Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 223

the largest evaporative loss would occur, is beneicial for plants that are growing in arid
conditions, such as succulents (Lambers et al., 2008 ).

The net CO 2 assimilation is the net result of the ixation of CO 2 , the photorespiration
(occurring when RuBisCO ixes oxygen rather than CO 2 ) and other respiratory pro-
cesses that are required to provide energy for growth, maintenance, and transport, so-
called dark-respiration (Lambers et al .¸ 2008 ). As photorespiration is directly linked
to the photosynthetic activity of RuBisCO, it is included in the gross assimilation rate
A g . Dark-respiration ( R d ) is not completely independent of light conditions and shows
a decay after the initiation of darkness (Byrd et al., 1992 ; Lambers et al., 2008 ). The
net assimilation rate is then decomposed as:

AAR
n

=−

(6.22) g d

The rate of CO 2 assimilation is determined by the most limiting part of the chain. For
both the light-dependent and light-independent processes this may be the tempera-
ture as both processes depend on enzymes. These operate optimally within a certain
temperature range only: a minimum temperature is needed to activate the enzymes
whereas temperatures beyond a certain maximum will cause denaturation (changes in
the three dimensional structure) (Gates, 1980 ). Furthermore, for the light-dependent
process the amount of PAR supplied to the leaf may be limiting, whereas for the light-
independent process the supply of CO 2 may hamper photosynthesis.
Figure 6.13 sketches each of these responses. At constant temperature and radi-
ation input, the net assimilation rate initially increases with internal CO 2 concen-
tration (within the leaf) until a plateau is reached at which the supply of CO 2 is no
longer the limiting factor ( Figure 6.13a ). For low internal CO 2 concentrations pho-
torespiration plus dark respiration dominate over gross photosynthesis, leading to a
negative net assimilation rate. The CO 2 concentration at which the net assimilation
is zero is called the CO 2 compensation point, denoted by Γ (Lambers et al., 2008 ).
At constant CO 2 concentration and temperature an increase of absorbed PAR ini-
tially leads to an increase of the net photosynthesis rate ( Figure 6.13b ). But at high
amounts of absorbed radiation the photosynthesis system becomes light saturated.
The small negative net photosynthesis rate at zero absorbed PAR is due to respira-
tion that is not related to the light-dependent process: dark respiration R d . The point
where A n = 0 is called the light compensation point (LCP; Lambers et al., 2008 ) and
the slope of the curve at the origin is called initial light use eficiency. In this slope
the photorespiration is included: it shows the net effect of adding extra light where
most of it is used to ix CO 2 and a small part is used to ix O 2 , under the release of
CO 2 . The light response is different for leaves that have developed in full sunlight
and leaves that are acclimated to shade (lower in the canopy): although the ini-
tial light use eficiency for both types of leaves is similar, sun-exposed leaves have