By Stephanos Koundouras

Irrigation is the most controllable vineyard management technique in determining grape and wine quality under semi-arid conditions

Traditionally grapevine was a nonirrigated crop covering extended areas in semi-arid regions. However, the current climate changes (significant rise in average growth season temperature and evapotranspiration) have increased drought impacts on vine productivity and grape quality. On the other hand, it is well known that, to produce wines according to the quality specifications of today’s wine market (especially red ones), grapevine varieties have to be grown under environmental stress that most commonly implies a limitation of water availability in certain stages of vine growth cycle. Extensive research and practical experience over the last two decades in vineyards worldwide have shown that moderate water deficit increases berry quality potential (via shoot growth cessation, reduction of berry size, steady ripening process and stimulation of phenolic and aroma compound synthesis) and winery net profits (despite reducing yields in most cases). Moreover, irrigation remains the single most controllable factor in determining grape and wine quality (especially in areas where summer rainfall is scarce) and, thus, is a key factor in vineyard management for achieving both high quality standards and substantial economies in water resources.

Recently, irrigation was introduced in Greece to minimize unfavourable impacts of excessive water stress and ensure the regular production of high quality grapes and wines. Moreover, proper irrigation has been proved to facilitate the expression of the distinctive characteristics of Greece’s numerous indigenous varieties, thus enhancing wine “typicity” which normally generates a higher selling potential (and which was until recently “buried” under the frequently oxidized flavour of average greek wines, mainly because of excessively stressful water conditions in the vineyard).

The irrigation strategy usually applied in order to reduce water supply during specific stages of vine growth has been termed regulated deficit irrigation (RDI). The principle of RDI is to apply a water deficit of variable intensity according to the objectives of grape production, i.e. white wines, light red wines to be drunk when young, long-ageing red wines etc. For example, for the production of aromatic white wines, only a light water restriction is desirable (especially in warm areas) to conserve grape varietal aromas. The same is also true for rose or fruity red wines. On the contrary, a more pronounced water deficit during green berry development (berry set to veraison) is necessary for premium-quality red wines in order to reduce berry size and yield, limit vegetative growth to create a more open canopy and impove grape microclimate and stimulate the accumulation of phenolic compounds which are essential for wine ageing.

However, due perhaps to the need for important investments in management equipment, the majority of vine-growers in Greece still decide irrigation application based on personal experience and habits, or, at best, according to seasonal weather conditions. Nevertheless, as water resources become limited and as wine quality is acknowledged as more important than yield in determining winery profits, the demand for accurate assessment of vine water status through the use of adapted indicators is becoming more often, especially in boutique wineries, oriented towards the production of high quality wines of distinctive character.

Among the various monitoring techniques used worldwide to define vine hydric condition, stem water potential has been proved to be the most reliable. The basic apparatus for the measurement of stem water potential is the so-called “pressure chamber (or pressure bomb)” which permits the placement of a leaf (formerly enclosed in opaque bags for at least one hour) in a sealed chamber leaving only the extremity of the petiole outside. Pressure is most commonly applied through the deliverance of inert gas (nitrogen) and the pressure necessary to extract the first drop of xylem sap from the leaf is recorded in a manometer, corresponding to the water potential of the stem bearing the leaf. The more negative the water potential, the greater the water deficit of the vine. Midday stem water potential values can thus be used as thresholds for irrigation management, depending on the variety and the desired wine style. Lowest (more negative) values are desirable for red varieties destined for the production of premium red wines while higher (close to zero) are better for aromatic white, rose/red wines.

Of course, research is continuing in scientific establishments to develop an easier and less time-consuming technique for water status assessment. However, the accuracy and reproducibility of this technique has not yet been surpassed. That’s why it remains an invaluable tool for vine growers and winemakers for irrigation management.

See Stephanos on Videos: Measuring water stress.

9.10.2009