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Knowing a tree crop’s seasonal growth and development as a function of heat accumulation can facilitate scheduling of irrigation, pesticide applications, and harvest. Our objective was to compare the goodness of fit of applied models and determine which provides the best description of pistachio nut growth as a function of thermal unit accumulation. Three fruit growth traits of pistachio—pericarp (hull) + endocarp (shell) size, endocarp thickening and hardening, and embryo (kernel) size—exhibited clear nonlinear dependence on heat accumulation. We tested three nonlinear models—Michaelis–Menten, three-parameter logistic, and Gompertz—fitted to fruit development data to create a tool to forecast pest susceptibility and harvest timing. Observation of development began at full bloom and ended at harvest. Data were collected from six pistachio cultivars in one experimental and eight commercial orchards over 3 years. Analyses of residual distribution, parameter standard errors, coefficient of determination (R2) and the Akaike information criterion (AIC) all demonstrated the Gompertz function was the best model. Cultivars differed significantly in all the three parameters (Asym, b, and c) for all three traits with the Gompertz model, demonstrating the Gompertz model can adjust to incorporate cultivar differences. The growth curve of the three traits together provided integrated information on nut biomass accumulation that facilitates predicting the critical timing for multiple orchard management practices.

The response of “Kerman” pistachio trees budded on three different rootstocks ( Pistacia terebinthus, Pistacia atlantica and Pistacia integerrima ) to regulated deficit irrigation (RDI) in shallow soils was studied for 3 years. The trees were either fully irrigated (C treatment) or subjected to deficit irrigation during Stage II of fruit growth with two water stress thresholds (T1 and T2). The irrigation scheduling for fully-irrigated trees and water-stressed trees was managed by means of midday stem water potential (Ψ stem ) measurements. The use of direct measurements of the water status allowed estimating accurately the irrigation requirements for pistachio trees, with water reductions ranging from 46 to 205 mm in fully-irrigated trees. The combination of the Ψ stem use and the RDI regime saved 43–70% in T1 and 48–73% in T2 of water compared to the calculated crop evapotranspiration (ET c ) for fully irrigated treatment (C). Deficit irrigation during Stage II significantly reduced the vegetative growth of the trees. Yield and fruit quality were not affected by any irrigation regime, except during the first year of the study. Thus, the results indicate that full irrigation scheduling and RDI can be achieved successfully using Ψ stem tool on pistachio trees growing in shallow soils. A Ψ stem threshold of −1.5 MPa during stage II (T1) was suggested for RDI scheduling, as it did not reduce the yield or the production value. However a Ψ stem threshold of −2.0 MPa (T2) resulted in a significant reduction and an extensive delay in the recovery of stomatal conductance (gl) , with negative effects on long-term pistachio production. P. integerrima showed a weaker capacity of adaptation to the study conditions compared to P. atlantica and P. terebinthus , having a tendency to get more stressed and to produce a lower quality crop. [ABSTRACT FROM AUTHOR]