The following summary concerns xylem vulnerability curves in relation to cavitation of plant organs such as stems, petioles, leaf veins or roots.
A xylem vulnerability curve (VC) is a plot showing the relative variation of xylem sap transport capacity as a function of xylem water stress.
Scope and limitations
In this section, we deal with methods specific to the xylem tissue and on the impact of water stress-induced cavitation on its transport capacity. Methods to assess whole leaf or whole plant vulnerability curves will be presented elsewhere. The only relevant measure of water stress for the construction of VCs is the xylem pressure potential (P, MPa), the key variable that determines the induction of cavitation during water stress.
Methods to construct VCs
A number of methods have been proposed to establish VCs. They differ by 1) the way xylem transport capacity is measured and 2) by the way xylem water stress is assessed or induced. We propose a nomenclature based on these two entries, the method for induction of cavitation will be discussed first. For instance Bench Dehydration – Hydraulic conductance means: cavitation induced by dehydrating cut branches on a bench and cavitation measured by measuring the loss of xylem hydraulic conductance. Methods to induce xylem cavitation by water stress DehydrationThis is the most straight forward and natural way of inducing cavitation in plants.
- Whole plant dehydration: Here the plant is intact and let to dehydrate in a pot or in situ. The xylem pressure can be measured with a pressure chamber on cover leaves or with stem psychrometers. The relevant pressure is the most negative pressure the plants have experience during the drought treatment. It is usually taken at midday on the driest day. (Ref : Tyree et al 1992; Bréda et al 1993)
- Bench dehydration: Here only a branch segment (typically a leafy branch) is cut from a plant and let to dehydrate freely in the air. The main advantage is that this method involves considerably faster dehydration of branches in comparison with the previous method. Very fast dehydration should be avoided because it can induce a high heterogeneity of water stress in the branch. The branches are best placed on the bench of a laboratory under ambient light conditions. Xylem pressure is measured as above. (Ref: Sperry et al 1988)
- Dehydration in a chamber at controlled air humidity. Here a small xylem sample was left to dehydrate in a close chamber at controlled humidity (by using a saturated solution of different salts for instance). Upon equilibrium, the xylem pressure in the sample equals the air water potential. (Ref : Kikuta et al 2003)
- Pressure sleeve. This one is also called “pressure collar“, or “double-end pressure chamber” technique, or “cavitation chamber” by PMS company (http://pmsinstrument.com). Here, a xylem segment cut at both end is inserted in a pressure chamber with the two ends protruding out of the chamber. Cavitation is induced by increasing the air pressure inside the chamber. This technique is frequently employed but biases associated with long xylem conduits have been reported. (Ref: Cochard et al 1992a; Salleo et al 1992)
- Single end air injection. Here a segment cut at both end has one end inserted in a regular pressure chamber, the other end protruding outside. (Ref: Sperry & Tyree 1990)
- Pressure chamber dehydration. This technique differs from the previous one because here a terminal leafy segment is inserted in the pressure chamber (cut at one end only). The sample is dehydrated by increasing the pressure in the chamber until the balance pressure is obtained. This is very similar to the pressure-volume curve technique. (Ref: Cochard et al 1992b)
- Here segments cut at both end are spun in a centrifuge. The dehydration is caused by the centrifugal force. Biases associated with long xylem conduits have also been reported with this technique. (Ref: Alder et al 1997; Pockman et al 1995)
Methods to measure xylem cavitation The methods for assessing xylem cavitation are very diverse. Some methods are direct, other indirect, some methods are quantitative while other are more qualitative.
- Observation of xylem water content
A number of methods have been employed to detect the effect of cavitation on the xylem lumen content (air versus water filled).
- Dye coloration: Water can move only in water-filled conduits. If water is colored with a dye then it is possible to visualize the percentage of water conducting conduits. Dye coloration is performed by absorption induced by leaf transpiration, or by infiltration through a xylem segment with a small gravimetric pressure. Safranine or Phloxine are frequently used in these experiments.
- Water content. Whole xylem water content can be an indication of xylem lumen water content. However this parameter is also influenced by the water content of the wood symplast. Water content can be measured gravimetrically (Ref: Heitz et al 2008), with a TDR (Ref: Holbrook et al 1992) or by gamma ray attenuation (Ref: Edwards & Jarvis 1983).
- A number of more sophisticated technologies have also been used for direct observations of xylem content: Cryo-SEM (Ref: Cochard et al 2000), RMN (Ref: Holbrook et al 2001) and X-ray Tomography (Ref: Fromm et al 2001).
- Measurement of xylem hydraulic conductance
These techniques measure the impact of cavitation on the xylem water transport capacity (embolism) which is best quantified by its hydraulic conductance. The percentage of loss of xylem conductance (PLC) is an indication of the percentage of xylem embolism.
- PLC sub-segment. Cavitation is measured here via its effect on the percent loss of conductance on small segments cut from a larger branch. The conductance is usually measured gravimetrically with a balance or a flowmeter (XYL’EM). The VC is hence constructed with many branches. This is the original -Sperry’ technique (Ref: Sperry et al 1988)
- PLC whole-segment. Here the entire VC is constructed on the same segment either placed in a pressure sleeve or in a centrifuge. The conductance is measured as above.
- PLC centrifugation. This is the Cavitron technique where the conductance is measured during centrifugation.
- Acoustic detection.
These technologies are base on the fact that the process of cavitation is associated with the emission acoustic emissions. The techniques differed by the frequency of these emissions
- Audible techniques. Here cavitation is detected with microphones in the audible range. (Ref: Milburn 1966)
- Ultrasonic emissions. Here only ultrasonic acoustic emissions are recorded. Specific apparatus have been commercialized for this detection (from Physical Acoustic Corporation). (Ref: Tyree et al 1984)
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