Plant-Based Methodologies and Approaches for Estimating Plant Water Status of Mediterranean Tree Species: A Semi-Systematic Review

Abstract

Global climate change presents a threat for the environment, and it is aggravated by the mismanagement of water use in the agricultural sector. Since plants are the intermediate component of the soil–plant–atmosphere continuum, and their physiology is directly affected by water availability, plant-based approaches proved to be sensitive and effective in estimating plant water status and can be used as a possible water-saving strategy in crop irrigation scheduling. This work consists of two parts: the first part extensively reviews the plant-based methods and approaches that are most applied to monitor the plant water status (PWS), the different technologies available, the gaps, and the possibility of further improvements in establishing a sustainable irrigation schedule. The various approaches are described, and the differences between conventional and recent improved methods are analyzed. The second part is an extensive dataset survey of 83 publications from 2012 to 2022 that applied the main monitoring methodologies and approaches for water status assessment in fruit and nut tree crops cultivated in a Mediterranean climate. The aim of this work is to serve as a practical reference to deepen reader knowledge on PWS and enhance researchers to identify gaps and potential advances in designing user-friendly monitoring technologies.

Plant Physiological Analysis to Overcome Limitations to Plant Phenotyping

“…Other physiological approaches can be used to gauge plant water content for physiological analysis, including stomatal conductance (G s ), leaf turgor, leaf thickness, water potential (Ψ), relative water content (RWC), and sap flow (SF) [156]. Nevertheless, the measurement of such parameters can be destructive and/or time-consuming.…”

“…However, nuclear magnetic resonance requires highly trained personnel, expensive equipment, detailed post-processing of data, and an extended period for each measurement, making it unsuited to low-cost, fast phenotyping applications. Measurement of the rate that sap flow ascends a plant can determine its transpiration rate, and can, thus, be used an indicator of plants water status [156]. These measurements can be made using two main approaches: the first calculates the sap flow rate through the heat balance method, which is destructive and unsuited for plant phenotyping., while the second method calculates sap flux density using the heat pulse or continuous thermal dissipation methods [172].…”

“…The water potential of the xylem (Ψ xylem ) reflects whole plant transpiration with soil and root/soil hydraulic conductivity. Stem water potential (Ψ stem ) is measured on a non-transpiring leaf, as when leaves do not transpire, their water potential is considered to correspond to stem water potential [156]. Several studies have shown that Ψ stem can be an effective water deficit indicator, and a more representative measure of plant water status than Ψ leaf [173][174][175].…”

Relating microtensiometer-based trunk water potential with sap flow, canopy temperature, and trunk and fruit diameter variations for irrigated ‘Honeycrisp’ apple

“…There are many plant-based sensors that can continuously and directly measure real-time trees' physiological responses and assess tree water status and fruit growth such as microtensiometers, sap flow sensors, thermoradiometers, and trunk and fruit dendrometers (Fernandez, 2017). All of them can be incorporated into decision-support systems for irrigation management and have shown a strong relationship with reference tree water status indicators such as the midday stem water potential measured with the Scholander pressure chamber and tree gas exchange (Fernandez, 2017;Noun et al, 2022). However, as has been previously reported by Garcia-Tejera et al (2021), the benefits of continuously monitoring tree water status for irrigation management based on threshold values depend on the possibility of rapidly triggering irrigation when the threshold value is exceeded, which in commercial orchards is not always possible.…”

Combining thermal imaging and soil water content sensors to assess tree water status in pear trees

“…In the same vein, stomatal conductance has been widely reported as a sensitive physiological indicator of tree water status. However, it has not been implemented in commercial orchards because it is also labor intensive and requires highly technical and expensive equipment ( Struthers et al., 2015 ; Noun et al., 2022 ). Other tree water status indicators such as canopy temperature (Tc) have also been identified ( García-Tejero et al., 2011 ; Gonzalez-Dugo et al., 2012 ; Ramírez-Cuesta et al., 2022 ).…”