Leaf Porometer Operator’s Manual
3. The Menus
21
cast days, for example, generally have lower conductances
than those taken on sunny ones. Additionally, leaves in
shadow or in shade usually have lower conductances than
those in greater light. Therefore, make sure to consider the
lighting conditions you will be measuring in.
Temperature.
A plant’s stomatal conductance is also
affected by temperature. For most plants, on a very hot or a
very cold day, photosynthesis will slow down or stop, and
the stomata will narrow or will not open, to prevent internal
water loss. Therefore, be sure to consider the temperature
before conducting measurements.
Humidity.
A factor that will cause the stomatal conduc-
tance to change is the humidity at the leaf ’s surface. As the
outside humidity increases, the stomata will open. Porome-
ters that dry the air around the stomata decrease conduc-
tance; likewise, those that allow surface humidity to increase
do the opposite.
Carbon Dioxide (CO
2
).
One purpose of the stomata is to
regulate the leaf ’s usage of CO
2
, which is important to con-
trolling the balance of humidity. An increase in CO
2
will
cause stomata to close, while a decrease will cause them to
open. Thus, it is important to consider the CO
2
levels of
your measuring environment. You, the Leaf Porometer
operator, can adversely affect readings by exhaling CO
2
directly on the plant you are measuring. This is particular
important in growth chambers or greenhouse, where CO
2
from an operator’s breath can double or triple ambient levels.
