Crop yield response to water - Cra

Figure 6Yield reduction with increasing water deficit quantified using (a) the integral of stem-waterpotential and (b) the difference between canopy and air temperature corrected by vapourpressure deficit. Sources: (a) Salón et al. (2005), (b) Grimes and Williams (1990).1.00.8r 2 = 0.810.60.4aRelative yield1.00 10 20 30 40 50Water stress integral (MPa day)0.90.8r 2 = 0.430.7b0.60.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Crop water stress indexof water deficit in season 1 had negative, neutral or positive effect on reproductive outcomein season 2 (Williams and Matthews, 1990). This diversity of responses is partially the result ofdifferences in varietal sensitivity, timing, intensity and duration of water deficit, interactions withother factors, and in same cases to poorly designed experiments. In a well-designed factorialexperiment looking at the combined effects of pruning and post-veraison irrigation on Shiraz,Petrie et al. (2004) measured statistically significant reductions in shoot number, bunch numberand yield in season 2 in response to reduction in irrigation rate in season 1 (Figure 7). In an equallywell designed experiment with Tempranillo, Intrigliolo and Castel (2010) found no carry overeffect of irrigation regime on bud fertility. Nevertheless, grape growers do have some capacity toregulate yield components by pruning and bunch thinning (Table 1, Figure 8).Many studies measured the effect of in-season water supply on yield and its components,as illustrated in Table 3 for three contrasting production systems. The combination of cultivar,environment and management resulted in yield of fully irrigated vines from 10 kg/vine for Bobalin Requena and Chardonnay in Niagara to 20 kg per vine for Shiraz in Riverland. Comparison ofrainfed and fully irrigated crops shows a large (up to twofold) benefit of irrigation in the drierenvironments (Riverland, Requena) in comparison to yield gains of only 10-25 percent in thecooler, humid environment (Niagara). Bunch number shows large variation among productionsystems, but is relatively stable in response to in-season water supply, as expected from thereproductive cycle of vines. In-season water deficit therefore reduces yield by reducing bunchweight, and the relative importance of its components, namely berry number and size, dependson the timing of water deficits. Water deficit around anthesis and berry set has the potentialto reduce berry number and size, whereas water deficit at later stages only reduces berry size.The post-veraison period is particularly critical because of the trade-off between maintenance468crop yield response to water