Background

Measurements in the field of plant hydraulics focus on hydraulic efficiency and safety parameters that quantify the amount of water transported in plants. The water transport system generally involves uptake at the root level, with the driving force for transport generated by transpiration via stomata at the leaf level during photosynthesis. There is general agreement that the principles behind water transport in plants follow the cohesion-tension theory, which means that water transported in the plant is usually under negative pressure (i.e., tension), and therefore in a physically metastable state. Due to water conduction under negative pressure, a universal assumption in plant hydraulics is that the water transport pathway can be interrupted by the entry of air bubbles, which may lead to cavitation, i.e. the spontaneous change from liquid to vapor phase. Cavitation results in the formation of a gas bubble (embolism), which reduces the capacity of the plant to deliver water to leaves. Plants can avoid extensive accumulation of embolism by refilling cavitated cells, although this refilling process remains poorly understood and has been an apparent paradox for many years.

Because of the inherent difficulty in observing and experimentally manipulating a transport system containing liquid under tension, many basic techniques to measure hydraulic conductivity and embolism provide variable and even contradictory results. Therefore, numerous basic questions surrounding water transport in plants are frequently challenged and remain unanswered or controversial.

Because water transport is tightly linked to vegetation productivity and distribution of plant species on a global scale, it is clear that expertise in accurately measuring plant hydraulics has far reaching consequences to various other research fields.

The goal of this workshop is to bring together leading experts working in the field of plant hydraulics in order to discuss a wide range of techniques that are being used for measuring plant water relations. While some techniques have become standard measurements for many years, there is an absolute need to evaluate which method is most suitable for particular plant species or growth forms (e.g. woody vs herbaceous), which techniques are most reliable and less prone to measurement artefacts, and how technical problems can be resolved. Progress that is critical to the field is only possible through an unbiased and open-minded mentality and a collaborative spirit among experts. It is therefore very clear that experts should come together to discuss technical details, possible misunderstandings, closer collaboration, and the development of new methods.

Topics to be addressed in the workshop

Proposed specific topics to be addressed during the workshop will include but are not limited to:

1. Hydraulic conductivity: What are the most suitable, standard protocols for measuring specific conductivity and leaf specific conductivity, and how comparable are these different techniques? What are the standard protocols for determining sample length, sapwood area, perfusion pressure, and pressure head? What is the effect of open vessels and stem length on hydraulic conductivity? What is the effect of segment tapering for long samples (e.g. 30 cm) on calculating specific conductivity, which may vary by a factor up to 2 depending on the mean or maximum sapwood area calculated? How do measurements on straight and branched segments scale across species? How do the values obtained scale with the hydraulically weighted conduit diameter? Which perfusion solutions should be used (e.g., 10mM KCl and 1mM CaCO2, MgSO4), especially with respect to the ionic concentrations and degassing of water?

2. Cavitation resistance and vulnerability curves: While the bench-dehydration technique is usually considered as the standard reference technique for determining the vulnerability of the hydraulic pathway to drought-induced cavitation and embolism formation, a controversy has developed in recent years regarding the shape of vulnerability curves obtained with the centrifuge method and the cavitron technique. Special attention will be given to the validity of exponential (R-shaped) vulnerability curves versus sigmoidal (S-shaped) curves and how artefacts can be avoided during centrifuge measurements. We will also discuss the relative importance of working on flushed vs. non-flushed samples. The development of non-destructive techniques for constructing vulnerability curves based on X-ray computed tomography (microCT), MRI, and the combination of sap flow meters with stem psychrometers. While some of these techniques are still being tested, it is important to discuss these alternative approaches and to compare their results with other techniques.

3. Embolism formation and refilling: What are the standard techniques for measuring the amount of embolism in xylem conduits? What are the advantages/disadvantages of releasing the tension before taking embolism measurements? Should we fully rehydrate branches before measurement, and how should this be done? How much refilling could occur during this rehydration phase and how does this vary between species? What is the available evidence for novel refilling under tension?

4. Leaf hydraulics: Generally, different techniques are used to determine leaf hydraulic parameters compared to stem segments. Similar to vulnerability curves for stems, the methods applied to determine cavitation resistance in leaves are prone to a range of methodological issues. Additionally, the hydraulic pathway of leaves involves both xylary and extra-xylary components, introducing further complexity into both methodology and interpretation of results. 

Outcomes of the Workshop

Outcomes from the workshop will include an invited review paper on plant hydraulic techniques in the journal Tree Physiology, and possibly a special issue in a peer reviewed journal dedicated to the topic. Results will also be disseminated through the Prometheus wiki (Protocols in ecological & environmental plant physiology), a resource for free open access resource that allows access to detailed protocols (http://prometheuswiki.publish.csiro.au/tiki-custom_home.php). The initiation of collaborative grant proposals will be encouraged to address the development of novel methods in plant hydraulics.