Beyá-Marshall et al. (2025) Water stress thresholds for walnuts: Stem water potential baselines to maximize yield and water productivity

Identification

• Journal: Agricultural Water Management
• Year: 2025
• Date: 2025-09-22
• Authors: Beyá-Marshall, Gustavo A. Lobos, Felipe A. Calvo, Jaime Otárola, Eduardo R. Trentacoste
• DOI: 10.1016/j.agwat.2025.109808

Research Groups

• Estación Experimental Lumina Terra, Santiago, Chile
• Instituto de Investigaciones Agropecuarias INIA Intihuasi, La Serena, Chile
• Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja (CRILAR - CONICET), La Rioja, Argentina
• Instituto de Agricultura Sostenible en el Oasis (Universidad Nacional de Chilecito), La Rioja, Argentina
• Instituto de Investigaciones Agropecuarias INIA Rayentu´e, Rengo, Chile
• Estación Experimental Agropecuaria La Consulta (Instituto Nacional de Tecnología Agropecuaria), Mendoza, Argentina

Short Summary

This study established cultivar-specific midday stem water potential (Ψₓ) baselines and detrimental thresholds for 'Chandler' and 'Serr' walnuts, demonstrating that optimizing irrigation based on these baselines can improve water productivity by 20–25% while sustaining high yields under water-limited conditions.

Objective

• To determine the water potential baselines (Ψₓ as a function of vapor pressure deficit (VPD) under non-soil-water-limited and non-stressed conditions) for 'Chandler' and 'Serr' walnut cultivars.
• To define water stress thresholds to maximize walnut water productivity, nut yield, and nut size in 'Chandler' and 'Serr' cultivars.

Study Configuration

• Spatial Scale: Multi-year, multi-location trials conducted in commercial walnut orchards across productive regions of Argentina (Guanchin-La Rioja, Los Sauces-Mendoza) and Chile (Illapel-Choapa, El Barraco-Salamanca, Cabrería-Limarí, Cuncum´en-Salamanca, I. Norte-Ovalle), located between 27°S and 40°S latitude.
• Temporal Scale: Multi-year trials spanning from 2014 to 2024, with data collected during the walnut growing season (October to May). Orchard ages ranged from 6 to over 17 years.

Methodology and Data

• Models used:
  • Linear mixed models (LMMs) for statistical analysis.
  • Fisher’s LSD test for multiple comparisons.
  • Regression modeling for yield loss categories.
  • Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) for model selection.
  • FAO-modified Penman-Monteith equation for reference evapotranspiration (ETo).
  • Equation for crop coefficient reduction (Kr = (2 * %Cover) / 100).
  • Murray (1967) method for hourly air vapor pressure deficit (VPD) estimation.
  • Quadratic polynomial regressions for relative nut weight and yield responses to Ψₓ deficit.
  • Myers (1988) integral water stress formula (adapted).
• Data sources:
  • Data collected from various irrigation experiments in commercial 'Chandler' and 'Serr' walnut orchards.
  • Midday stem water potential (Ψₓ) measurements using Scholander pressure chambers (every 10–20 days during the irrigation period).
  • Hourly meteorological data (air temperature, relative humidity, VPD, ETo) from automated weather stations.
  • Yield components including total yield, average fruit weight, kernel yield, and commercial nut size classification.
  • Water productivity (WP) calculated as total yield divided by the sum of irrigation water applied and effective rainfall.
  • Crop load and yield efficiency normalized based on Trunk Cross-Sectional Area (TCSA).

Main Results

• Midday stem water potential (Ψₓ) showed a strong linear correlation with vapor pressure deficit (VPD), with R² values ranging from 0.29 to 0.65.
• Full Irrigation Baseline: For both 'Chandler' and 'Serr' cultivars, Ψₓ decreased linearly with increasing VPD at a slope of approximately -0.07 MPa per kilopascal (kPa), ranging from -0.3 MPa to -0.5 MPa within a VPD range of 0.5–4.0 kPa.
• Non-Stressed Baseline:
  • For 'Chandler', the non-stressed Ψₓ ranged from -0.55 MPa to -0.70 MPa within a VPD range of 0.5–4.0 kPa.
  • For 'Serr', the non-stressed Ψₓ ranged from -0.62 MPa to -0.80 MPa within a VPD range of 0.5–4.0 kPa.
  • Optimizing irrigation based on this non-stressed baseline improved water productivity by 20–25% compared to fully irrigated treatments.
• Optimal Ψₓ Deficit for Maximum Yield and Fruit Size:
  • 'Chandler' achieved maximum yield and fruit size with a Ψₓ deficit ranging from -0.10 MPa to -0.30 MPa below the full irrigation baseline, peaking at -0.22 MPa.
  • 'Serr' performed optimally with a Ψₓ deficit ranging from -0.15 MPa to -0.40 MPa below the full irrigation baseline, peaking at -0.29 MPa.
• Detrimental Ψₓ Thresholds (Yield Loss):
  • 'Chandler': 0–5% yield loss occurred at Ψₓ values from -0.55 MPa to -0.79 MPa; 5–15% yield loss from -0.55 MPa to -0.80 MPa; and 15–25% yield loss from -0.56 MPa to -0.80 MPa.
  • 'Serr': 0–5% yield loss occurred at Ψₓ values from -0.62 MPa to -0.83 MPa; 5–15% yield loss from -0.66 MPa to -0.91 MPa; and 15–25% yield loss from -0.74 MPa to -0.95 MPa.
  • Yield losses increased progressively beyond these optimal thresholds, reaching 25% or more at higher water deficits.
• 'Serr' demonstrated higher maximum water productivity and greater tolerance to water deficit compared to 'Chandler'.
• Over-irrigation (maintaining Ψₓ at or above the full irrigation baseline) was found to negatively impact root health, increase the risk of Phytophthora infestation, and reduce fruit size and yield.

Contributions

• This study is the first to establish a non-stressed Ψₓ baseline for walnuts, validated with multi-location production data, providing a robust guideline for optimal yield and fruit size.
• It provides cultivar-specific (for 'Chandler' and 'Serr') Ψₓ baselines (full irrigation and non-stressed) and detrimental thresholds associated with 5%, 15%, and 25% yield losses.
• The research offers robust, practical, and science-based guidelines for walnut irrigation management by standardizing Ψₓ measurements against VPD, thereby accounting for environmental variability.
• It demonstrates that maintaining moderate Ψₓ deficits, rather than full irrigation, is the optimal approach to maximize water productivity and sustain high yields, challenging previous assumptions.
• A predictive equation for the full irrigation Ψₓ based on air temperature and relative humidity is provided, enhancing practical application for growers.
• The methodology incorporates standardization for crop load and tree size (Trunk Cross-Sectional Area) in yield analysis, offering a clearer understanding of the direct impact of water status on yield performance.

Funding

Not explicitly stated in the provided text.

Citation

@article{BeyáMarshall2025Water,
  author = {Beyá-Marshall and Lobos, Gustavo A. and Calvo, Felipe A. and Otárola, Jaime and Trentacoste, Eduardo R.},
  title = {Water stress thresholds for walnuts: Stem water potential baselines to maximize yield and water productivity},
  journal = {Agricultural Water Management},
  year = {2025},
  doi = {10.1016/j.agwat.2025.109808},
  url = {https://doi.org/10.1016/j.agwat.2025.109808}
}