Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 222

replacement resistance for two parallel resistances: (/

1 r + 1 / r )

− 1

. A typical value
for the cuticular resistance is 4000 s m -1 (Leuning, 1995 ; Ronda et al., 2001 ).

s cut

6.4.2 Photosynthesis
Plants ingest CO 2 through their leaves and - using the energy from sunlight - ix this
CO 2 in the form of carbohydrates. The process consists of two steps:
The light-dependent process: formation of energy-storage molecules (ATP and NAPDH) •

using the energy of absorbed photosynthetically active radiation (PAR). PAR is part of the
shortwave radiation, covering the wave length range of 0.4-0.7 μ m. In terms of energy
luxes, PAR represents 40-50% of global radiation, depending on season, sky conditions
and location (see review in Papaionnou et al., 1996). Because individual photons inter-
act with the photosynthesis mechanism, PAR can also be expressed in terms of number
of photons per unit of time and area. Typically, the ratio of PAR to global radiation is
2 μ mol J -1 (Jacovides et al., 2007 ).
The light-independent (or dark) process: the ixation of CO •

2 into carbohydrates, using
the energy supplied by ATP and NADPH. This process is called the Calvin cycle and is
catalysed by the enzyme RuBisCO.

Photosynthesis mainly takes place in the mesophyll cells (see Figure 6.12 ). Three dif-
ferent mechanisms of photosynthesis can be distinguished:

• C 3 carbon ixation : Carbon ixation takes place in the mesophyll cells and the irst car-
bohydrate produced in the ixation process is a three-carbon organic acid. This in turn is
used to produce glucose. RuBisCO, which catalyzes the carbon-ixation is also able to
catalyze oxygen ixation. The relative importance of carbon ixation and oxygen ixation
depends on temperature: at higher temperatures the balance shifts to oxygen ixation,
even to the point that more CO 2 is produced than taken up: net photorespiration. This
makes C 3 plants unsuited to grow under hot conditions.
• C 4 carbon ixation : The CO 2 is ixated in two steps and two locations. The irst car-
bohydrate produced - in the mesophyll cells - is a four-carbon organic acid. For this
production the enzyme PEP carboxylase is used, which has a strong preference for CO 2
and also works at relatively low concentrations of CO 2 . Subsequently, this product is
transported to the bundle sheath cells (photosynthetic cells arranged around the veins of
a leaf). In the latter cells, the reaction is reversed to produce CO 2 from the organic acid.
Subsequently, this CO 2 is used in the regular Calvin cycle to produce glucose or other
carbohydrates. Because in the bundle sheath cells the oxygen concentration is low, the
ixation of oxygen by RuBisCO is nearly absent.
• CAM carbon ixation. The CO 2 is ixated in two steps at two different times. In
CAM plants (crassulacean acid metabolism) the uptake of CO 2 and photosynthesis are
asynchronous. During the night these plants open their stomata to ingest CO 2 that is
stored in organic acids. During the day, CO 2 is extracted from the organic acids and used
in a regular photosynthesis process. This arrangement, where stomata are closed when