Elsevier

Agricultural Water Management

Volume 271, 1 September 2022, 107794
Agricultural Water Management

Optimal irrigation management for avocado (cv. 'Hass') trees by monitoring soil water content and plant water status

https://doi.org/10.1016/j.agwat.2022.107794Get rights and content

Highlights

  • Water savings of 29 % were reached for avocado orchard managed with frequency domain reflectometry (FDR) probes.

  • Breaking point (BP) defined in FDR probe is effective to increase water productivity without compromise fruit size and yield.

  • Greater water inputs or water content criteria over the BP do not improve any yield components.

  • Validated non-stressed baseline is first reported for avocado.

Abstract

Irrigation scheduling based on soil water content (Ɵw) sensors requires that Ɵw be maintained within a range (management lines) that is optimal for plant growth. The lower limit or “breaking point” is determined following the soil water content dynamics on the transition of a rapid rate of depletion to a slower, under similar reference evapotranspiration. Although this criterion is practical, its implementation should be validated with plant water status measurement that contemplate weather condition, such as stem water potential “non-stressed” baseline (Ψₓ as a function of vapor-pressure deficit (VPD) in Ɵw conditions that do not limit yield). A study was conducted on a mature cv. ‘Hass’ avocado orchard in Central Chile during two seasons. There were 5 irrigation treatments: T1, Control; T2 and T3 with 29% less and 25% more of what was applied in T1, respectively; T4 and T5 same as Control until first and second fruit drop abscission, respectively, and then with 29% less. T1 trees were irrigated using a continuous frequency domain reflectometry (FDR) probe to maintain the root zone between field capacity and the breaking point. There was biweekly monitoring of the Ɵw prior to irrigation, Ψₓ and VPD. The Ψₓ decline proportional to the intensity and the timing of water restriction; however, no treatment affected the crop load in either season. T2 did not show significant detrimental in fruit size, production and maturation, despite that frequently reached water content levels at the limit of the breaking point, and showed lower levels of stem water potential than Control, being the treatment with the highest water productivity. The results confirm that breaking point is an effective criterion to establish irrigation management. Additionally, when comparing the baseline for our non-stressed trees with a baseline from full irrigation treatments obtained from the literature, 30% water savings were achieved.

Introduction

Avocado industry has a high relevance in Chile, both in surface and production, since Chile ranks fourth in the world regarding avocado exports (Muñoz et al., 2020). However, in the last decade, Chile has been affected by a severe decrease in water availability (between 25% and 45% less), especially in semi-arid to arid regions (center-north of the country) (Garreaud et al., 2017, Garreaud et al., 2020). Additionally, over-exploitation of water resources combined with a poor agricultural irrigation management have led to a water crisis, reducing agricultural productivity, increasing water access problems for the population and causing potential ecological damage. Under this scenario, the avocado is one of the most affected and controversial species (Muñoz et al., 2020) due to its high-water requirements (Fereres, 2012). Given the competition for water resources that Chile faces, it is necessary to develop agronomic management strategies that increase irrigation Water Productivity (WP) (kg of fruit/m3 of water applied) (Beyá-Marshall et al., 2018). In recent years, irrigation scheduling and monitoring technologies (Scholander pressure chamber and capacitance probes linked to online platforms, mainly) have been gradually introduced to the sector, although there is still a significant technology gap among farmers (Callejas et al., 2019, Callejas et al., 2014).

One of the methods to define irrigation scheduling has been the establishment of plant water status thresholds. Among these, midday stem water potential (Ψₓ) thresholds have been reported for fruit trees (Ferreyra et al., 2007, McCutchan and Shackel, 1992, Naor, 2006, Naor et al., 2008, Shackel et al., 1997, Williams et al., 2012). Although Ψₓ can be used to monitor plant water status, it varies widely based on the environmental conditions when the measurement is taken. In this sense, its normalization with environmental variables, such as vapor-pressure deficit (VPD), has proven to account for this variation (McCutchan and Shackel, 1992, Naor, 2006, Ferreyra et al., 2007, Gálvez et al., 2014, Corell et al., 2016, Shackel et al., 2021).

The response curve of the Ψₓ as function of VPD, at non-limiting soil water content level, is called the fully-irrigated baseline (Shackel et al., 1997), and it has been implemented as an effective criterion to determine the optimal irrigation frequency (Corell et al., 2016, Gálvez et al., 2014, Shackel, 2007, Shackel et al., 2021). However, it is a discontinuous method and requires a high labor to fulfill the frequency of measurements required.

