Sarah D. Taylor-Laine1, Susana Espino1, Alec Downey2, and H. Jochen Schenk1.
1Department of Biological Science, California State University Fullerton, 2ICT International, Armidale, Australia.
Plant hydraulic conductance, defined as liquid flow divided by the pressure difference driving the flow, is functionally related to transpirational flux, sap flow, stomatal conductance, photosynthetic rate1, and sensitivity to drought2 or freezing3. Hydraulic conductance declines when embolisms form in xylem, and measurements of conductance as a function of stem water potentials therefore characterize a stem’s vulnerability to embolism formation4.
Current methods of determining hydraulic conductance and xylem vulnerability are both destructive and labor intensive, as plant stems must be repeatedly cut, hydrated and/or dried4. There is a need for a system that allows for hydraulic conductance to be determined under more natural conditions, preferably in intact plants in the field. Previous methods for measuring hydraulic conductance in intact plants have used a pressure chamber5 to determine water potentials and a sap flow gauge for flow determination6,7, but this approach is still destructive and provides no real-time logging.
We propose a novel system to determine stem hydraulic conductance in situ in intact plants, which is much less destructive than previous methods, allows logging in real-time, and can be used for an extended period. This is accomplished by using a Heat Balance sap flow gauge8,9 to measure volumetric sap flow, coupled with two temperature-corrected stem psychrometers10, all situated on one plant. By subjecting plants to drying cycles from fully hydrated to severely drought-stressed, the system can in principle be used to measure xylem vulnerability curves in situ.
To test the system, it was installed on a potted and a free-growing shrub of Malosma laurina (Nutt.) Abrams (Laural Sumac, Anacardiaceae). Our set-ups were designed to test if hydraulic stem conductance could be determined in situ and if our method would detect a decline in hydraulic conductance with decreasing stem water potential (Ψstem), as observed for Malosma in previous studies11,12.
– Two Malosma plants were studies, one potted in the CSUF greenhouse complex in February-March 2012 and one free-growing in the Fullerton Arboretum during March-April 2012.
|Fig. 1: Potted Malosma at
the CSUF greenhouse.
|Fig. 2: Free-growing Malosma
at the Fullerton Arboretum.
Determining Stem and Leaf Water Potentials
– One stem psychrometer (ICT, Australia) was placed at the base of the stem, near the ground, and one on a leaf (after gently removing the leaf cuticle with carborundum abrasive paste). Each was set to take measurements every 15 minutes. The free-growing plant had two additional psychrometers installed below (mid) and above the sap flow gauge (upper).
Determining Sap Flow and Hydraulic Conductance
– To determine sap flow (F), one Heat Balance sap flow gauge (Dynamax, USA) was placed on a stem directly between the psychrometers. It was set to take measurements every 15 minutes. At the conclusion of the experiment, the gauge was calibrated on the stem using a Masterflex Precision Pump (Cole-Parmer, USA).
– To determine hydraulic conductance (K), the following equation was used: K = F / (Ψin – Ψout), where Ψin = water potential below and Ψout = water potential above the gauge.
Fig. 3: Leaf and stem water potentials, volumetric sap flow and stem hydraulic conductance for a potted Malosma between February 11, 2012 and February 18, 2012. Occasional negative conductance was caused by leaf water potentials that were greater than stem water potentials.
Fig. 4: Stem water potentials at two stem positions, volumetric sap flow and stem hydraulic conductance for a free-growing Malosma between March 26, 2012 and March 31, 2012. Stem hydraulic conductance calculated between “lower” and “upper” stem.
Fig. 5: Average hydraulic conductance versus upper stem water potential per hour for a free-growing Malosma from Marth 26 to 31, 2012. 50% maximum conductance (Ψ50) occurred at about -1.5 MPa. Points represent the average for each time period over the six day period. Error bars represent standard deviation.
Fig. 6: Stomatal conductance and sap flow for a free-growing Malosma from June 15 to 16, 2012. Stomatal conductance determined by repeated measurement of leaves using a LI-1600 Steady State Porometer (LI-COR, USA). The period from sunset to sunrise is marked by the shaded area. Error bars represent standard deviation.
• Our system was successful in measuring hydraulic conductance in real-time (Fig. 4). Highly regular diurnal fluctuations of water potential, sap flow, and conductance were found (Figs. 3 and 4). The latter have been previously observed for Malosma using destructive hydraulic methods11.
• Leaf water potentials (Ψleaf) may not be suitable for calculating stem hydraulic conductance in Malosma, because their fluctuations do not translate directly to changes in sap flow or conductance (Fig. 3). This may be because leaves are somewhat hydraulically decoupled from stems1. Ψleaf occasionally exceeded Ψstem, probably because of dew formation (Fig. 3). Stem psychrometers installed at the base of the trunk and on an upper branch gave much better readings for conductance calculations (Fig. 4).
• There was a much more complicated relationship between Ψstem and hydraulic conductance in Malosma than predicted by lab-generated vulnerability curves (Fig. 5)12. As predicted, hydraulic conductance declined with declining Ψstem. This was not caused by stomatal closure, as Malosma does not show midday stomatal depression (Fig. 6).
• Hydraulic conductance recovered at around -0.6 MPa (Fig. 5). 50% maximum conductance (%PLC) occurred at approximately -1.5 MPa, which agrees closely with previous observations of Ψ50 for Malosma12.
• Recovery of hydraulic conductance at -0.6 MPa was evidence for embolism repair under tension in Malosma, which has been previously documented for plants in the field11. This period of recovery coincided with very low sap flow, even though the stem water potential gradient persisted (Fig. 5).
• Our new system for measuring hydraulic conductance in situ works well. The main drawback is the relatively high price (about $10K including logger).
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We thank Ed Read (CSUF greenhouse), and Chris Barnhill (Fullerton Arboretum), as well as Ken Shackel (UC Davis) for his advice on leaf psychrometry. We also thank Matthew Taylor-Laine, Emily Nguyen Wieber, Miguel Macias, Darren Sandquist, Diane Tran, An Ly, and Donald Quick for their contributions to the success of this project. Support from NSF grant IOS-0943502 to HJS is gratefully acknowledged.