The use of dendrometers to measure the stem diameter (SD) of trees provides information about their actual water stress levels. The Scholander chamber is currently the gold standard for measuring stem water potential and thus for quantifying the water status of trees, despite being a laborious method, especially for apple trees. The aim of this study was to analyze dendrometer data to assess the water stress in drip-irrigated ‘Gala’ apple trees (Malus x domestica Borkh). A trial was performed in Grens, Switzerland, during the 2022 season. Three different irrigation treatments were applied (T1 = 100 % soil moisture-based irrigation, T2 = 30 % less than T1, and T3 = without irrigation). The irrigation treatments notably affected fruit production. Trees in T3 exhibited a significant yield loss of 26 %, whereas treatment T2 yielded 13 % less than the reference treatment T1. Although daily changes in the SD correlated with vapor pressure deficit (VPD) (r2 = 0.84), limiting the amount of water available to the plant disrupted this correlation (r2 = 0.27), with stem water potentials dropping by −1.7 MPa and a noticeable shift in stem movement (shrinking/expanding). Finally, we developed a suitable linear regression model that compared the shift in slope and offset of regression lines fitted for SD and VPD during initial non-stressed conditions (reference) with those under stressed conditions in order to estimate stem water potential. By integrating dendrometer parameters with VPD, the model effectively predicted stem water potential values. These findings suggest that dendrometers are effective indicators of water stress in apple trees. Further refinement of the model in field conditions could enhance the use of these sensors for irrigation management, providing more precise guidance on the timing and amount of water applied.
