Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 215

VPD . It is thought that as long as water is available, there is
no selection pressure against those plants that can keep their
stomata open at night. There may be a slight increase in the
potential for bacteria or viral entry, especially if moisture is
present on the leaves, but this should not occur during times
of high VPD unless artificial irrigation is used. Other
researchers also have observed nighttime transpiration in
willow (Iritz and Lindroth 1994) and Populus spp. (Hogg
and Hurdle 1997), both plants commonly installed at
phytoremediation sites. An important distinction to keep in
mind is that nocturnal sap flow is a consequence of transpi-
ration, not photosynthesis. For some C 3 and C 4 plants, how-
ever, stomata closure does not occur during the night (Caird
et al. 2007).
When surface soils become dry, it is possible that the
deeper water that is taken up by a plant from groundwater
can be redistributed to the shallower soils during the night.
This is driven by a gradient in water potential from the
wetter xylem to the drier soil, so it is a passive process.
This process is called nocturnal reverse flow or hydraulic
redistribution (Hultline et al. 2003). Its occurrence is
inversely related to the VPD during nighttime and does not
occur after the surface soils are rewetted during precipita-
tion. Hydraulic redistribution essentially is the allocation of
water from wetter areas in contact with deeper roots to drier
areas in contact with shallower roots from the same plant.
To summarize, the phenomenon of nighttime transpira-
tion has implication for phytoremediation projects as far as
the estimation of the total water budget. Even though this
process of water loss is small, it still accounts for an effect on
a site's water budget that may be beneficial to the hydrologic
goals set for the site and should, therefore, not be ignored.

are based on diffusion. In the past, plant physiologists have
relied on a number of approaches to quantify stomatal size in
order to relate it to both steady state and dynamic gas
exchange, be it water, CO 2 ,orO 2 . These approaches have
ranged from simple observation of leaf surfaces in response
to different conditions of VPD to photographic imaging
systems (Weyers and Meidner 1990). Another method
includes the addition of various fluids to a leaf surface to
measure the amount of time for uptake, the uptake presum-
ably occurring through open stomata. For example, the
cobalt chloride method was a standard method used in the
field prior to the 1960s (van Bavel et al. 1965).
The fluids in the above method were applied under nor-
mal pressure gradients but were under a pressure nonethe-
less. This progression led to the idea in the early 1900s of
using a gas rather than a liquid under pressure to measure
stomatal opening and stomatal conductance. Today, the
accepted standard to measure leaf conductance is to use a
gas-flow porometer. This device can be used to measure the
effect of changes in humidity on sap flow by using a cup that
contains a humidity sensor placed over a leaf for a short
period of time. These measurements may have more rele-
vance to trees such as poplars, which have stomata on both
upper and lower surfaces of leaves, compared to other trees
that possess only one stomatal surface. In these cases, some
researchers have found no transpiration from the upper sur-
face of a leaf but it is found on the lower surface (Zhang et al.
1999). However, porometer measurements on single leaves
are not likely to be representative of the whole tree
(McDermitt 1990), due to equipment-induced changes in
the humidity and wind circulation patterns around sampled
leaves. For example, attachment of equipment to poplar
leaves prevents leaves from fluttering in response to wind
that under natural conditions can remove large amounts of
water vapor due to disruption of a humid boundary layer
around the moving leaves.
Some standard pieces of equipment that are field portable
include the LI-1600 steady-state porometer

9.1.4 Stomatal Conductance

As was described in Chap. 3, the stomata represent a struc-
tural compromise between the need for the mesophyllic
cells to be open to the diffusive entry of CO 2 into a wet
boundary layer, while at the same time restricting the pas-
sive loss of water vapor to the atmosphere. Stomata help
regulate the evaporation of water from a leaf such that
transpiration is more complicated than evaporation of
water from an exposed water surface. The stomata are
regulated in turn by the water potential of the leaf cells.
Hence, knowledge of the impact of stomatal conductance, or
leaf resistance, to water-vapor flow to the atmosphere, is
important

(LI-COR

Biosciences , Lincoln, NE) or a dynamic, transit-time

porometer (AP4, Delta-T Devices, Cambridge, UK), or the
SC-1 (Decagon Devices, Pullman, WA). These devices use a
constant stream of gas (dry air) over the leaf surface to
determine transpiration through the stomata. These devices
calculate the rate of gas exchange by measuring the differ-
ence in humidity between the inside and outside of the leaf.
For example, inside the leaf the humidity is assumed to be
100%, and outside the leaf the ambient humidity is measured
by the sensors. In all cases, the results for stomatal conduc-
tance will be a function of the stomatal characteristics of the
plant leaves, such as number, surface location, and degree of
opening, as well as leaf temperature and atmospheric humid-
ity. The units of measurement of stomatal conductance are
expressed as a conductance (in millimoles per square meter

™

in understanding the water dynamics in

phytoremediation projects.
Most studies of stomatal (leaf) conductance have been
performed on deciduous trees. Stomatal conductance ulti-
mately determines the rates of the processes of transpiration,
photosynthesis, and cellular respiration, as these processes