September/October 2017

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The Pressure Chamber – Does It Have a Place in Your Tool Box? By: Allan Fulton, Luke Milliron, and Richard Buchner Among the most basic observations that can be used to manage irrigation in orchard crops are visual cues of crop water stress. However, these cues can be somewhat subjective and are often expressed after plant stress is higher than desired. Measuring midday stem water potential (SWP) using a pressure chamber is a quantitative method for evaluating plant water status, and relationships have been established between pressure chamber measurements and tree growth and productivity. From these relationships, guidelines have been developed to assist growers in making irrigation scheduling decisions. This is an abbreviated discussion of pressure chamber operation and interpretation of measurements based upon peer-reviewed UC ANR Publication 8503, Using the Pressure Chamber for Irrigation Management in Walnut, Almond, and Prune (2014). Plant Water Status During plant transpiration, water moves from the soil into fine root tips, up through the vascular system, and out into the atmosphere (fig.1). Water flows through the tree from high potential in the soil (about –0.1 bar) to low potential in the Figure 1. Illustration of how water moves from the soil through an irrigated tree and into the atmosphere, from both a whole-tree and cellular perspective. SWP measures the water potential gradient that drives this movement of water through the tree. Source: Adapted from Pearson 2008. Upper Saddle River, NJ: Pearson Education Inc. 4 Progressive Crop Consultant September/October 2017
atmosphere (less than –40 bars). Low potential is created at the leaf surface through small openings called stomata that open and close to regulate photosynthesis, gas exchange, and plant water loss. Simultaneously, water held in the soil enters root tissue and begins its journey to the leaves. This creates a vacuum, or continuous column of tension within the water-conducting system of the tree. The amount of tension depends on the balance between available soil moisture and the rate at which water is transpired from leaves. Midday Stem Water Potential (SWP) Concept A pressure chamber measures plant water tension by applying pressure to a severed leaf and stem enclosed in an airtight chamber (fig. 2). The sample leaf is covered with a foil-laminate bag for at least ten minutes before it excised from the tree. The leaf remains in the bag while the measurement is taken. requiring more pressure to force water out of the cut surface of the leaf stem. Stem water potential is a direct measure of water tension (negative pressure) within the plant and is given in metric units of pressure, such as bars (1 bar is about 1 atmosphere of pressure, or 14.5 psi). Technically, SWP should always be shown as a negative value (e.g., –10 bars), but in conversation and because the gauge does not indicate negative values, we often omit mentioning “negative” before the value. A larger number on the gauge indicates more tree water stress. Figure 3. Four examples of commercially available pressure chambers. Example A is a pump-up style by PMS Instruments in which a foot pump creates air pressure in the rectangular chamber. Example B, by Specialty Engineering, also has a rectangular chamber, employs a tripod to hold the pressure chamber, and uses compressed carbon dioxide (CO2) gas to supply the pressure. Example C is a suitcase-style pressure chamber. It has a cylindrical chamber that uses nitrogen gas, to pressurize the chamber. Both PMS Instruments and Soilmoisture Equipment Corp. offer this style of pressure chamber. Example D is a consoleor bench-style pressure chamber by Soilmoisture Equipment Corp. that also uses nitrogen gas to pressurize the chamber. Photos: Courtesy of Plant Moisture Stress (PMS) Instrument Company, Albany, OR (A, C); Specialty Engineering, Waterford, CA (B); and Soilmoisture Equipment Corp., Santa Barbara, CA (D). A C B D bers for measuring stem water potential (SWP). All have the same basic components and operate on the same concept. The choice of a pressure chamber depends largely on preferences. Several pressure chamber styles and designs are available, ranging from simple manual pump-ups to consoles with more advanced features (fig. 3). Compressed nitrogen gas is a convenient, relatively inexpensive, and inert (safe) gas to use as a source of pressure. Carbon dioxide gas is also used to supply pressure with some pressure chambers. Compressed air is used with some pressure chamber models. The cost may range from about $1,000 to about $7,000, depending on the style, design, and whether the unit is new or used. Figure 2. Schematic showing how water potential is measured in a severed leaf and stem (petiole) using a hand-held pump-up pressure chamber. Source: Adapted from Plant Moisture Stress (PMS) Instrument Company. The pressure required to force water out of the stem of a severed leaf equals the water potential and is measured by a pressure gauge. As soil moisture is depleted, more tension develops in the plant, Pressure Chamber Selection A number of companies produce durable, portable, and relatively inexpensive pressure cham- Costs to Monitor Plant Water Status Growers and consultants who already use the pressure chamber and midday stem water potential as one of their management tools indicate the cost to integrate it into their management ranges from about $10 to $20 per acre annually. This is a Continued on Page 6 September/October 2017 www.progressivecrop.com 5
Page 1 and 2: September/October 2017 The Pressure
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September/October 2017

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