The Regulation of Fruit Growth, Fruit Composition, and Wine Composition by Water Management
This summary will constitute a brief overview of what we have learned about water stress in winegrape production in California. This project was designed to extend our initial study with hillside Cabernet franc and Sauvignon blanc (1984-1988) by extending our understanding of the role of vine water status in determining yield, fruit and wine composition, and wine sensory attributes. Experiments and sites were designed to test several questions including: whether vine water status can be readily controlled at different sites and soils (Carneros and Lodi areas); whether other important red winegrape varieties, Pinot noir and Cabernet Sauvignon, respond similarly to seasonal water deficits; whether water stress at veraison is particularly critical in determining the composition of harvested fruit; whether the changes in fruit composition caused by water stress are due directly to water stress or to indirect effects of changes in the cluster microclimate; and which specific flavor and aroma compounds are responsible for the sensory differences caused by water stress. We report that soil and vine water status is readily controlled in drip-irrigated vineyards of Pinot noir (Carneros) and Cabernet Sauvignon (Lodi). For the Pinot noir site, differences in water status of vines receiving the standard or a supplemented rate of irrigation were greater than we obtained in the hillside Cabernet franc and Sauvignon blanc we used in our earlier study. This shows that our earlier results can be extrapolated to valley floors and are not indicative only of hillside vineyards with shallow soils and high exposures. The results also show the utility of drip irrigation for control of vine water status and the prevalence of water deficits in winegrape production in the North Coast. Thus, many growers can control vineyard water status; we are working to identify how much stress should occur and when to meet grower and winemaker objectives. The results from three sites and four varieties over several seasons indicate some clear generalizations. Regulation of vine water status is effective in controlling fruitfulness, and many vineyards may be under-irrigated for maximum fruitfulness. Early season stress is more effective in decreasing berry size and fruitfulness in the following season. Several aspects of fruit ripening are to some extent controlled by vine water status. Properly timed water stress increases the color of red winegrape juice, the concentration of phenolics (tannins), and the concentration of amino acids in juice of red and white varieties. As long as the water stress is moderate, these changes occur without significant effects on sugar accumulation and titratable acidity. The timing of water stress is important. For color and phenolics, stress early in the season (before veraison) is more important. However, early season stress also leads to significant decreases in the concentration of malate (increasing the tartrate:malate ratio) which may be perceived as positive or negative, and leads to inhibited yields in the following season. Water stress after veraison (a common practice), is more important than early stress in increasing the concentration of amino acids in the harvested juice and also decreases the relative amount of proline in the total amino acids. This is important because proline is largely unavailable for yeast during fermentation. Postveraison stress is less effective in increasing color, phenolics, and the tartratermalate ratio. If late stress is severe enough, delayed ripening and inhibited yield in the following season occur. These differences in fruit composition lead to significant differences in the appearance, aroma, and taste of wines. The results of sensory analyses indicates that all stresses are not equal. That is, the appearance, aroma, and taste of a wine made from vines that experienced and early stress are different from those attributes of a wine made from vines that experienced water stress only after veraison. We are now testing whether these changes in fruitfulness, fruit composition, and wine sensory attributes are due to water stress directly or are due to the effects of water stress on the canopy microclimate. If the latter is true, water stress may not be necessary to obtain these differences. Early season water deficits increased light and air temperature in cluster zone throughout much of the season. Postveraison water deficits can also cause an increase in light in the cluster zone, apparently due to leaf abscission which we intend to quantify in 1992. Another approach was to include treatments that counteracted the effects of the irrigation treatments on microclimate. In some control vines, leaves were removed to open the canopy and make conditions similar to those experienced under the Early deficit treatment. Likewise, in some Early deficit vines clusters were bagged with shade cloth to decrease light penetration. The results suggest that changes in fruit growth and composition that occur in response to water stress are unlikely to be due to changes in the canopy; rather fruit responses appear to be due to changes in the physiology of the berry caused by water stress conditions. However, these treatments, partial defoliation of control vines to create “Early deficit-like” canopies and shading of Early deficit clusters were only partially successful in accurately recreating the alternative cluster environments. The plan for 1992 includes modifications in these treatments to more accurately reproduce their alternative cluster environment goals and sensory analysis of Pinot noir and Cabernet Sauvignon wines from different water stress treatments.