Development of Irrigation Management Strategies to Improve Fruit Quality

This project seeks to develop an approach to vine water management appropriate to cooler growing regions that will provide growers the tools they need to know when to begin to irrigate, when to schedule subsequent irrigations and how much water to apply each time they irrigate. This research project utilized midday measurements of leaf water potential (LWP) as a threshold or trigger point to determine when to begin supplying irrigation water. After a threshold LWP has triggered the start of the irrigation season, water was supplied at a fraction of full vine water use. It was our goal to use water management, defined as the timing and quantity of applied water, to impose vine water deficits as a means of producing desirable must and wine characteristics. In order to fully evaluate such an approach, vine and must/wine measurements must be taken over numerous years. Irrigation management strategies for red winegrape varieties, which rely on methods that can insure repeatable results, have not been developed for mature vines in cool regions. Climatic water demand and soil moisture loss variables are commonly used to schedule irrigations in the warmer growing regions in the state; however, these can grossly over estimate the amount of water necessary to insure high fruit quality. Growers are aware of this and realize they must “deficit irrigate” their vines in order to maximize fruit quality.

This project will give them the tools they need to select an irrigation management strategy that integrates climate variables common to the cooler growing areas such as spring rains and summer fog.

Development of Irrigation Management Strategies to Improve Fruit Quality

This project seeks to develop an approach to vine water management appropriate to cooler growing regions that will provide growers the tools they need to know when to begin to irrigate, when to schedule subsequent irrigations and how much water to apply each time they irrigate. This research project utilized midday measurements of leaf water potential (LWP) as a trigger to determine when to begin supplying irrigation water. After a threshold LWP has triggered the start of the irrigation season, water was supplied at a fraction of full vine water use. It was our goal to use water management, as defined as the timing and quantity of applied water, to impose vine water deficits as a means of producing desirable must and wine characteristics. In order to evaluate fully such an approach, vine and must/wine measurements were taken over the term of at least three years. Irrigation management strategies for red winegrape varieties, which rely on methods that can insure repeatable results, have not been developed for mature vines in cool regions. Climactic water demand and soil moisture loss variables are commonly used to schedule irrigations in the warmer growing regions in the state; however, these can grossly over estimate the amount of water necessary to insure high fruit quality. Growers are aware of this and realize they must “deficit irrigate” their vines in order to maximize fruit quality. This project will give them the tools they need to select an irrigation management strategy that integrates climate variables common to the cooler growing areas such as spring rains and summer fog.

Grape Maturity and Vine Water Status: Effects on Tannin Composition in Red Grapes

Grape berry samples were collected from a carefully replicated vineyard trial on Vitis vinifera cv. Cabernet Sauvignon at the H block in the Tokalon vineyard. Samples were divided into skins and seeds, and extracted with 75{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} acetone. The quantity and characterization of extracted flavan-3-ols and procyanidins was determined in seeds and skins over the course of ripening and at different levels of vine water status. In the seed, the per berry extractive yield of all polyphenolics decreased with maturity, but increased in the skin. The apparent size of the seed and skin polyphenols were measured by chromatographic analysis and by thiolytic degradation. The relative proportion of procyanidin extension and terminal units was also measured by thiolysis. Chromatographic analysis showed an increase in size with maturity, while the average degree of polymerization, determined by thiolysis, declined slowly with maturity. In seeds, The relative decrease in the subunits varied with subunit type, with the fiavan-3-ols decreasing the most rapidly followed by the procyanidin extension units and then the terminal units. Increased water stress increases the rate of decline for flavan-3-ols and their relative proportion. This is the first clear description of fundamental changes in polyphenol structure during the important period of the harvest maturity window. Due to the possible sensory implications of these changes, these observations may lead to polyphenols ripeness parameters for red grapes.

