Crop yield response to water - Cra

oth irrigation level and crop load. However, as fruit size is a major attribute of fruit quality,growers are interested in larger fruit up to a certain limit, where oversize apples will fetch alower price. The yield of large fruit is affected by both irrigation and crop load (Figure 4). At lowcrop loads, there is no advantage of increasing irrigation, as similar crop yields were obtainedin one experiment where irrigation levels between June and harvest ranged from 46 percent to119 percent of ET o (Naor et al., 1997).Figure 4 Effect of the number of fruit per tree (1 250 trees/ha) on apple yield (>70 mm in diameter) atdifferent irrigation levels (ET o ) from mid-June to harvest. Bars denote Standard Error (Naoret al., 1997).80Crop yield >70 mm in diameter (tonne/ha)604020Irrigation coefficientrelative to ETo0.470.650.841.011.1900100200300 400500Fruit per treeThe water deficits may reduce tree size in the following year, but do not negatively affectflower bud number or fruit load (Girona et al., 2008). As crop load increased (double numberof fruit per tree), yield of large fruit increased with increasing irrigation levels. This indicates anincreased limitation in assimilate availability that cannot be overcome by supplying additionalwater above full requirements. At the highest crop load, yield of large fruit did not respond toa seasonal irrigation level above 84 percent of ET o , which is more or less equivalent to apple ET cduring the irrigation period (see below).Fruit size increased with increasing midday stem-water potential (SWP: Figure 5) where differentresponse curves were observed for different crop loads. The threshold of midday SWP to reachmarketable size fruit shifted to higher stem-water potentials with increasing crop load. For croploads up to medium levels, maximum fruit size can be achevied by improving tree water status.However, at extremely high crop load, marketable size fruit cannot be reached even undernon-stress conditions. Therefore, there is an upper limit of crop load that would enable largefruit size, suggesting that both irrigation and fruit-thinning practices should be employed tomaximize crop yield of large, marketable fruit. In some cases, summer pruning can help achievemarketable sizes, as for peach.APPLE 337