Instrumentation & Measurement Magazine 23-2 - 38

Hydraulic Quantities Modelling Water
Transport in Plants
In trees, the physiological mechanism of water transport from
soil into the root and the ascent through the stem, up to the
leaves is described by the cohesion-tension theory [5]. Due
to evapotranspiration, surface tension (capillarity) creates
the tension in the fluid at the air-water interfaces (menisci) of
leaves' cell-wall surfaces at stomatal openings. Consequently,
this tension is transmitted throughout a complex network of
xylem conduit vessels/tracheids, extending all the way to the
root [2]. Adhesion of water to conduit walls and cohesion of
polar water molecules to each other acts as the main pulling
mechanism, enabling the water to ascend upward, opposing
the Earth's gravitational force (Fig. 1a). Thus, water transport
through the xylem network in vascular plants can be described
as a complex physio-chemically controlled hydraulic system
[6].
Hydraulic quantities commonly used to describe the water
uptake, transport and storage include: xylem water potential, xylem water (i.e., sap) flow, hydraulic conductivity of the
plant's tissues, and content of water residing/stored in the tissue, analogous to an electrical voltage, current, resistance, and
capacitance, respectively (Fig. 1b).

Xylem's Water Potential
Xylem's water pressure/potential Ψ is a state-variable, measuring the energy state of the water, at a given point in the
xylem, on given environmental conditions. It is defined
as energy required to push/pull a cubic meter of water
through the xylem. Consequently, it is expressed in Pascals
[Nm / m3 = N / m2 = Pa].
Factors contributing to the water potential Ψ are Ψp turgor pressure, Ψs osmotic potential, Ψm matric potential, and
Ψg gravity potential. Thus, plant water potential is usually
expressed as their summation: Ψ = Ψp+ Ψs+ Ψm+ Ψg, jointly representing individual contributions of complex interactions of
plant-transpiration, environmental temperature, humidity,
solar irradiation, plant height, soil moisture, nutrient concentration, homeostatic stomatal regulation in isohydric species,
etc. Thus, plant water potential should be interpreted as a resultant energy-state variable.
Plant water potential is usually measured as a relative pressure in xylem vessels of a root, stem or individual leaves. Pure
water at the root level is arbitrarily assigned zero potential. In
transpiring leaves/stem, evapotranspiration-induced tension
(suction) dominantly lowers the Ψ below zero. Positive difference of water potentials (i.e., pressure gradient) between two
points enables the water flow between them (i.e., from root to
leaves) [2]. Thus, Ψ can be considered analogous to the voltage
in electrical circuits.

Xylem Sap Flow
Xylem sap (water) flow caused by the pressure gradient is usually quantified either as mass flow [kg h−1], or volume flow
[cm3 h−1]. They are frequently expressed as "flux density," indicating that the mass or volume flow is normalized per (xylem)
38	

Fig. 1. (a) Plant's water transport system ([7], licensed under Creative
Commons BY 4.0). (b) Simplified hydraulic model of a plant (reproduced from [6]
with permission, © American Society of Plant Biologists, 1999).

surface (i.e., [cm3 cm−2 h−1]). The range of sap flux densities
ranges from -10 (negative flow) up to 80-110 cm3 cm−2 h−1 (max.
positive flow) [8]. Water flow is considered analogous to current in electrical circuits. Physiologically, it is observed, that
the xylem sap flow tends to lag after the changes in transpiration rate, owing to the hydraulic capacitance of the stem and
other plant tissues [8].

IEEE Instrumentation & Measurement Magazine	

April 2020



Instrumentation & Measurement Magazine 23-2

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