Exploring the water status of drip-irrigated apple trees based on variation in predawn stem water potential

Determining irrigation timing based on plant water status is essential for drip-irrigated apple orchards. Stem water potential (Ψ stem ) has been suggested as a tool for precision irrigation in fruit trees, but clear thresholds for drip-irrigated apple trees are lacking. In 2023 and 2024, this study used PSY1 technology to continuously monitor Ψ stem in apple trees under fully irrigated (NS), moderately water-deficient (MS), and severely water-deficient (SS) treatments on the Loess Plateau. The 2023 data were collected from a single tree per treatment and are therefore descriptive only. The results showed that Ψ stem exhibited a sinusoidal diurnal pattern, with NS treatment exhibiting the highest Ψ stem values in both years. Descriptively, SS reduced predawn and midday Ψ stem (Ψp and Ψm, respectively) by 0.13–0.14 MPa in 2023; in 2024, MS and SS reduced them by 0.22–0.33 and 0.22–0.34 MPa, respectively. Ψp exhibited a higher discrimination coefficient and significantly stronger signal intensity than Ψm. Moreover, as soil water potential (Ψ soil ) declined, the dynamics of Ψp exhibited three distinct phases: a gradual decline (Ψ soil ≥ −0.10 MPa), a sharp drop (−0.14 < Ψ soil < −0.10 MPa), and a rebound (Ψ soil ≤ −0.14 MPa), with critical thresholds identified at −0.51 MPa (95% CI: −0.78 to −0.39 MPa) and −0.92 MPa (95% CI: −1.05 to −0.82 MPa), respectively. These values can serve as indicators of water stress in apple trees and the need for irrigation. The findings provide important reference values for the formulation of orchard irrigation strategies.