficiency and potential grape and wine composition. Since the mid-1980?s significant emphasis has been placed on increasing cluster exposure to sunlight to improve fruit quality, particularly in coastal regions. However, in many cases these vineyards have experienced quality problems related to excessive exposure of the fruit to sunlight. A particular concern is the over-exposure of clusters located on the western or southern exteriors of the canopy. These clusters are exposed to direct sunlight during the afternoon and frequently contain sunburned or slightly desiccated berries with reduced color. In order to examine the effects of sunlight on berry composition more closely, the influence of cluster light exposure on the microclimate and composition of Cabernet Sauvignon fruit was examined in four premium winegrowing regions of California (Napa, Lodi, Gonzales and Paso Robles). Canopy density in the fruiting zone was manipulated following berry set by varying the amounts of basal leaf and lateral shoot removal. Treatment effects on the afternoon sun (south or west) and afternoon shade (north or east) portions of the fruiting zone were recorded separately. A negative, near linear relationship was found between leaf layer number (LLN) in the fruit zone and the percentage of total clusters located on the canopy exterior. Berry temperature and cluster sunlight exposure increased as LLN in the fruiting zone declined in all regions. Berry weight was generally least for fully exposed clusters and greatest for shaded clusters. Soluble solids were generally lowest in fully exposed berries and increased as LLN increased. A strong correlation between cluster exposure and titratable acidity was not found, while malic acid content declined as cluster exposure to sunlight increased. Juice pH also declined with increased cluster exposure to diffuse light, while increased amounts of direct sunlight generally increased juice pH. Treatment effects on skin anthocyanin content generally reflected differences in berry temperature due to sunlight exposure. In all growing regions, fully exposed clusters on the afternoon sun side of the canopy were lower in anthocyanins than partially or completely shaded clusters. An exception was noted in Gonzales, where afternoon berry temperatures were more moderate. Skin phenolic content increased with cluster exposure regardless of cluster position within the fruiting zone. Results from this and other recent studies suggest that the optimum fruit zone light environment contains high levels of diffuse light with minimal amounts of direct sunlight, particularly on the afternoon sun side of the canopy. Chemical and sensory analyses of wines produced from each location will be completed by July 1, 2003.