The greatest source of variability in the measurements of stem potentials with the FloraPulse microtensiometer is the variation in the contact between the sensor chip and the xylem of the plant. The xylem is a complex tissue made up of elongated, active water pipes of varying size, older inactive plugged pipes and additional living generally spherical parenchyma cells, often in radial files called rays. Examples of this complex structure can be seen below.
Diagrams of representative soft (left) and hardwood (right) wood structure from the book Wood Characteristics, 2015, by Christopher Richter. The surface facing to the right is where the microtensiometer is pressed.
Below is a cross section of a grape cane showing the alternation of varying sized water vessels and radial “rays”of small living cells important in reserve storage (courtesy of M. Goffinet, Cornell University). Note also the irregularity of the grape stem that commonly occurs with the pith not being centered, so the depth of the xylem varies.
It is quite clear that any sensor installation will be interfacing with a very complex tissue. The photos below show the xylem tissue in three woody crop species in the plane that the sensor edge is pressed.
A study at Cornell University (S. Zhu, PhD thesis) installed microtensiometers at several radial depths in apple trunks. She found that positioning the sensor in the outer xylem gave the most correct mid-day stem potentials. This was in the portion of the xylem that had been shown to have the highest water conductivity (blue band) in a prior study (Dragoni 2008).
Effects of relative radial xylem position (r) from the center pith (to the left) to the cambium layer (to the right) in an apple trunk.
Below is a cross section of a grapevine stem (from Sun et al., 2013) showing radial files of xylem vessels and parenchyma rays. A diagram of the microtensiometer installation is shown to indicate desired position in the outer xylem, but clearly inside the cambium zone of stem cells. Arrow indicates the sensor edge where the water exchange occurs between the sensor and the xylem.
Another concern about positioning is that it appears that the cambium layer is active in wound response both in producing cells such as undifferentiated callus tissue, but also gums, gels and other liquids that can interfere with the xylem/sensor interface. So it appears to be important to not be too close to the cambium. There may be also specific cells in the xylem itself that can produce other materials. These vary with species, so the type of wound reaction of each species may cause different challenges and opportunities for long-term monitoring. Since there seem to be infinite variations, it will require experience and testing in each species.
Due to the natural complexity and variability of the xylem tissue, drilling a hole into wet wood generally leaves some raggedness. We use a flat-ended drill bit, but there are tough fibers in most wood. So to make a good hydraulic contact with an uneven surface we inject a hydraulic paste ahead of the sensor to fill the irregular voids and dramatically improve the sensor-xylem interface. This process works remarkably well and generally allows the FloraPulse microtensiometer to accurately measure plant water potential, despite the myriad challenges.