As a counterpart, monitoring soil water content through sensors, mainly FDR and TDR (frequency and time domain reflectometry, respectively) capacitance probes and, most recently, linked to telemetry and ICTs (information and communication technology), have garnered great interest among farmers (Callejas et al., 2019, Vera et al., 2019, Vera et al., 2013). Their use is based on the determination of limits or management lines (ML), which are defined visually following the dynamics of the soil water content (Ɵw). Full point, field capacity and breaking point or irrigation threshold are worthy of note. The full point is defined as the maximum desirable water storage without deep percolation. Field capacity is established with the traditional definition, which is “the amount of water stored in the soil after an abundant irrigation and a free drainage between 24 and 72 h depending on the soil texture, finding a balance between water and oxygen in the soil (Veihmeyer and Hendrickson, 1950)”. The breaking point (threshold, refill), the lowest limit, is slightly above the level where a crop begins to experience water stress. Thus, maintaining the crop within these levels (Field capacity and breaking point) ensures a suitable water status and, at the same time, avoids loss due to deep percolation (Campbell and Campbell, 1982). The most relevant ML is the breaking point, which is determined when apparent water uptake rate change at least 20 % respect the day before, at a similar referential evapotranspiration. Apparent water uptake is calculated as the difference between the maximum and minimum Ɵw values recorded during the daylight of a given day, after one or two days irrigation or rainfall(Abrisqueta et al., 2012; Starr and Paltineanu, 1998a; Thompson et al., 2007a, Thompson et al., 2007b). Although this criterion is practical for irrigation management, its implementation must be validated with objective measurements of the plant water status, especially if there are water restrictions in phenological periods sensitive to water scarcity. However, few studies, and to the best of our knowledge, none on avocado, have related the progressive decline of soil water content to plant water status measurements (Abrisqueta et al., 2012, Thompson et al., 2007a, Thompson et al., 2007b) and its impact on yield, either in a complete crop cycle or in periods sensitive to water scarcity. Furthermore, there are several tree species that produce more and better-quality fruit when they experience moderate stress as more carbon is partitioned to fruits and only vegetative sinks may slow down; so, a decline of water uptake may not correspond to detrimental water stress. In addition, establishing management lines for a crop and given irrigation cycle differs remarkably according to the soil depth, and its properties, and the method used to set ML (Thompson et al., 2007b); therefore, the adequate use of this technology requires to be checked with evaluations of plant water status of the monitored tree and others from the same management area. Because this technology is expensive and most of the farmers use one sensor per uniform area of management, it is worth having another tool to support irrigation management decisions.

The most critical phenological period of avocado is at the end of spring and beginning of summer, when shoots and the roots grow vigorously, and the fruit set and final fruit size is defined mainly by the amount of carbohydrates available (Ferreyra and Selles, 2012, Silber et al., 2019). In fact, the greatest rate of cell division of the fruit occurs during the first 100 days after flowering (Cowan et al., 1997). This period (end of spring and beginning of summer) is consistent with the projected declines in irrigation water availability at basin scale (irrigation channels) (Garreaud et al., 2017, Garreaud et al., 2020) during the season, which is why it is relevant to assess the impact of a water restriction during this period.

For this, the aims of this study were (i) to evaluate the management lines effectiveness determined with a FDR probe on productivity and fruit size in ‘Hass’ avocado; based on greater profitability and water productivity, (ii) generate a non-stressed base line to check the management lines, and (iii) to evaluate the impact of a reduction in water input in periods critical to avocado phenology.

Section snippets

Site and orchard characteristics

The trial was carried out between September 2017 and October 2019 (two seasons) in a commercial avocado orchard (Persea americana Mill) cv. ‘Hass’ grafted on ‘Mexícola’, 12 years old, 4-meter height, planted at 6×2 m in 2005, NW-SE orientation, located in Peumo, Region of O′Higgins, Chile (34°24'21.69"S; 71°10'31.43"W). The climate of the study area is Mediterranean, with rainy winters, temperatures that vary on average between a maximum of 29.5 ºC (January) and a minimum of 4.6ºC (July) with

Seasonal conditions

The reference evapotranspiration (ETo) during the period of greatest demand for irrigation (September-May) varied between 1020 and 971 mm for 2017–2018 and 2018–2019 (Fig. 3). The maximum ETo occurred in December, with a maximum average of 5.6 mm/day. The maximum vapor-pressure deficit (max VPD) varied from 1.3 to 1.4 kPa in September to 3.0–3.6 kPa between December and February. There was no precipitation during the summer. In terms of the minimum temperature, although frosts appeared during

Concluding remarks

The information collected in this study, associated with physiological indicators of the plants, yield components and fruit quality, made it possible to evaluate irrigation strategies based on management lines established with the capacitance probes in mature avocado trees. In general, the results allow to conclude that:

  • The criterion of the breaking point, based on following the soil water content dynamics on the transition of a rapid rate of depletion to a slower, under similar reference

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors wish to thank the O′Higgins Regional Government, Chile for funding this study as part of project FIC-R (IDI 30474710). We also wish to thank Rodrigo Gómez and José Ortega (Agrícola Comercial Huerto Los Molinos Ltda) and his team of professionals for granting access to the property where this study was conducted. The authors declare there is no conflict of interest with respect to the publication of this study.

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