Impact of Vine Deficit Irrigation on Wine Quality Parameters

This study evaluated the impact of conservative irrigation management on premium wine quality by relating quantitative results on tannin development in the vineyard to the perception of astringency in experimental wines from deficit-irrigated fruit. On a per berry basis, the skin tannins increase slightly between veraison and harvest. There is a trend that the deficit-irrigated treatments are in general higher in tannins than the fully irrigated treatment. In the seeds, the tannins decrease between veraison and harvest. There were no statistically significant differences in primary amino nitrogen for all treatments on a per berry basis. A sensory evaluation of the 1998 wines revealed significant differences between all treatments in flavor by mouth. A correlation of the sensory results with the analytical data is in progress.

Modification of Wine Characteristics through Irrigation Management

This project seeks to develop an approach to vine water management appropriate to cooler growing regions that will provide growers the tools they need to know when to begin to irrigate, when to schedule subsequent irrigations and how much water to apply each time they irrigate. This research project will utilize measurements of xylem (leaf) water potential (XWP) as a trigger to determine when to begin supplying irrigation water. After a threshold XWP has triggered the start of the irrigation season, water was supplied at a fraction of full vine water use. It is our goal to use water management, as defined as the timing and quantity of applied water, to impose vine water deficits as a means of producing desirable must and wine characteristics. In order to evaluate fully such an approach, vine and must/wine measurements must be taken over the term of at least three years. Irrigation management strategies for red winegrape varieties, which rely on methods that can insure repeatable results, have not been developed for mature vines in cool regions. Climate demand and soil moisture loss variables are commonly used to schedule irrigations in the warmer growing regions in the state; however, these can grossly over estimate the amount of water necessary to insure high fruit quality. Growers are aware of this and realize they must “deficit irrigate” their vines in order to maximize fruit quality. This project will give them the tools they need to select an irrigation management strategy that integrates climate variables common to the cooler growing areas such as spring rains and summer fog.

Modification of Wine Characteristics Through Irrigation Management

We propose to develop an approach to vine water management appropriate to cooler growing regions that will provide growers the tools they need to know when to begin to irrigate, when to schedule subsequent irrigations and how much water to apply each time they irrigate. This research project will utilize measurements of xylem (leaf) water potential (XWP) as a trigger to determine when to begin supplying irrigation water. After a threshold XWP has triggered the start of the irrigation season, water will be supplied at a fraction of the full vine water use. It is our goal to use water management, as defined as the timing and quantity of applied water, to impose vine water deficits as a means of producing desirable must and wine characteristics. Irrigation management strategies for red winegrape varieties have not been developed for mature vines in cool regions. Climate demand and soil moisture loss variables are commonly used to schedule irrigations in the warmer growing regions in the state; however, these can grossly over estimate the amount of water necessary to insure high fruit quality. Growers are aware of this and realize they must “deficit irrigate” their vines in order to maximize fruit quality. This project will give them the tools they need to select an irrigation management strategy that integrates climate variables common to the cooler growing areas such as spring rains and summer fog.

Identifying the Potential to Use Vineyard Water Status to Alter Anthocyanins and Other Phenolic Compounds in Red Winegrapes

The first of two main objectives in this project is the separation and analysis of the phenolics present in approximately 300 fruit samples of Cabernet Sauvignon that were obtained from vines grown near Lodi and subjected to various irrigation regimes. The major phenolic compounds that change in concentration during ripening were identified by HPLC and comparison to standards. We identified approximately 20 compounds in skin extracts and juice samples that are of potential interest because of significant responses to changes in vine water status. These are categorized into phenolic classes, cinnamates, flavonols, and anthocyanins on the basis of their absorption spectra. For those compounds identified as present in significant concentrations, the seasonal pattern, the relationship of the concentration to the accumulation of sugars in the berry, and changes in these patterns caused well-defined differences in vine water status that were imposed before and after veraison were analyzed. The second specific objective was the establishment of a new field trial in a commercial Cabernet Sauvignon vineyard located in the North Coast. A field trial was established in a mature, uniform vineyard in Knight’s Valley on a light soil in which five irrigation regimes can be imposed that create early season water deficits and varying degrees of late season water deficits. The late ripening at the site will facilitate extended seasonal analysis of phenolic responses to vine water status.

