Geoscience Reference
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concentration) to maintain a high inlux. At a given demand for CO
2
(determined by the
available amount of radiation) the plant can regulate the internal CO
2
concentration by
regulating the stomatal aperture. The regulation happens through the sensitivity of the
guard cells to the internal CO
2
concentration (Willmer and Fricker,
1996
). It turns out
that the ratio of internal to external CO
2
concentration is rather conserved variable (see
Eq. (
6.26
)).
•
External CO
2
concentration.
If the external CO
2
concentration is increased, the irst-
order effect will be that the CO
2
lux into the stomata is increased. If the inlux becomes
too large, the plant will no longer be able to use all the CO
2
for photosynthesis and the
internal CO
2
concentration will increase. This in turn causes an increase of the stomatal
resistance (Jarvis and Davies,
1998
).
•
Leaf water potential.
When the leaf water potential decreases (becomes more nega-
tive) the stomata close to prevent further water loss. Because the leaf water potential
is the net result of water uptake and transport towards the leaves and loss of water
through transpiration, any process that inluences one of these luxes will have an
effect on stomatal aperture (e.g., excessive evaporative demand, lack of root water
uptake). Though maintaining turgor should be an important strategy for the plant, sto-
matal closure also seems to be modulated directly by signals from the roots (Davies
and Zhang,
1991
).
The responses sketched in the preceding text generally lead to a clear diurnal cycle
in the stomatal resistance with low values during daylight and ininite values during
the night. However, under dry and hot conditions the vapour pressure deicit may
increase beyond its critical value around midday, leading to midday closure of the sto-
mata. This prevents excessive water loss during the time of maximum temperatures
and vapour pressure deicit.
When comparing the responses of the stomatal resistance to environmental factors
in
Figure 6.15
to the responses of the photosynthesis rates in
Figure 6.13
it appears
that they are related: the responses to CO
2
concentration, radiation and temperature
are such that low resistances are linked to high net photosynthesis rates. This relation-
ship - expressed also in Eq. (
6.29
) - is exploited in one of the models for the canopy
resistance that is discussed in
Section 9.2.4
.
6.4.4 CO
2
Exchange at the Ecosystem Level
In
Section 6.4.2
photosynthesis and respiration have been discussed on the level of
individual leaves. However, the exchange of CO
2
between the atmosphere and the
land surface is determined not only by the net photosynthetic activity of plants. Res-
piration by organisms in and on the soil is an important source of CO
2
. This includes
respiration by plant roots, as well as microbes and animals feeding on organic matter
within and on top of the soil. The respiration by plants is called autotrophic respi-
ration because the plants use their own carbohydrates, whereas the respiration by
microbes and animals is called heterotrophic respiration.
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