Geoscience Reference
In-Depth Information
Figure 6.14
CO
2
concentration along the pathway from atmosphere to cell for C
3
and C
4
plants. Due to a lower intracell concentration (
q
cc
) and a smaller mesophyll
resistance, C
4
plants maintain a lower CO
2
concentration in the stomata (
q
ci
) than C
3
plants. As a result, the stomatal resistance for a C
4
plant can be higher than that for a
C
3
plant for a given assimilation rate (thus limiting water loss). Note that the slope of
the
q
c
-line is proportional to the resistance at that location.
C
4
plants can have a higher stomatal resistance (see
Figure 6.14
) and hence a lower
transpiration. This implies that the water use eficiency (mass of ixed CO
2
per mass
of water lost) is signiicantly higher for most C
4
plants.
Question 6.5:
Given the following data:
q
e
= 9 g kg
-1
,
q
i
= 12 g kg
-1
q
ce
= 385 mg kg
-1
and
q
ci
= 300 mg kg
-1
,
ρ
=1.1 kg m
-3
and
r
s
= 70 s m
-1
.
a) Determine the transpiration and assimilation luxes
E
and
A
n
.
b) Determine the water use eficiency (using the deinition of a plant physiologist: kg
CO
2
ixed per kg water lost).
c) C
4
plants are able to maintain a lower internal CO
2
concentration (
q
ci
) than C
3
plants.
Explain why this leads to higher water use eficiency (as deined earlier).
d) Explain what happens to the transpiration and the water use eficiency if the leaf
temperature increases.
6.4.3 Stomatal Behaviour
In
Section 6.4.1
it was shown that the exchange of both water vapour and CO
2
between plants and the atmosphere is - to an important extent - related to the stoma-
tal resistance, and hence the stomatal aperture: the larger the aperture, the smaller the
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