SVERIGES LANTBRUKSUNIVERSITET - Epsilon Open Archive - SLU

60
-, Different approaches for calculating tissue parameters from pressure
volume curves have lately been outlined and discussed by Kikuta and
Richter (1986) and Stadelmann (1984). The advantage of the pressure
volUIne method is that apoplastic water does not interfere in the determination
of l/J 7r and also the elasticity of the tissue can be quantified. An
useful ecological interpretation of pressure volume curves is given by Jane
and Green (1983) and Bannister (1986).
Measurement of stornatal conductance and gas exchange
The epidermis of leaves creating the boundary between le af interior and
the atmosphere is covered by a relatively impermeable cuticle except at
stomata openings through which gases (C0 2 and water vapour) mainly
exchange between the leaf interior and the atmosphere. When water
deficits develops in the leaves the turgor regulated stornatal cells close with
the consequent reduction in transpiration and CO 2 uptake. Thus it is
important to measure stornatal opening directly from stornatal impressions
or by measuring the rate of gas exchange.
Stornatal impressions can be obtained by applying a quick - drying
substance to the leaf surface followed by microscopic countings and
measurements of stornatal opening on the impressions (Rice et al., 1979).
Stornatal opening can be converted into diffusive resistance (Meidner and
Mansfield, 1968).
Stornatal conductance can be calculated from measurements of water
vapour loss in leaf chambers under laboratory conditions (Sestak et al.,
1971). However, for field conditions more simple portable technique is
required in order to obtain many determinations and hereby reduce the
statistical uncertainty. This claim has been met by the introduction of
diffusive porometers and most work on stornatal resistance is now carried
out with such porometers based on a electronic moisture - sensor system.
These instruments measure the diffusion of water vapour away from the
leaf and are of two basic types the "transient" and "steady state" porometer.
In both types the leaf is enclosed in a cuvette which may be ventilated.
Ventilation reduces the boundary resistance layer to a low value. However,
ventilation may increase the evaporative loss and induce stornatal closure.
Non -ventilated transient porometers are relatively cheap and instruments
are commercially available (e.g. "Delta T Automatic Porometer Mk III",
Delta-T Devices, Cambridge, U.K.).
In steady state porometers, which are always ventilated, the relative
humidity is maintained at or near ambient humidity. Stornatal resistance
is derived either from the increase in humidity in the air passing the leaf
or from the rate of inflow of dry air needed to maintain a constant
humidity. These instruments are also commercially available (e.g. "LI -1600
Steady State Porometer", LiCor Inc.).