Establishing a Reference Baseline for Midday Stem Water Potential in Olive and Its Use for Plant-Based Irrigation Management

Abstract

Midday stem water potential (SWP) is rapidly becoming adopted as a standard tool for plant-based irrigation management in many woody perennial crops. A reference or “baseline” SWP has been used in some crops (almond, prune, grape, and walnut) to account for the climatic influence of air vapor pressure deficit (VPD) on SWP under non-limiting soil moisture conditions. The baseline can be determined empirically for field trees maintained under such non-limiting conditions, but such conditions are difficult to achieve for an entire season. We present the results of an alternative survey-based approach, using a large set of SWP and VPD data collected over multiple years, from irrigation experiments in olive orchards located in multiple countries [Spain, United States (California), Italy, and Argentina]. The relation of SWP to midday VPD across the entire data set was consistent with an upper limit SWP which declined with VPD, with the upper limit being similar to that found in Prunus. A best fit linear regression estimate for this upper limit (baseline) was found by selecting the maximum R2 and minimum probability for various upper fractions of the SWP/VPD relation. In addition to being surprisingly similar to the Prunus baseline, the olive baseline was also similar (within 0.1 MPa) to a recently published mechanistic olive soil-plant-atmosphere-continuum (SPAC) model for “super high density” orchard systems. Despite similarities in the baseline, the overall physiological range of SWP exhibited by olive extends to about −8 MPa, compared to about −4 MPa for economically producing almond. This may indicate that, despite species differences in physiological responses to low water availability (drought), there may be convergent adaptations/acclimations across species to high levels of water availability. Similar to its use in other crops, the olive baseline will enable more accurate and reproducible plant-based irrigation management for both full and deficit irrigation practices, and we present tentative SWP guidelines for this purpose.

Prediction of Stem Water Potential in Olive Orchards Using High-Resolution Planet Satellite Images and Machine Learning Techniques

“…The high variation in Ψ stem (particularly in July and August) is in line with the results found in the literature [8]. Previous studies have highlighted that, as the olive tree is considered a drought-resistant species, a wide range of Ψ stem in olive orchards can be expected under different cultivated conditions [44] and different cultivars [45]. In addition, Giorio et al [46] highlighted that the olive tree is characterized by low hydraulic conductivity, which can be responsible for a high degree of variation in Ψ stem in response to changing environmental factors [9].…”

Monitoring Stem Water Potential with an Embedded Microtensiometer to Inform Irrigation Scheduling in Fruit Crops

“…Mid-day minimum stem potential values integrate the soil potential limitations, the evaporative demand, and other factors that may affect gas exchange and stomatal opening, complicating interpretation. To help differentiate these effects, a baseline water potential has been defined as the mid-day stem water potential that would be expected for plants with non-limiting soil moisture, with an adjustment for the evaporative demand due to vapor pressure deficit (VPD) [36,52]. If the measured stem potential is more negative than the baseline after adjusting for VPD, that indicates that soil water has become limited to some extent, and irrigation may be needed.…”

A new empirical framework to quantify the hydraulic effects of soil and atmospheric drivers on plant water status

“…(Guo et al., 2019) examined the relationships between Ψ soil and VPD but did not employ a generalizable metric like the stringency metric proposed here. A ‘baseline’ relationship between VPD and stem Ψ md (i.e., the Ψ of a non‐transpiring leaf) was developed by (Shackel, 2011) for irrigation management (Shackel et al., 2021), but its application is confined to non‐limiting soil water supply.…”