Crop yield response to water - Cra

The postharvest period is important for peach trees, as it is during this period when nextseason's flower buds are initiated (usually around August) and when these buds are clearlydifferentiate from vegetative buds they start to develop floral organs (Handley and Johnson,2000). Another important feature of the period between harvest and leaf fall is theaccumulation of carbohydrate reserves, needed for continuous bud development processesuntil bloom, and also because fruit set is highly dependent upon carbohydrate availability(Arbeloa and Herrero, 1991). Heat and water stress during post harvest enhances the formationof abnormal fruit (Naor et al., 2005).Responses to Water DeficitsPeach water relations have been studied in more detail than most other deciduous fruit treespecies. As for most plants, vegetative growth is extremely sensitive to water deficits, andseveral studies have shown that leaf and young shoot expansive growth are slowed by mildwater deficits that are difficult to detect.Peach trees are mostly grown to produce fresh fruit, where both size and some qualitycharacteristics are important. However, while there is a premium paid for large fruit sizein most markets (or a penalty for the small sizes), quality features, other than size, do notgenerally influence growers’ revenue, although many consumers are well aware of theimportant differences in quality among and within peach and nectarine varieties.Final fruit size depends directly on the number of cells and the average cell size of the mesocarp.The number of cells is primarily determined during Stage I and several experiments (somein container-grown trees) have demonstrated that this is a very sensitive period for waterdeficits in terms of final yield and revenue. One field study (Girona et al., 2004) with mild tomoderate stress at Stage I demonstrated that fruit dry matter was affected at high fruit loads,but that fresh weight could recover if the water supply during Stages II and III was adequate.Nevertheless, risks of inducing damaging stress levels at Stage I are low because the initial soilprofile is usually full, evaporative demand is low and the canopy development process has notbeen completed (and thus the crop coefficient, K c , is below the maximum that will be achievedwhen canopy growth is near completion). Thus, peach transpiration (Tr) is relatively low atthis time, which also coincides with seasonal spring rains in many peach growing regions. It istherefore not difficult to avoid water deficits in Stage I, even inadvertently. Nevertheless, inshallow soils and/or very dry environments, or in drought years when the soil profile is dry, itis possible to induce significant levels of water stress that will impact negatively on fruit sizeand yield (Girona et al., 2004).As the fruit continue to grow and the pit starts to harden, the rate of vegetative growthaccelerates and fruit growth slows at the onset of Stage II. The duration of this phase variesfrom only a few days in early varieties (and thus is almost impossible to detect) to about 60days in the very late varieties. Water deficits during Stage II affect primarily lateral shootexpansion and trunk growth while having minimal or no impact on fruit growth. Figure 2bshows the impact of water deficits in shoot extension growth and the negligible influencethat it has on fruit growth and final fruit size (Girona et al., 2003). This differential sensitivitybetween vegetative and fruit growth formed the basis for the successful application of waterstress in Stage II in peach first described by Mitchell and Chalmers (1982). These authors andpeach 397