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Continuous Measurements of Water Status in Deeply Rooted Southern California Chaparral Shrub Species.
Much of Southern California experiences a Mediterranean climate, with long, hot summers and cool, wet winters. Characteristic plants include the deeply-rooted shrubs collectively known as chaparral. Such species, including Malosma laurina (Laurel Sumac, Family Anacardiaciae) and Heteromeles arbutifolia (Toyon, Family Rosaceae), have the ability to tap cool, deep waters. Chaparral species face severe drought stress during the summer, which can be quantified in measurements of water status. Measuring water status continuously in situ can prove challenging, partially because some instruments are extremely sensitive to the extreme temperature gradients experienced by chaparral shrubs during the summer, and often require complex protocols for installation and maintenance. This study was conducted to develop two methods, stem psychrometry and measuring air flow into xylem, for using plant water status sensors on deeply-rooted chaparral shrubs. It was found that temperature gradients in large basal stems caused by ascent of cool xylem sap from deep soil layers can lead stem psychrometers to report overly-negative water potential readings midday. Attempts to correct for these temperature effects turned out to be unsuccessful, leading to a recommendation to install psychrometers on smaller branches up high. Air flow into xylem occurs during embolism spread or repair, possibly as stable nanobubbles are pulled through pit membranes, driven by the pressure gradient between air and xylem sap, and subsequently dissolved into the iii xylem sap. Using a liquid flow gauge to measure air flow into xylem, one set of measurements out of many attempts was completed, demonstrating the validity of the approach.
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