Written by Victor Blanco, Postdoctoral Research Assistant WSU TFREC- Horticulture; Jenny L. Bolivar-Medina, ITT WSU Extension; Erica Casagrande-Biasuz Postdoctoral Research Assistant WSU-TFREC Horticulture; Noah Willsea, Graduate Student Horticulture WSU-TFREC Horticulture; and Lee Kalcsits, Endowed Chair WSU Horticulture. May, 2022.
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Irrigation has never been considered an exact science; however, with new technological approaches, it can be.
Irrigation is crucial during late spring and summer in Washington State for high-quality fruit production. Fruit trees from high-density apple orchards need water for canopy development, fruit growth, and also for cooling down the leaves as a mechanism of protection against heat to maintain photosynthesis. The root systems of trees planted to high densities are smaller and are more responsive to water deficits. Poor irrigation during canopy establishment may slow tree development and consequently, may limit long-term productivity and economic sustainability of the orchard. In adult apple trees, water can be used as a tool to control fruit size, either maximizing fruit weight for smaller fruited cultivars or limiting fruit size for cultivars susceptible to disorders.
In this sense, measuring tree water status provides direct information on water limitations, which, in combination with growers’ experience and knowledge, can be used to make irrigation decisions. Irrigation scheduling based on direct measurements of water relations is advantageous over irrigation scheduling strategies based on time, environmental conditions, or soil water content because these options do not directly consider tree demand but only the factors that affect it. There have been several attempts to continuously measure the tree water status, however, none of them directly measure stem water potential. Microtensiometers could be a step forward in continuously monitoring tree water status (Figure 1).
What is a microtensiometer and what does it measure?
The microtensiometer is a plant-based sensor embedded into the trunk of the tree (Figure 2). It continuously measures water tension that indicates how tightly the woody tissue holds water. More tension indicates less free water in the stem which occurs under water limited conditions. This measurement is known as the stem water potential. Stem water potential is the most reliable indicator of water stress in trees, vines, and shrubs. This measurement involves the complete water pathway from the roots, which absorb the water available in the soil, to the stem whereby water is transported through the xylem, to fruit and leaves where it evaporates into the air (transpiration).
How can microtensiometers help you in making irrigation decisions?
Traditionally, the stem water potential has been a time-point, destructive, and labor-intensive measurement, which requires covering leaves for two hours prior to their measurement (to limit their transpiration). In fruit trees, it is usually measured at midday, when the highest water demand takes place, and it requires the use of the Scholander pressure chamber which cannot be automated (Figure 3).
The greatest advantages of microtensiometers are that they are continuously measuring stem water potential, are automated, and can be integrated into the irrigation decision-making process, so you will be able to monitor the water status of your tree at every moment from your phone, and you will water your trees when they need it and with the exact quantity they need (Figure 4). The measurements provided by the microtensiometers have been validated by comparing them with those measurements provided by the Scholander pressure chamber (Figure 5).
How do you interpret stem water potential measurements?
Generally, in fruit trees, we are interested in the daily minimum value recorded at midday or early afternoon. A low stem water potential value, below -2.5 MPa for apple trees (Figure 6) means that the water (sap) cannot freely flow throughout the tree and it is tightly held by the plant, which will cause processes such as transpiration, photosynthesis, and fruit growth to sharply decrease. If this water deficit situation worsens (with values below -4.0 MPa), it would cause structural damage to the tree, which would limit not only the current season’s yield, but possibly also affect next year’s yield. On the other hand, high stem water potential values (higher than -0.7 MPa) may stimulate excessive vegetative growth. Thus, it must be said that the best irrigation strategy is that which allows trees to achieve high yields and full potential fruits (according to the market price) consistently across the productive years of the orchard.
What do you need to consider?
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Tree selection is an important determining factor in reflecting the water status of your orchard block.
The tree should be representative of the majority of trees within the orchard. It should not be too big, not be too small, and should be located in a representative area within your orchard, avoiding borders, low spots, knolls and steep slopes.
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The minimum trunk diameter for installation should be greater than 1.6 inches.
Smaller trunks make installation of the microchip sensor difficult.
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Monitoring of plant water needs requires multiple data inputs across several years.
What works one year might not work under different conditions. Factors such as weather, crop load, rootstock/cultivar combination, and canopy development will affect tree response, so it is always preferable to schedule the irrigation according to the tree water status and phenology than to the calendar.
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You must know your goal.
Microtensiometers provide information but they are a tool to guide decisions to achieve target goals such as vigor control, bitter pit, etc.
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Information is power, but knowledge without action is useless.
Sensor numbers should match management zones, and data from the microtensiometers should inform irrigation decisions.
Additional information:
Acknowledgements 
Washington Tree Fruit Research Commission – Project: Validation of plant-based sensors for making irrigation decisions
Contacts
Victor Blanco, Ph.D.
Postdoctoral Research Associate
Tree Fruit Physiology- Kalcsits Lab
WSU Tree Fruit Research & Extension Center
Wenatchee, WA
Fundación Seneca
(Región de Murcia, Spain, 21261/PD/19)
email: victor.blanco@wsu.edu
Jenny L. Bolivar-Medina, Ph.D
ITT- WSU Extension- – Tree Fruit Horticulture
WSU-Irrigated Agriculture Research and Extension Center
24106 North Bunn Road
Prosser, WA 99350
phone: 509-786-9201
email: j.bolivarmedina@wsu.edu
Lee Kalcsits
Associate Professor
Endowed Chair
Tree Fruit Environmental Physiology and Management
WSU Tree Fruit Research & Extension Center
Wenatchee, WA
phone: 509-293-8764
email: lee.kalcsits@wsu.edu
Erica Casagrande-Biasuz
Postdoctoral Research Assistant
WSU-TFREC Horticulture
e.casagrandebiasuz@wsu.edu
Noah Willsea
Graduate Student Horticulture
WSU-TFREC Horticulture
noah.willsea@wsu.edu
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