Assessing microtensiometers for monitoring stem water potential in mandarin (Citrus reticulata Blanco) orchard under different irrigation regimes

Stem water potential (Ψ stem ) is the standard indicator of crop water status, commonly estimated using the Scholander pressure chamber (PC). However, it does not allow continuous monitoring, limiting real-time decision-making. The recently developed microtensiometer (MT) offers continuous Ψ stem monitoring, overcoming the PC limitations. This study assesses the MT’s reliability in monitoring Ψ stem dynamics in a mandarin orchard in Sicily, Italy, under two irrigation regimes: i) a frequent irrigation regime via subsurface drip irrigation (SDI) system, employed to ensure well-watered conditions, and ii) an ordinary irrigation management typical of the area, provided by a micro-sprinkler (SPR) system. Over two seasons, Ψ stem measurements obtained with the MT (Ψ mt ) were compared with those from the PC (Ψ pc ). Shading trials were also conducted to evaluate the sensor’s ability to detect the plant reactions to external perturbations. A step-change in solar radiation imposed by artificial shading revealed different signal dynamics of MT sensors compared to pressure chamber Ψ pc measurements, characterised by a 4-fold increase in signal time constant. This led to a consequential impact on Ψ stem estimation, resulting in both a substantial time lag and attenuation, consistent with first-order sensor dynamics. Across both seasons and irrigation regimes, daily Ψ mt overestimated Ψ stem compared to Ψ pc , along with a clear time lag coherent with the first-order response shown by the sensor-tree-complex. Under the SDI irrigation regime, Ψ mt was significantly correlated with vapour pressure deficit, indicating a strong plant-atmosphere coupling under stable soil moisture conditions. Normalised cross-correlation analysis between reference evapotranspiration and Ψ mt showed variable time lags, specifically in the SPR plot, subjected to alternating soil moisture conditions. These findings suggest that MTs are a promising tool for real-time monitoring of mandarin water status. However, the first-order dynamics of the sensor-tree-complex response signal preclude the use of Ψ mt as a direct replacement for pressure-chamber Ψ stem measurements. Consequently, suitable compensation protocols accounting for a large time constant should be developed to apply Ψ mt readings to currently published crop-specific Ψ stem thresholds for irrigation scheduling protocols.