Crop yield response to water - Cra

Season-long stressSeveral studies have imposed deficit irrigation throughout the season. Some of these imposedirrigation regimes based on tensiometer readings. A value of 70 kPa as a threshold was usedwhich caused delayed bloom, flower opening, and decreased fruit set with cv. Valencia(Davies and Bower, 1994). In a study with the cv. Shamouti where tensiometers were used,with 35 percent less applied water than fully irrigated trees, flowers per tree increased by52 percent but the flower abscission rate was high. This resulted in a 20 percent lower yieldbut higher sugars and acid (Moreshet et al., 1983). Another study showed that allowing 80percent depletion of available water in the surface 1 m in the summer and winter and 60percent depletion in spring and autumn did not reduced yields with navels (Wiegand andSwanson, 1982). The same regime reduced fruit weight in cv. Valencia. No applied water datawas given and it was difficult to judge the level of stress that the depletion of soil waterinduced in the trees. Another approach to produce season-long stress is to irrigate at fractionsof ET c . For grapefruit, applying 35 percent less than full ET c delayed maturity and reduced yieldby 13 percent due to both smaller fruit and lower fruit load. The sugar and acid content washigher while there was no impact on the sugar/acid ratio (Eliades, 1994). For navels, irrigatingwith 22 and 46 percent less than full ET c reduced yields by 7 and 22 percent, respectively (Brychand Luedders, 1988). Finally, for clementines the application of 55 percent of tree water needsduring the entire season reduced yields by only 17 percent (Gonzalez-Altozano and Castel,1999).Indicators of tree water statusThe established indicator of tree water status is the stem-water potential (SWP) as measuredwith the pressure chamber. A simpler and less time-consuming approach is to measure thewater potential of shaded leaves. Both Goldhamer with navels, and Castel with clementines,found the slope of regression lines very close to unity and high correlations between thismeasurement and SWP (slopes of 0.97 and 0.98; R 2 of 0.95 and 0.96, respectively). MiddaySWP values above – 1.0 MPa indicate the absence of water stress for a typical summer day ofET o about 6-7 mm/day. Values between – 1.0 and -1.5 MPA are indicative of mild stress. Citruscan withstand more negative SWP than other fruit trees and vines. Trees show hardly anyvisual symptoms of stress with SWP of -2.0 to -2.5 MPa. Severely stressed orange trees thathad predawn LWP around -6.5 MPa recovered their water status within a week of rewateringalthough they experienced severe defoliation and took several weeks more for completefunctional recovery (Fereres et al., 1979).Water Use RequirementsOrchard transpiration, the primary component of ET c , depends directly on stomatalconductance and scientists recognized early on that the stomatal behaviour of citrus differssignificantly from that of most other crop plants. An early comparative study found thatmaximum stomatal conductance (Gl) of soybean, wheat, and orange under fully irrigatedconditions was 1.0, 0.60, and 0.48 cm/s, respectively, and occurred at about 09:00 hours. Theyattributed this to differences in stomatal opening and densities (Meyer and Green, 1981).A comparison of citrus Gl against that of almond and pistachio under similar conditions inthe San Joaquin Valley of California carried out by Goldhamer showed that citrus reached aGl maximum of around 0.4 cm/s at 08:00 hours and declined steadily thereafter. By contrast,CITRUS 321