Crop yield response to water - Cra

environments, rainfall pulses and limited buffering capacity of soils drive marked wet-dry cycles.The frequency, duration and severity of the dry spells in these cycles are largely unpredictable.Owing to drying trends and reliability of quality wine supply required by globalized markets,supplemental irrigation is likely to increase even in these areas that have been traditionally rainfed.Figure 4 compares the plant water status of rainfed vines from contrasting climates and soils withina given region; examples from irrigated vines are also included. It is clear that irrigation stabilizespredawn leaf water potential at a level rarely found naturally over the growing season in theseareas, including the very cool climate regions of the Loire Valley in France and the Rheingau inGermany. Figure 4 also shows that the differences in water status between vineyards within eachof the regions could be larger than the differences in general water status between differentclimate zones. It is also clear that variation in water status during a particular season increasesfrom warm and dry climates to summer rainfall because of the irregularity of the frequency andintensity of summer precipitation in the latter.Growth and yieldCrop responses to water deficit depend on the intensity, duration and timing of stress. Intraspecificvariation has been reported for major traits related to the development, water and carboneconomy of grapevine including phenology, susceptibility to embolism, stomatal density andstomatal conductance in response to both soil water content and vapour pressure deficit, biomassper unit transpiration, dry mater partitioning, and rootstock response to water deficit, salinity andsoil-borne diseases.466Figure 4 Seasonal courses of predawn leaf water potential from different vineyard sites in contrastingenvironments. Left panel is Syrah from two warm, dry areas in southern France: Pic St. Loup areanorth of Montpellier (Schultz, 2003) and Aude region (Winkel and Rambal, 1993). Central panelis Cabernet franc from vineyards with three contrasting soils in the cool, summery rainfall LoireValley of France (Morlat et al.,1992), the warm, summer rainfall St. Emilion region of France (vanLeeuwen and Seguin, 1994), Loire, sandy clay the warm, dry Napa Valley of California for an irrigated treatmentand a water deficit treatment Loire, sand after on deep veraison clay (Schultz and Matthews unpublished). Right panel isNapa Valley, irrigatedWhite Riesling from the cool, summer rainfall Rheingau region in Germany collected in 1999 (openNapa Valley, not irrigatedRheingau, shallow soilsymbol and dotted line) and 2002 (closed symbols); adapted from Gruber and Schultz (Gruber andpre-dawn leaf water potential (MPa)0.0-0.2-0.4-0.6-0.8-1.0-1.2-1.4Pic. Schultz, St. Loup, irrigated 2005). All treatments clay layerwere rainfed, unless otherwise indicated.Pic. St. Loup, calcareous soilAude, calcareous soilSyrahcrop yield response to water-0.4aSt. Emilion, sand onSt. Emilion, gravelyLoire, sand on sandstoneCabernet francRheingau, deep loess soilRheingau, med. loamirrigatedRheingau, med. loamRieslingPic. St. Loup, calcareous soilLoire, sandy clayAude,Loire,calcareoussand on deepsoilclay -0.60.0Napa Valley, irrigatedNapa Valley, not irrigated Syrah-0.8St. Emilion, sand onPic. St. Loup, irrigated -0.2clay layer160 200 240 280 160 Pic. St. 200 Loup, calcareous 240 280 soil 160 St. 200 Emilion, 240 gravely 280 -1.0Aude, calcareous soilLoire, sand on sandstoneday of year -0.40.0-1.2SyrahCabernet franc-0.6-0.2-1.4r potential (MPa)-0.6bawn leaf water potential (MPa)-0.8-1.0cpre-dawn leaf water potential (MPa)Pic. St. Loup, irrigated0.0-0.2-0.4Pic. St. Loup, irrigatedPic. St. Loup, calcareous soilAude, calcareous soilLoire, sandy clayLoire, sand on deep clayNapa Valley, irrigatedSyrahNapa Valley, not irrigatedSt. Emilion, sand onclay layerSt. Emilion, gravelyLoire, sand on sandstoneRheingau, Cabernet shallow soil francRheingau, deep loess soilRheingau, med. loamirrigatedRheingau, med. loamRieslingLoire, sandy clayLoire, sand on deep clayNapa Valley, irrigatedNapa Valley, not irrigatedSt. Emilion, sand onclay layerSt. Emilion, gravelyLoire, sand on sandstoneCabernet francRheingau, shallow soilRheingau, deep loess soilRheingau, med. loamirrigatedRheingau, med. loam160 200 240 280 160 200 240 280abcday of yearRiesling