Crop yield response to water - Cra

Responses to Water DeficitsIn areas of winter and spring rainfall, water stress conditions rarely occur before harvest inearly cultivars, particularly when soils are deep and with high water-holding capacity. Theeffects of reduced soil water availability, as well as the level of stress experienced by plants,depend on the intensity and duration of the water deficit, and on the plant phenologicalstage. Generally in June, July and August (months with high evaporative demand) the effectsof water deficit are more evident.Apricots, like most stone fruit trees, are sensitive to water shortages during the entire fruitdevelopment period. The early stages of fruit growth are of great significance not only forfruit size but also for the accumulation of some phloem-immobile nutrients (e.g. calcium,Ca). About 85 percent of fruit Ca content at harvest is gained within the early four weeks ofdevelopment (Montanaro et al., 2010). Hence optimal soil water supply during these weeks isessential to avoid reduction of water (and nutrients) uptake.Water deficits during the later stages of fruit growth lead to smaller fruit at harvest. However,it has been reported that for the cv. Búlida, recovery from water stress (-1.0 MPa predawnleaf water potential (LWP)) during Stage II of fruit growth, induced a compensatory fruitgrowth rate during the final stages, which allowed the fruit to reach a similar diameter asfruit from fully irrigated plants (Torrecillas et al., 2000). In the same experiment, water deficitsapplied during Stages I and II that imposed mild to moderate stress from mid-March to mid-May (predawn LWP of -1.1 MPa) caused a yield decline of about 15 percent in the last threeyears of a four-year experiment. Surprisingly, this difference was not statistically significantfrom the yield of a fully irrigated control (predawn LWP of -0.4 MPa) (Torrecillas et al., 2000).For mature trees water deficits (-1.0 MPa predawn LWP) negatively affected trunk growthduring the drought period (Perez-Pastor et al., 2009). However, upon recovery of optimal soilwater condition trunk circumference may easily recover (Torrecillas et al., 2000).Water stress (-1.5 to -2.2 MPa of predawn LWP) occurring during the early postharvest period(~30 days after harvest), could have detrimental effects on the potential yield of the followingyear, particularly for early cultivars. This is because water deficits at this time negatively affectbud induction and the floral differentiation process, which happen in the early postharvestperiod.Water RequirementsThe water requirements for irrigation depend primarily on the annual water deficit of theenvironment, than on cultivar and yield target. For example, the amount of irrigation waterneeded to produce 1 kg of fruit in southern Italy, where the seasonal water deficit (ET o -rainfall)is around 850 mm/year, is about 160 litre (~30 tonne/ha yield). For the same cultivar, this valuedecreases to about 40 litre/kg in northern Italy (44°08’ N; 12°44’ E) where the seasonal waterdeficit is only around 160 mm/year.There have been very few measurements of the consumptive use (ET c ) of apricot trees. Cropcoefficients (K c ) for apricot orchards are similar to those of other stone fruit such as plumapricot 435