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• W2161442364 abstract "‘… commonly used techniques as well as the application of novel methods in studies on xylem transport may suffer from the “observer effect”, indicating that measurements of the hydraulic system cannot be made without affecting this system.’ A major challenge with integrating plant hydraulic data into other disciplines concerns discrepancies and ambiguity of hydraulic data collected and published by researchers studying plant–water relationships. Comparability of results from research groups is increasingly important to address some of the most challenging questions related to water transport, and is also crucial for any interdisciplinary efforts and the compilation of large datasets. Because there is often variation in the fine details of how hydraulic measurements of the xylem tissue are carried out, methodological issues have been a concern for many years. These technical problems are frequently associated with the inherent difficulty in observing and experimentally manipulating the xylem transport system containing liquid under tension. Therefore, both commonly used techniques as well as the application of novel methods in studies on xylem transport may suffer from the ‘observer effect’, indicating that measurements of the hydraulic system cannot be made without affecting this system. The field of plant hydraulics is particularly concerned about measurement artefacts, which have the potential to hinder progress and confuse those outside the field. The major goals of the workshop were to: (1) recognize different opinions and viewpoints on technical issues; (2) stimulate collaboration and inspire new approaches; and (3) make plant hydraulics more accessible to students and nonexperts outside the field. During the first day of the workshop, oral presentations were held by Mel Tyree (Northwest A&F University, China), John Sperry (University of Utah, Salt Lake City, UT, USA), and Hervé Cochard (INRA, Paris, France), as well as several videoconference presentations and 20 poster presentations. Breakout sessions during two additional days were held to discuss methodological issues in smaller focus groups, including protocols related to vulnerability curves, leaf hydraulics, sapflow, hydraulic conductivity, soil water, and wood anatomy. Later, we report only on xylem vulnerability curves. One of the major issues debated concerned the accuracy of measuring vulnerability of the plant hydraulic pathway to the entry of air bubbles that block conduits and impede water delivery to transpiring plant tissues. As emphasized during the presentation by Hervé Cochard (INRA), the debate about whether xylem conduits are filled with water or air goes back to the pioneers of microscopy in the seventeenth century. Over the last decades, vulnerability to embolism has been defined by plotting the percentage loss of hydraulic conductivity (PLC, in %) against decreasing xylem water potential (ψx, in MPa). The shape of the resulting vulnerability curve (VC) is found to vary from sigmoidal (s-shaped) to logarithmic (r-shaped) (Fig. 1). The main methodological concern arises from the fact that in some species the same plant material exhibits a fundamentally different curve shape depending on the technique used for measurement. The physiological implications of these differences are enormous and must be viewed in the context of the total water relations of a particular species. The r-shaped curves imply a high amount of embolism formation or high native PLC levels, with embolism refilling occurring on a daily basis. The s-shaped curves, however, predict that embolism formation does not occur below a certain water potential threshold, probably only during periods of severe drought, and embolism refilling is either nonexisting or assumed to occur on a seasonal rather than daily basis. Therefore, the shape of VCs strongly influences the xylem water potential corresponding to 50% loss of conductivity (ψ50, in MPa; Fig. 1c,f). Although consensus was not achieved on this issue, several general guidelines were highlighted during the workshop, and various possible explanations were discussed to better understand why the plant hydraulic community is facing these current discrepancies. First, variation in the shape of VCs can be explained by the method applied. While the bench dehydration method is generally considered as the oldest, ‘gold standard’ method, this technique is labour intensive and could be prone to some artefacts (see later). Faster methods include the air injection and centrifugation technique, but these can also be prone to errors (Cochard et al., 2005; Choat et al., 2010; Ennajeh et al., 2011). New progress in understanding the ‘open vessel artefact’ was recently reported by Wang et al. (2014) for Robinia pseudoacacia: after spinning samples for 4 h in a centrifuge, PLC levels were significantly higher in stem segments that have a high mean vessel length than stem segments with a shorter mean vessel length. This finding may suggest that the open vessel artefact is due to bubbles moving from the stem base towards the centre, where the tension is highest, causing them to expand and cavitate. However, there are several versions of the centrifuge technique and some researchers report that the original ‘static centrifuge’ method is unaffected by the