Identifying the Potential to Use Vineyard Water Status to Alter Anthocyanins and Other Phenolic Compounds in Red Winegrapes

The data from field experiments with Cabernet Sauvignon grown near Lodi show that clear differences in vine water status were obtained at different stages of development by withholding water. Although water status of vines irrigated every 2-3 days (Continual treatment) decreased until veraison, when water was withheld from the beginning of the season (Early Deficit) or from times closer to veraison (Veraison Stress and Moderate Veraison Stress) vine water status decreased significantly more rapidly resulting in differences in vine water status of about 3 bars at veraison. When water was withheld after veraison (Late Deficit), vine water status rapidly decreased to values that were about 4 bars more stressed than the Continual vines. This was sufficient stress to cause some leaf senescence and abscission. The general response to water deficits was for phenol ics and anthocyanins to increase in concentration although this is not uniformly the case. For anthocyanins, the accumulation that occurs at veraison was induced earlier in vines that were water stressed. The minor anthocyanins were more responsive to early stress, whereas the major anthocyanin compound, malvidin-3-glucoside, accumulated to the highest concentration in Late Deficit fruit. The responses of juice phenolics in general and cinnamates specifically to water stress were complex and depended on the specific compound. Some accumulated during ripening and some decreased in concentration after veraison. In general, most juice cinnamates were present at harvest in highest concentrations in Late Deficit fruit. Nonanthocyanin skin phenolics were predominately flavanoids which accumulated during ripening. The accumulation was more rapid and to higher concentrations in vines that were exposed to early stress (Early Deficit and Veraison Stress) and lowest in vines that were exposed to high water status early in fruit development. Analysis is continuing of the specific nonanthocyanin skin phenolics. A new field trial has been established in a premium Cabernet Sauvignon vineyard (Knight’s Valley). In this trial similar data will be developed but more samples will be collected late in fruit ripening for determination of the relationship of the concentration of specific compounds with Brix, TA, and “hang time” and their responses to early and late season water deficits.

The Regulation of Fruit Growth, Fruit Composition, and Wine Composition by Water Management

This project quantified the role of cluster microclimate in water stress responses, tested the importance of stress at veraison, and extended what we learned about seasonal water stress in hillside Cabernet franc and Sauvignon blanc production in Napa Valley to additional sites and varieties. Soil and vine water status is readily controlled in drip-irrigated vineyards of Pinot noir (Carneros) and Cabernet Sauvignon (Lodi). Changes in vineyard water status, yield, and fruit and wine composition caused by pre- (Early Deficit) and post-(Late Deficit) veraison stress were similar to our earlier results. Thus, those results, showing control of yield and fruit composition, can largely 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 and, because of this, color, phenolics, malate, and amino acids, and other compounds in the fruit. Light penetration into the canopy increased in Early Deficit vines early in the season compared to other treatments. After veraison light penetration increased slowly in C vines and rapidly in Late Deficit vines, in part due to differences in the rate of leaf drop. The role of microclimate in the stress responses of fruit development was investigated with reciprocal treatments that, e.g., created in some well-irrigated vines a canopy that mimicked the canopy of Early Deficit vines and vice versa. Temperature in the cluster zone did not differ significantly among any treatments, and, therefore, was unlikely to explain the differences that we observed in fruit and wine composition. However, differences in the light environments were important in establishing part (less than 50{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}) of the decreased pH and and increased color of Early Deficit wines. This shows that water deficits can be used to improve canopy structure. Water stress at veraison was not found to be critical for controlling fruit composition. However, the timing of the water stress was important in determining sensory attributes because judges could easily discriminate between wines made from Early Deficit and Late Deficit vines.

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.