Dito et al. (2026) Establishing hazelnut stem water potential baseline to improve water management

Identification

• Journal: Frontiers in Plant Science
• Year: 2026
• Date: 2026-03-27
• Authors: Giacomo Dito, Sofia Russo, Nik G. Wiman, Sergio Tombesi
• DOI: 10.3389/fpls.2026.1771736

Research Groups

• Department of Sustainable Crop Production, Università Cattolica del Sacro Cuore, Piacenza, Italy
• North Willamette Research and Extension Center, Oregon State University, Aurora, OR, United States

Short Summary

This study established the first species-specific stem water potential (SWP) baseline for hazelnut (Corylus avellana L.) under non-limiting soil moisture conditions, revealing a stable, linear relationship between SWP and vapor pressure deficit (VPD) to improve irrigation management.

Objective

• To establish a species-specific stem water potential (SWP) baseline for hazelnut (Corylus avellana L.) under non-limiting soil moisture conditions, primarily driven by vapor pressure deficit (VPD), to optimize irrigation management and prevent over-irrigation.

Study Configuration

• Spatial Scale: Experimental hazelnut orchards in Piacenza, Italy (45°02′03.2″N, 9°43′51.7″E; 61 m a.s.l.) and commercial hazelnut orchards in the Willamette Valley, Corvallis, Oregon, USA (44°26′46.9″N, 123°18′15.6″W; 44°26′08.3″N, 123°13′26.6″W).
• Temporal Scale: 2024 growing season (May to October in Italy, May to August in Oregon), supplemented with measurements from previous trials. Weekly measurements were conducted between 12:00 and 14:00 local time.

Methodology and Data

• Models used:
  • FAO Penman–Monteith equation for reference evapotranspiration (ET0).
  • Crop evapotranspiration (ETc) calculated as ETc = ET0 × Kc × Kr.
  • Linear regression for SWP-VPD baseline: SWP (MPa) = −0.09 × VPD − 0.18.
  • Nonlinear regression for leaf transpiration (E) vs. VPD.
  • Third-degree (cubic) polynomial regression for stomatal conductance (gs) vs. SWP.
• Data sources:
  • Field measurements of Stem Water Potential (SWP) using a Scholander chamber (model 3005H07G4P40, Soil Moisture Equipment).
  • Field measurements of stomatal conductance (gs), leaf transpiration (E), and net photosynthesis (An) using a portable gas analyzer (LCpro T, ADC BioScientific) or porometer (LI-600, LI-COR Biosciences).
  • Vapor Pressure Deficit (VPD) derived from atmospheric features.
  • A large dataset of SWP–VPD pairs collected from different hazelnut cultivars (Tonda di Giffoni, McDonald, Jefferson), grafting types (self-rooted, grafted on Corylus colurna), and irrigation managements (fully irrigated, rain-fed) in Italy and Oregon, supplemented with data from previous trials.

Main Results

• A stable and linear relationship between SWP and VPD was established for hazelnut under non-limiting soil water conditions, with the baseline equation: SWP (MPa) = −0.09 × VPD − 0.18 (R² = 0.80, p < 0.0001).
• The baseline was derived from the upper 0.07 fraction of SWP values within 0.5-kPa VPD classes, covering a VPD range of 0.2 to 3.5 kPa and an SWP range of −0.22 to 0.55 MPa for the baseline data.
• The overall dataset had a VPD range from 0.2 to 5 kPa and an SWP range from −0.05 to −1.4 MPa.
• Hazelnut exhibited a less steep baseline slope (−0.09) compared to olive (−0.18) and Prunus spp. (−0.12), suggesting a relatively buffered SWP response to increasing VPD when soil water supply is adequate.
• Stomatal closure in hazelnut occurs at relatively high SWP values (around −1.4 MPa), supporting its classification as an isohydric species.
• The relationship between leaf transpiration (E) and VPD was positive, with a steeper increase between 0.5 and 2 kPa, followed by a tendency to flatten at higher VPD values. The highest E observed was 11 mmol s⁻¹ m⁻².
• The relationship between stomatal conductance (gs) and SWP was non-linear, tending to saturate after −0.8 MPa, with gs approaching closure at −1.4 MPa and reaching maximum at −0.45 MPa. Optimal gs values were approximately 0.2–0.6 mmol m⁻² s⁻¹ at −0.4 MPa.
• No statistically significant differences were detected in the SWP–VPD relationship between grafted and own-rooted plants under well-watered conditions.
• SWP values close to the baseline indicate non-limiting soil water conditions, negative deviations reflect increasing water stress, and values exceeding the baseline may signal excessive soil moisture, potentially indicating transient waterlogging.

Contributions

• Provides the first species-specific stem water potential (SWP) baseline for hazelnut (Corylus avellana L.) under non-limiting soil water conditions, developed across different environments and cultivars.
• Offers a practical, plant-based threshold for direct application in field irrigation management, enabling optimization of water application timing and amount.
• Establishes a robust physiological reference to distinguish between atmospheric-driven and soil-driven reductions in plant water status.
• Helps growers avoid both water deficit and excessive irrigation, which can lead to root inefficiency or waterlogging stress.
• The baseline can be integrated with other plant-based indicators (e.g., Crop Water Stress Index) into advanced irrigation decision-support frameworks, enhancing sustainable water use.
• The multi-site and multi-cultivar approach contributes to the robustness and generalizability of the established baseline.

Funding

• The National Recovery and Resilience Plan (PNRR) (J53C22002020001)
• Helix a.r.l.
• Portus project, funded by the Romeo and Enrica Invernizzi Foundation
• PhD in Agro-Food System (Agrisystem) of the Università Cattolica del Sacro Cuore (Italy)

Citation

@article{Dito2026Establishing,
  author = {Dito, Giacomo and Russo, Sofia and Wiman, Nik G. and Tombesi, Sergio},
  title = {Establishing hazelnut stem water potential baseline to improve water management},
  journal = {Frontiers in Plant Science},
  year = {2026},
  doi = {10.3389/fpls.2026.1771736},
  url = {https://doi.org/10.3389/fpls.2026.1771736}
}