Instrumentation & Measurement Magazine 23-2 - 44

Table 1 - Currently used and prospective future technologies for measuring water stress
Measurement
technique

Physical phenomena

Accuracy

Maturity

Challenges

Pressure chamber

±0.1 MPa [1]

Commercial, most
used

Destructive, laborintensive

Psychrometers

±0.1 MPa [13]

Commercial, some
field trials

Installation,
environmental factors

Implantable
tensiometers

±0.5 kPa [13]

Early stage, patented

Installation, unknown
field performance

Thermal, continousheating

10-15% error [19]

Thermal, pulseheating

5% error [20]

Commercial, many
field trials

Calibration, specificity,
power-hungry

Xylem water content

Capacitive

±0.1% error [25]

Early stage, academic

Field performance
specificity

Xylem embolization

Passive ultrasonic

∼10-20% error versus
hydraulic conductivity
[32]

Used in lab and some
field trials

Processing,
interpretation, metrics

Xylem water potential

Xylem sap-flow

modelling of the tree branches as acoustic waveguides [31].
To accurately assess the true frequency spectrum of xylem's
ultrasonic emissions, it is necessary to decouple it from the frequency response of the resonant piezosensors. Thus, recent
studies use broadband sensors with extended frequency range
[30]. Nevertheless, the area of advanced time-frequency signal
analysis remains open for further research.
To assess how early warning signals of water stress induced plant mortality are the xylem's ultrasonic emissions,
it is important to physiologically interpret emission data. In
this sense, correlation of the ultrasonic activity to the known
plant-hydraulic quantities, such as water potential and loss of
hydraulic conductivity is under investigation [32]. While there
have been proposed emission signal parameters which correlate well to underlying physiological features and processes
of gymnosperms (conifers) [29], interpretation of so-far investigated signal parameters obtained on angiosperm plants
remains inconclusive. Thus, it remains open how to integrate
them into the irrigation of economically viable cultures, such
as grapevine.
Apart from this, a key engineering challenge to bring
the instrumentation for xylem emission detection from the
laboratory into the field remains and requires the clever, energy-efficient design of the signal chain for conditioning,
acquisition and processing, due to large signal bandwidth
and requirements for always-on operation [28]. This becomes
even more pronounced with implementation of near-sensor
processing tasks involving separation of different emission
sources or emission localization.

References
[1]	 C. Acevedo-Opazo, S. Ortega-Farias and S. Fuentes, "Effects of

Future Challenges

grapevine (Vitis vinifera L.) water status on water consumption,

Conclusively, the most prospective plant water stress measurement technologies for integration in future precision
irrigation systems are summarized in the Table 1.
44	

For botanists, an accurate measurement of xylem water potential has always been the holy grail of water stress
monitoring. However, direct xylem water potential measurement remains largely inaccessible, leaving a lot of room for
methodological uncertainty, even in state-of-the-art plantphysiological research. Therefore, early implantable MEMS
xylem pressure sensor prototypes [13] look encouraging to
change this and inspire similar future designs.
On the other hand, thermal anemometric sap-flow measurements reached their maturity in the metrological sense,
with several commercial spinoffs. Quite like semiconductor gas sensors some years ago, this area is now open to take
its next technological step towards miniaturization in MEMS
technology, and optimization of power consumption, in order
to reach the goals for autonomous field-operation. Especially
interesting research directions are minimally invasive techniques and microfluidic designs.
Finally, embolism detection is crucial in the prevention of
the drought-induced mortality of plants [3]. Passive ultrasonic
monitoring is one of the most prospective technologies in its
early stages of technological readiness. In addition to many
open signal-processing challenges, intermittently appearing
ultrasonic acoustic emissions call for novel electronic designs
of dedicated wake-up triggering interfaces. Also, this also
opens the space for novel technological design of tailor-made
piezosensors and passive piezoelectric MEMS arrays for passive ultrasonic event detection.

vegetative growth and grape quality: An irrigation scheduling
application to achieve regulated deficit irrigation," Agricultural
Water Management, vol. 97, pp. 956-964, 2010.

IEEE Instrumentation & Measurement Magazine	

April 2020



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