Introduction
The AP4 is a cycling porometer.
It
is used for measuring the stomatal
resistance of plant leaves. This is a measure of the resistance to loss
of water vapour through the stomata and is an indicator of the
physiological state of the plant.
Stomata are sensitive to light, carbon dioxide, pollutants and water
stress and pathogens. The AP4 provides an important tool for
unravelling and understanding the intricate web of mechanisms that
control stomatal behaviour.
It
also has an important role to play in
the comparison of the performance of different varieties and breeds of
crops in response to environmental variations and stresses.
The instrument works by measuring the time
it
takes for a leaf to
release sufficient water vapour to change the relative humidity in a
small chamber by a fixed amount. This is compared with a calibration
plate of known resistance in order to derive the stomatal resistance or
conductance of the leaf.
The method is well established.
It
is supported by a good theoretical
understanding of a cycling porometer (Monteith, Campbell
&
Potter,
1988)
and backed up by our considerable manufacturing experience of this type
of instrument. Many hundreds of Delta-T cycling porometers have been in
use throughout the world over the last decade.
The design has been significantly improved by the use of a
microprocessor to release the user from the burden of frequently
replotting calibration graphs. The process is now much simpler and
quicker.
It
also needs doing less frequently because the AP4 can correct
for changes in temperature. Readings are automatically converted into
units of resistance or conductance, and can be annotated, displayed,
saved and output to a computer or printer. Menu driven software, a 40
character by
8
line liquid crystal display and dedicated function
buttons are thoughtfully integrated to make the use of the instrument as
intuitive and simple as possible.
Beginners can be up and running quickly without a deep knowledge of all
the available options, whereas more experienced users have easy access
to the default settings via the use of a context sensitive SET button.
A
HELP
button displays context-sensitive help messages, and a laminated
single page Quick Guide is provided which you can tuck in the carrying
bag.
A
single button on the porometer head permits one-handed use of the
instrument in the field. The software is arranged so that successive
readings can be taken, evaluated and stored just by pressing this one
button. Stable readings are denoted by a the sound of a double "beep",
so you do not need to look at the instrument at all.
Two porometer cup shapes are provided for different leaf shapes. The
head and case are relatively light, and a padded shoulder bag with a
waist-strap is provided as standard.
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