Water stress thresholds for regulated deficit irrigation in pistachio trees: Rootstock influence and effects on yield quality

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 (Psi(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 Psi(stem) use and the RDI regime saved 43-70% in T1 and 48-73% in T2 of water compared to the calculated crop evapotranspiration (ETc) 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 Psi(stem) tool on pistachio trees growing in shallow soils. A Psi(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 Psi(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. (C) 2015 Elsevier B.V. All rights reserved.