Vaccaro et al. (2025) Assessing microtensiometers for monitoring stem water potential in mandarin (Citrus reticulata Blanco) orchard under different irrigation regimes

Identification

• Journal: Agricultural Water Management
• Year: 2025
• Date: 2025-10-08
• Authors: Girolamo Vaccaro, Mariachiara Fusco, Vincenzo Alagna, Loris Franco, Antonio Motisi, Massimo Iovino
• DOI: 10.1016/j.agwat.2025.109873

Research Groups

• Department of Agricultural, Food and Forest Sciences (SAAF), University of Palermo, Palermo, Italy
• Irritec S.p.A., Capo D’Orlando, Italy

Short Summary

This study assessed the reliability of microtensiometers (MTs) for continuous stem water potential (Ψstem) monitoring in a mandarin orchard under two irrigation regimes. It found MTs promising for real-time monitoring, but their first-order response dynamics (characterized by significant time lags and signal attenuation) necessitate the development of compensation protocols for direct application of readings to existing crop-specific Ψstem thresholds.

Objective

• To assess the reliability and performance of microtensiometers (MTs) for continuous stem water potential (Ψstem) monitoring in mandarin trees (Citrus reticulata Blanco) under Mediterranean climate conditions and different irrigation regimes.
• To specifically analyze: (i) the accuracy of MT measurements (Ψmt) in comparison with the established Scholander pressure chamber (Ψpc), (ii) the MT’s ability to detect plant reactions to external perturbations induced by shading trials, and (iii) the sensitivity of Ψmt values to varying climatic conditions.

Study Configuration

• Spatial Scale: A 30-year-old mandarin orchard (Citrus reticulata Blanco cv. Tardivo di Ciaculli grafted onto Citrange “Carrizo”) spaced 5 m × 5 m, located near Palermo, Italy (38°4′53.4′′ N, 13°25′8.2′′ E). The orchard was divided into two plots: one with subsurface drip irrigation (SDI) and one with micro-sprinkler (SPR) irrigation. Soil water content was monitored from the surface to a depth of 0.60 m.
• Temporal Scale: Two irrigation seasons (June-October) in 2022 and 2023. Continuous MT and soil water content data were recorded every 20 minutes. Meteorological data were recorded every 30 minutes. Pressure chamber measurements were taken every 3 hours (2022) or 2 hours (2023) from 06:00 to 18:00/20:00. Shading trials were conducted for 24 hours in August and September 2023.

Methodology and Data

• Models used:
  • FAO-56 Penman-Monteith equation for estimating hourly grass reference evapotranspiration (ETo).
• Data sources:
  • Microtensiometers (MTs): Four commercial FloraPulse microtensiometers, with two installed in the trunk of a representative tree in each plot, providing continuous stem water potential (Ψmt) data every 20 minutes.
  • Scholander pressure chamber (PC): Manual measurements of stem water potential (Ψpc) on two healthy stems per tree, following standard protocols.
  • Soil Water Content (SWC) probe: A 0.60 m long "Drill & Drop" probe (Sentek Pty Ltd.) measuring volumetric soil water content and temperature at 0.10 m intervals from the surface to 0.60 m depth, recorded every 20 minutes.
  • Weather station: A WatchDog 2000 series weather station (Spectrum Technologies, Inc.) recorded air temperature (T), relative humidity (RH), vapour pressure deficit (VPD), precipitation (P), solar radiation (SR), and wind speed (u) every 30 minutes.
  • Soil hydrological characteristics: Determined using pressure plate extractors for volumetric SWC at field capacity (θfc) and wilting point (θwp).
  • Soil organic matter: Determined using the loss-on-ignition method.
  • Irrigation systems: Subsurface drip irrigation (SDI) with self-compensating drippers (2.1 L/h at 100 kPa) and micro-sprinkler (SPR) irrigation with two micro-sprinklers per tree (200 L/h at 150 kPa).

