The role of soil water holding capacity and plant water relations in zone/terroir expression

The spatial variability in soil type and depth and water holding capacity is very high in many viticultural regions of the world. Differences in rooting depths and water extraction profiles and their seasonal dynamics add additional variability and it is extremely difficult to deduct direct causal relationships between these factors and fruit composition even within small units of climatic zones, and much less so over larger climatic trans-sects. The influence of water status on grape composition has been studied intensively for many years, yet indirect effects caused by changes in plant water status have been largely neglected. For example, vineyard sites with limited water supply will be more prone to early leaf drop causing substantial changes in the light environment of the fruit, which in itself will change fruit temperature. Additionally, there is almost certainly a different link between plant water status and fruit and wine composition for red and white cultivars and within each respective group between varieties of different geographic origin. Another unresolved problem is the coupling of soil to plant water status. Many plant water status indicators such as stem, or midday or pre-dawn (ΨPD) leaf water potential are difficult to link to quantitative soil water data. We have recently started to use the concept of total transpirable soil water (TTSW) and the fraction thereof (FTSW), originally proposed for herbaceous plants, to evaluate the coupling between soil water availability and plant water status measurements for contrasting vineyard sites. Even for soil water holding capacities over the root profiles between 380 and 100 L/m2, and a TTSW varying from 50 to 175 L/m2, respectively, we found a single common relationship between ΨPD and FTSW for all vineyards, irrespective of water extraction profiles and canopy systems (Gruber and Schultz 2004 in press). This relationship has also been proven stable across different wine regions in Europe. This system may provide a platform to better link quality parameters to plant and soil water status. Some recent results also suggest that indirect effects of changes in water supply may be more important than previously thought for fruit composition. These effects seem not restricted to changes in canopy microclimate or co-limiting factors such as nitrogen, but seem to extend to substances influencing micronutrient metabolism of yeasts, which may alter aromatic expression. It is clear and has been proven many times that water relations are important in quality formation and in the expression of terroir characters, yet it is still difficult to provide conclusive linkages between all the involved parameters.