December 2021 - 34

technically speaking
BY ERIK RUNKLE
Water Vapor-Pressure Deficit
The amount of water in the air (water vapor) influences the
rate of water loss by plants, growing substrates and other
surfaces. Relative humidity (RH) quantifies the water vapor
concentration relative to the maximum humidity possible at a
particular temperature and is usually expressed as a percentage.
A limitation of RH is that the amount of water vapor that air can
hold increases with temperature. Because of this dependence
on temperature, RH is not a consistent metric to describe the
potential loss of water by plants to the air.
A better way to describe the driving force for water
loss is the water vapor-pressure deficit (VPD). The VPD
is the calculated difference between the saturating
pressure of water (when water vapor is at an equilibrium
with condensation) and the actual water vapor pressure
of the air. These
measurements are
as pressures, and for
plant applications, we
usually use the unit
kilopascal (kPa). VPD
can be calculated
if you know the
canopy temperature
(or air temperature
as an approximate
substitute) and RH,
or the wet- and
Erik Runkle is
professor and
floriculture
Extension specialist
in the department
of horticulture at
Michigan State
University. He can be
reached at runkleer@
msu.edu.
dry-bulb temperature, using
a psychometric chart or an
online VPD calculator such
The VPD is the
calculated difference
plants are grown at a very low VPD, there is little movement of
water through the plant and thus, little uptake of nutrients from
the substrate. Leaves stay wet for a long time and the substrate
dries very slowly, which increases the virulence of many plant
pathogens.
Low VPD (0.3 to 0.7 kPa). This is the minimum desirable VPD
for most plant growth applications. This VPD range occurs when
the RH is 64 to 85% at 63° F, 70 to 88% at 68° F, and 75 to 90%
at 73° F. The gradient that exists between leaves (which are
practically saturated with water) and the air means that plants
transpire at a moderately slow rate. This VPD range can be
desirable during propagation once young plants have developed
an initial root system. Leaves don't stay wet for so long and there
is a modest uptake of water and nutrients from the substrate.
However, for some crops,
especially those susceptible
to tip burn, this VPD is too
low.
Moderate VPD (0.7 to
between the saturating
pressure of water and
the actual water vapor
pressure of the air.
one under the biology and then horticulture category at www.
omnicalculator.com.
When the air is saturated with water vapor, the VPD is zero. This
may occur when a fogging system is used inside a greenhouse,
such as early during propagation of vegetative cuttings. Since
the air is saturated with water, there is no driving force for water
loss and so cuttings remain hydrated. When the water vapor in
the air decreases, or as temperature independently increases,
the VPD increases. As it increases, water loss increases through
evaporation from the substrate and transpiration from the leaves.
Below are subjective VPD classifications, values and
descriptions when growing plants in controlled environments.
Corresponding RH values are provided; note how these values
change with temperature.
Very low VPD (<0.3 kPa). In this situation, water vapor is at or
near saturation. This low of a VPD occurs when the RH is above
85% at 63° F, 88% at 68° F, and 90% at 73° F. This environment
is only desirable when we want slow-drying conditions, such
as during propagation or cold storage of plant material. When
1.5 kPa). This water VPD
can be considered a target
for most plant growth
applications. The air is
neither too dry nor too wet,
so substrates and plants do
not stay wet for long, nor do
they dry very quickly. The
driving force for water and
nutrient movement from the
roots through the shoots is
moderate but usually not excessive. This VPD occurs when the RH
is 24 to 64% at 63° F, 36 to 70% at 68° F, and 46 to 75% at 73° F.
High VPD (>1.5 kPa). This can be considered a " dry air "
environment and so water loss from plants and their substrates
is relatively rapid. This occurs when the RH is less than 24%
at 63° F, 36% at 68° F, and 46% at 73° F. In some production
situations, such as when plant canopies are large (e.g., high-wire
vegetable production), plants may not be able to sufficiently
translocate water, which can cause plants to partly close their
stomata, thus decreasing water loss but also uptake of carbon
dioxide. As a result, the rate of photosynthesis decreases, and
the growth rate slows down.
VPD generally only becomes a crop production issue when at
extremes. When the VPD is too low (less than 0.7 kPa, except
during propagation), some pathogens can be more problematic
and nutrient uptake may not be sufficient to meet the needs of
plant growth. When the VPD is high (more than 1.5 kPa), the rate
of water loss is high and in response, photosynthesis of some
crops can decrease.
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