Instrumentation & Measurement Magazine 23-2 - 39

Hydraulic Conductivity
During the periods of drought, excessive evapotranspirative
tension (extreme water potential) can lead to formation of cavitation bubbles in xylem conduits. Accumulation of cavitation
bubbles in xylem vessels can cause the embolization of parts of
the xylem tissue, disrupting the sap flow. Although physiological xylem refilling mechanisms make the process reversible
[9], embolization is considered as one of the major causes of
plant's mortality caused by water stress [3]. It can be visualized
in-vivo by high-resolution x-ray micro-CT [9]. It is quantified
by measuring the hydraulic conductivity.
Hydraulic conductivity K is derived from the water flow
and the water potential. It is defined as the mass flow of the
xylem water, divided by the pressure difference (tension gradient) causing it [10]. Thus, it is analogous to the electrical
conductivity, or more precisely, admittance. It is determined
by the tissue's physical dimensions and by the structure of its
membranes and xylem water conduit vessels (type, number,
length, diameter, etc.). Usually it is expressed normalized to
the maximal conductivity, as a "percentage of loss of conductivity" (PLC).

Water Content
Water balance within plant tissues constitutes of the "bound
water" residing within cellular structures, and the "free water"
which is transported along the xylem vessels. Water content is
most commonly expressed as relative volumetric water content (VWC) [% = cm3 / cm3]. Water content measurement is
also important in context of quantification of the level of xylem embolization. Water content can be considered analogous
to electric capacitance.

Measuring Xylem Water Potential
Pressure Probes for Direct Xylem Pressure
Measurement
Due to the significance in physiological interpretation, direct
field-measurement of xylem water potential is a holy grail of
instrumentation for plant-hydraulic measurements. However,
it is all but trivial to measure.
Water potential in individual xylem conduit elements (tracheids in gymnosperms, xylem vessels in angiosperms) can be
directly measured by introducing a conical glass micropipette
ending with a sharp 10-50 μm tip directly into a xylem vessel
[11] (Fig. 2a). The microcapillary is filled with a liquid hydraulic medium and sealed at its outer end, forming a microbaric
chamber. Thus, any tension introduced by the xylem water potential is transferred onto the liquid hydraulic medium within
the microcapillary and measured using a liquid-coupled pressure sensor capable of measuring tensions down to at least -1.0
MPa.
Insertion of the probe into a vessel is the most challenging aspect of direct measurement of the xylem pressure. The
probe is optically guided into the vessel using the micromanipulator. Insertion may fail for several reasons. Firstly,
insufficiently precise positioning may damage (rupture) the
April 2020	

vessel. Secondly, the tip may clog during advancement of the
probe through the tissue or once a vessel has been impaled.
Thirdly, an air bubble may form in the tip of the microcapillary.
Finally, due to the metastable state of xylem water, insertion
may itself cause cavitation, either by leakage induced at the insertion point of the glass micropipette or by the presence of a
gas phase in the interior of the probe [12].
A clear advantage of the method is the direct measurement
principle. However, the method exhibits limited potential for
field application. First, measurements are only possible in soft
tissue (i.e., leaf petioles of the young twigs, soft root, etc.). Meticulous preparation of the probe is required (cleaning, filling,
degassing) in order to prevent cavitation of its hydraulic-medium. Still, the measurement range is limited by the tension
at which cavitation of the liquid medium occurs [12]. As the
method requires delicate positioning and mechanical stability,
continuous long-term field measurements would require design of the specialized portable equipment for field-insertion
and mechanical fixtures to hold the sample in place.

Implantable Xylem Pressure Sensors
Shortcomings of the pressure probe technique motivated the
design of a recent concept of an implantable pressure sensor for direct measurement of stem water potential [13]. This
MEMS device (Fig. 2b) miniaturizes a working principle of a
macroscopic soil tensiometer sensor. It consists of an enclosed
container (enclosure) filled with water. The container is exposed to the environment through a permeable membrane.
When the water potential of the environment is lower than the
potential of the pure water in the container (i.e., drier, containing more solutes, etc.), water from the container permeates the
membrane and leaks out, creating tension (negative pressure).
Tension at the membrane is sensed on the flexible strain-sensing diaphragm by a micro-tensiometer forming a Wheatstone
bridge with four piezoresistors.
The reported measurement range spans from 0-10 MPa of
tension (negative pressure), covering the dynamics of xylem
water potentials in conditions of extreme drought. The response is linear, with the reported sensitivity of approximately
0.5 mV/Vin/ MPa, invariant to temperature changes in range
of 0-20 °C [13].
Main advantages of the method are direct water potential
measurement principle, small dimensions (the latest reported
dice size 2x5 mm excluding packaging), linear response, and
temperature stability. In comparison to pressure probes, this
device enables a wider measurement range and measurements
in hard tissues of plant trunks. A disadvantage is that this proprietary technology still appears in the very early stage of
technological readiness.

Indirect Xylem Water Potential Measurements
With shortcomings of direct xylem pressure measurement
methods, de-facto standard in practical agronomy [1] is the
indirect, "pressure bomb" method (Scholander's chamber).
It is a high-pressure chamber, confining a cut-off leaf. The externally applied pressure required to invoke secretion of the

IEEE Instrumentation & Measurement Magazine	39



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