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(a)
(b)
(c)
max
max
max
0
0
0
0
0
Internal CO
2
concentration
Absorbed PA R
Leaf temperature
Figure 6.13
Dependence of photosynthesis rate on various environmental condi-
tions (all other factors kept constant). (
a
) CO
2
concentration in the air space of the
leaf. (
b
) The supply of photosynthetically active radiation. (
c
) Temperature. (After
Gates,
1980
)
higher maximum assimilation rates than shaded leaves (the plateau in
Figure 6.13b
is at a higher level). At the same time, sun-exposed leaves have a higher respiration
rate, leading to a higher light compensation point: a downward shift of the curve in
Figure 6.13b
(Lambers et al.,
2008
). Finally, the temperature of the leaf is a limit-
ing factor if it is either too low or too high for optimal functioning of the enzymes
(
Figure 6.13c
). However, species may acclimate to the temperature regime of their
habitat, leading to optimum temperatures that can range from below 10 °C to above
30 °C (Lambers et al.,
2008
).
The pathway for CO
2
is similar to that of water vapour, except for the fact that -
provided that assimilation dominates over respiration - it is directed in the opposite
direction. Besides, as the CO
2
is used for photosynthesis inside the cells (in contrast
to water vapour that originates mainly from the intercellular space) it has to pass
a number of extra interfaces, relected by the mesophyll resistance
r
m
(see
Fig-
ure 6.12
) The
r
m
is suficiently high to have a strong impact on the CO
2
uptake. Fur-
thermore,
r
m
is highly dynamic, reacting to environmental factors (e.g., temperature
and radiation) on timescales that are shorter than the reaction time of the stomatal
resistance (seconds to minutes rather than tens of minutes; Flexas et al
.¸
2008
). The
internal CO
2
concentration in the substomatal cavity is determined by the combina-
tion of the net CO
2
demand from the photosynthesizing cells (
A
n
) and the various
resistances. It turns out that this interplay results in a
q
ci
that, at high light levels,
has a rather constant relationship to the external CO
2
concentration (Zhang and
Nobel,
1996
).
If again we focus on the path through the stomata only, the net assimilation
A
n
can
be written as:
AF
qq
r
−
=− =
ρ
ce
ci
(6.23)
n
c
s,c
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