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

Abstract

Stem water potential (Ψstem) 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 Ψstem monitoring, overcoming the PC limitations. This study assesses the MT’s reliability in monitoring Ψstem 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, Ψstem measurements obtained with the MT (Ψmt) were compared with those from the PC (Ψpc). 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 Ψpc measurements, characterised by a 4-fold increase in signal time constant. This led to a consequential impact on Ψstem estimation, resulting in both a substantial time lag and attenuation, consistent with first-order sensor dynamics. Across both seasons and irrigation regimes, daily Ψmt overestimated Ψstem compared to Ψpc, along with a clear time lag coherent with the first-order response shown by the sensor-tree-complex. Under the SDI irrigation regime, Ψmt 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 Ψmt 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 Ψmt as a direct replacement for pressure-chamber Ψstem measurements. Consequently, suitable compensation protocols accounting for a large time constant should be developed to apply Ψmt readings to currently published crop-specific Ψstem thresholds for irrigation scheduling protocols.