open vessel artefact (Hacke et al., 2015). The reliability and comparability of the various centrifuge techniques thus remains a contentious issue (Cochard et al., 2013). Additional variation in VCs can be caused by the age of the plant material and anatomical variation of the xylem tissue, especially the vessel length distribution and connectivity between vessels both within and across growth rings. Seasonality may exert influence on VC shape via cavitation fatigue causing a lower resistance to embolism formation after earlier drought- or frost-induced cavitation events (Christensen-Dalsgaard & Tyree, 2014). Ideally, different techniques for constructing VCs should be tested using plant material from the same age, similar size, same plant population, and within the shortest possible time frame, and the exact age of the plant and the collecting date should be reported in publications. Shifts in VCs could also be due to changes in the xylem sap composition and the perfusion solution that is used when conducting hydraulic measurements (Sperry & Tyree, 1990; Herbette & Cochard, 2010), but may also depend on which ‘reference point’ is used to standardize PLC values. Different methods are applied to measure hydraulic conductivity either at low or high pressure, which might result in PLC errors and hamper comparability. Therefore, VCs are suggested to be expressed in terms of xylem specific conductivity (Ks). This parameter is generally normalized to the conductive xylem area in order to reflect the actual conducting capacity. Variation in Ks, however, can be influenced by several factors other than embolism, depending on the device and protocol used. Although no ‘gold standard’ protocol is available for Ks measurements, several tests and recommendations to assure comparability of results were discussed at the meeting. An important question in the s- and r-shaped VC debate concerns the functional starting point, that is whether or not VCs should start with all vessels in a stem segment filled or not. In practical terms, this means whether or not samples should be flushed before measuring Ks. While all vessels (except for those that have not fully developed yet) should be included for juvenile stem segments that are < 1 yr old, there was no general agreement about how we should deal with the ‘throw away strategy’ of ring-porous species. Except for tracheids, functional vessels in species with ring-porosity are mainly limited to earlywood vessels of the current year, because embolized earlywood vessels do not refill after frost. Interestingly, a VC that excludes these previously embolized earlywood vessels from the measurement is always sigmoidal, which might be considered artefactual. The bench dehydration method is generally considered as a reference technique for validation of other VC methods. However, this method is also indirect and destructive, and recent work has shown that it could be prone to artefacts caused by cutting xylem under tension (Wheeler et al., 2013; Torres-Ruiz et al., 2014). For many years, there has been general agreement to be careful with cutting plant material while the xylem sap is under tension. However, it is not clear how we should relieve the tension of the xylem sap while preventing any refilling due to capillarity (Trifilò et al., 2014), and whether or not multiple cuts within a short time frame are sufficient to avoid the excision artefact. Finally, the field of plant hydraulics should take advantage of novel techniques to measure VCs, in addition to improving current methods. Alternative VC protocols discussed at the workshop included the application of acoustic emission methods, noninvasive and direct measurement by X-ray microtomography (microCT), magnetic resonance imaging (MRI), and sap flow methods to estimate whole-tree VCs. An easy and novel tool was also presented by students from Rafael Oliveira’s laboratory (State University of Campinas, Brazil), who demonstrated how VCs could be constructed by a vacuum meter method that is applied to the proximal site of an intact branch. The field of plant hydraulics will benefit from standardized protocols, definitions of parameters and units, and more intense collaboration to develop, share, and modify techniques. One possibility to achieve this goal will be the publication of a plant hydraulics handbook, which is inspired by similar efforts from plant ecologists (Pérez-Harguindeguy et al., 2013). In addition, individual protocols will be uploaded on the PrometheusWiki (Protocols in ecological and environmental plant physiology; http://prometheuswiki.publish.csiro.au/tiki-custom_home.php). Therefore, we are currently inviting colleagues working in the field of plant hydraulics to share detailed instructions and protocols of robust methods that have been tested repeatedly. The authors thank all participants for valuable presentations, discussions, and comments during the workshop, and the German Research Foundation (DFG) for financial support. B.C. acknowledges the Humboldt Foundation for his Humboldt Fellowship." @default.
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• W2161442364 date "2015-01-12" @default.
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• W2161442364 title "Current controversies and challenges in applying plant hydraulic techniques" @default.
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