Crop yield response to water - Cra

(see Chapter 4), as in other species, there is a good correlation between midday shaded LWP(faster to measure) and stem-water potential (Goldhamer et al., 2005).One factor that complicates taking LWP measurement on pistachio leaves is that at the onsetof gas injection into the chamber, exudates, presumably from the phloem, appear at the cutend of the petiole. These can interfere with identifying the instant xylem fluids appear. Oneapproach to eliminating this problem is to use a cotton swab to soak up these exudates priorto the appearance of the xylem fluid. Another approach to eliminating this problem usesblotting paper positioned at the cut end of the petiole that absorbs only xylem fluid butexcludes the other interfering fluids. A third approach is not to use individual leaves but small,interior shaded spurs that may have one to four leaves. The procedure involves covering thespur with a damp cloth just prior to excision. A few millimiters of bark is removed at the cutend with either a small knife or a thumbnail. The entire spur is placed in the chamber afterthe cloth is removed and the reading is taken. It is quite easy to identify the appearance ofthe xylem since there is no interference of phloem exudates and the cross-sectional area ofview is larger than the leaf petiole. Goldhamer also found a good correlation between spurwater potential and midday shaded LWP. The slope of the relationship was about unity butthe intercept indicated that the spur water potential differs from the shaded LWP reading byabout -0.7 MPa.Water RequirementsRelatively few studies have quantified pistachio ET c . Research in Iran with Ohadi on BadamiZarand rootstock (Kermani and Salehi, 2006), concluded that 600 and 1 200 mm per seasonshould be applied with drip and flood irrigation, respectively, although 910 mm was reportedas a ‘previously determined’ irrigation amount for mature pistachio trees (Kermani and Salehi,2006). Early studies found that trees irrigated with a K p (pan evaporation) value of 0.50produced equally as well as those irrigated with a K p of 0.75 for Larnaka on P. integerrimarootstock (Monstra et al., 1995). In Southeast Turkey, the K c values for Antep and Uzun varietiesrose from 0.49 in May to 0.80 in August and continued at this magnitude through the firstweek of September when they declined to 0.32 during October because of leaf senescence(Kanber et al., 1993). However, it was noted that while all irrigation regimes began the seasonwith a nearly full soil water profile, they all ended with it nearly depleted. The researcherssuspected that there was insufficient irrigation to meet ET c for their most heavily irrigatedtrees (Kanber et al., 1993).A soil water balance approach with arrays of neutron probe access tubes to a depth of 3 mand ET o estimates from a nearby weather station was used to calculate bimonthly K c valuesfor mature Kerman on Atlantica rootstock (Table 1) (Goldhamer et al., 1985). A unique aspectof this approach was to make use of soil hydraulic conductivity data obtained in a separateexperiment to eliminate one of the shortcomings of the water balance approach to determineET c : deep percolation below the deepest depth monitored.PISTACHIO 425