Crop yield response to water - Cra

Table 4Yield, fruit sugar concentration and sugar yield of irrigated and rainfed Riesling on a steepslope vineyard close to Geisenheim (50 o N), Germany. Values are mean ± standard deviation foreight consecutive years since 2002. Combination of an irrigation threshold of -0.3 MPa predawnwater potential and weekly irrigation decision interval resulted in 7.4 ± 3.4 irrigation events perseason, and applied water 29.3 ± 12.2 litre/m 2 (Gruber and Schultz, unpublished).Response variable Irrigated RainfedYield (tonne/ha) 7.6 ± 3.22 5.0 ± 2.16sugar conc. (g/litre) 212 ± 19.5 204 ± 29.7sugar yield (kg/ha) 1 182 ± 460.1 728 ± 280.1Berry and wine attributesWine quality is an elusive concept and attempts to quantify it are bound to be controversial(Box 1). Quantitative assessments of berry and wine attributes in response to water deficit are,however, essential for irrigation scheduling. Indeed, regulation of grapevine water relations isan important tool for quality management in irrigated viticulture. There is a significant body ofliterature dealing with the effects of water relations on the composition of red grapes, especially onphenolic compounds; information regarding the effects of plant water status on the compositionof white varieties is scarce.Red varietiesFigure 9 illustrates a typical, although not universal, relationship between wine quality andirrigation for red varieties. The negative association between water supply and wine qualityis partially mediated by an apparent trade-off between yield and quality attributes of berries(Figure 10, Figure 11). The negative associations between rate of accumulation of sugar andanthocyanins and yield components in Figure 10b probably reflect a high fruit-to-canopy ratio,rather than high yield per se. If this hypothesis is correct, manipulation of this ratio by irrigation,BOX 1 Wine qualityThe elusiveness of ‘wine quality’ stems from the complexity of wine attributes compoundedby the complexity and variability of human smell and taste sensitivity. Temporal andregional variation in wine quality has been assessed with price and vintage ratings(Cicchetti and Cicchetti, 2009 and Almenberg and Dreber, 2009). The drawbacks of eachof these approaches are many, including marketing factors influencing price beyondspecific quality parameters, and vintage scores derived from expert, albeit subjectiveevaluations (Sadras et al., 2007). Views on vintage scores range from “…controversial,potentially misleading and essentially impossible to get consistently correct…” (Fullerand Walsh, 1999) to the proposal of ratings that “…express the likelihood of what mightreasonably be expected from a wine of a given year…” (Stevenson, 2005). Individualattributes of berries and wine such as colour or content of many critical compounds,on the other hand, can be measured with accuracy. The challenge to this approachis however, the integration of individual measurements into a complete measure ofquality. There is no doubt that wine quality is a controversial concept, and there is nodoubt either that, however imperfect, quantification of key berry and wine attributes isessential to irrigation management.grapevine 471