Main Results

• Microtensiometers (MTs) are a promising tool for real-time monitoring of mandarin water status, but their first-order dynamics (sensor-tree-complex response) lead to a substantial time lag and signal attenuation, precluding their direct use as a replacement for pressure chamber (PC) Ψstem measurements.
• Ψmt consistently overestimated Ψstem compared to Ψpc, with an average offset of 0.66 MPa in the afternoon and a time lag of up to 2 hours for minimum values, regardless of the irrigation regime.
• Shading trials revealed that MTs responded to external perturbations, but with a slower, more gradual response compared to Ψpc. The estimated time constant (τ) for the MT sensor-tree complex was approximately 4 times higher (122-125 minutes) than for Ψpc (29-32 minutes).
• Under the frequent subsurface drip irrigation (SDI) regime, Ψmt remained relatively stable and showed a strong correlation with vapour pressure deficit (VPD) (R² = 0.57 in 2022, R² = 0.70 for the refilling phase), indicating strong plant-atmosphere coupling under stable soil moisture conditions.
• Under the ordinary micro-sprinkler (SPR) irrigation regime, Ψmt exhibited greater variability and weaker correlations with VPD, reflecting a stronger influence of fluctuating soil moisture conditions.
• Predawn Ψmt values in the SPR plot showed significant recovery (up to 40% of pre-irrigation values) within 24 hours post-watering, highlighting the tree's rapid response to water availability.
• The time constant (τ) of Ψmt demonstrated sensitivity to different irrigation regimes, remaining stable under SDI but showing distinct patterns within SPR irrigation cycles.
• Practical limitations included sensor failure due to water infiltration into insulation and internal disconnections, leading to unrealistic Ψmt values.
• A preliminary predawn Ψmt threshold of −0.51 ± 0.09 MPa was suggested for well-watered mandarin orchards.

Contributions

• This study represents the first comprehensive assessment of microtensiometers for continuous stem water potential monitoring in mandarin trees under Mediterranean climate conditions and different irrigation regimes.
• It quantitatively characterized the first-order dynamics (time lag and signal attenuation) of the microtensiometer sensor-tree complex response in mandarin, providing critical insights into its implications for direct Ψstem estimation.
• The research demonstrated the sensitivity of the MT time constant to varying irrigation regimes, suggesting its potential as an informative indicator of tree physiological status beyond merely a measurement limitation.
• It provided valuable insights into the temporal coupling between plant water potential and evaporative demand in mandarin, highlighting the importance of considering dynamic responses in irrigation scheduling.
• The study proposed a preliminary predawn Ψmt threshold for well-watered mandarin orchards, contributing to the development of precision irrigation protocols for this crop.

Funding

• European Union - FESR or FSE, PON Research and Innovation 2014–2020 - DM 1062/2021 (CUP: B7521002300001)
• CN_00000022 “National Research Centre for Agricultural Technologies (Agritech)” financed by D.R. n. 1032 on 17.06.2022, PNRR MUR - M4C2 −1.4 - “Centri Nazionali” - D.D. n. 3138 on 16/12/2021, CUP: B13D21011580004. Spoke 3 Enabling Technologies and sustainable strategies for the smart management of agricultural systems and their environmental impact.
• PRIN 2022, “Smart Technologies and Remote Sensing methods to support the sustainable Agriculture WAter Management of Mediterranean woody Crops (SWAM4Crops)”, CUP: B53D23018040001.

Citation

@article{Vaccaro2025Assessing,
  author = {Vaccaro, Girolamo and Fusco, Mariachiara and Alagna, Vincenzo and Franco, Loris and Motisi, Antonio and Iovino, Massimo},
  title = {Assessing microtensiometers for monitoring stem water potential in mandarin (Citrus reticulata Blanco) orchard under different irrigation regimes},
  journal = {Agricultural Water Management},
  year = {2025},
  doi = {10.1016/j.agwat.2025.109873},
  url = {https://doi.org/10.1016/j.agwat.2025.109873}
}