Environmental Engineering Reference
In-Depth Information
Rubisco's abundance is not only a matter of its ubiq-
uity but also of the slowness of its reactions (catalyzing
just 3 molecules/s compared to 1000 molecules/s for a
typical enzymatic process) and its reversibility. The en-
zyme acts as a carboxylase (as just described) and, once
the CO
2
concentrations inside the leaf drop to about 50
ppm, as an oxygenase. In the latter role Rubisco is the
catalyst for the binding of O
2
to RuBP to produce not
only PGA but also 2-phosphoglycolate, a 2-C com-
pound, during the C
2
oxidative photosynthetic cycle
that releases CO
2
(this photorespiration is an entirely dif-
ferent process from night-time mitochondrial respiration
in leaves). Additional energy is needed to remove 2-
phosphoglycolate by converting it first to glycolic acid
and then to glycine and serine. This oxygenation appears
to be of no benefit for the plant, yet photosynthesis and
photorespiration are inextricably linked: there is just one
CO
2
and one O
2
pool, and the C
3
and C
2
cycles create a
necessary balance for net exchange of the gas (fig. 3.2).
Because of the relatively low CO
2
and high O
2
levels
in today's atmosphere, some species use about half of
photosynthetic energy in the C
2
cycle (Tolbert 1997;
Hall and Rao 1999). Only a drastic reduction of atmo-
spheric O
2
(to about 2%) or greatly elevated ambient
CO
2
levels would eliminate C
2
-cycle losses in C
3
plants.
But some plants can avoid the photorespiration losses.
Kortschak tried to replicate the Calvin-Benson sequence
with sugarcane, but his first product was not PGA but
rather the 4-C acids malate and aspartate, which he
assumed to have a common precursor in oxaloacetate,
another 4-C acid. Hatch and Slack then unraveled the
details of this distinct process. Instead of reducing CO
2
with Rubisco, they used phosphoenol pyruvate (PEP)
carboxylase in the mesophyll cells to form oxaloacetate
(fig. 3.3) (Hatch 1992).
This acid is reduced to malate, transported into
chloroplasts of the bundle sheath cells where CO
2
is
regenerated, and only then used in the Calvin-Benson
cycle (Sage and Monson 1999). These C
4
plants also dif-
fer structurally from C
3
species. The latter have no signif-
icant differentiation in mesophyll and bundle sheath,
and the vascular conducting tissue of C
4
species is sur-
rounded by a bundle sheath of large thick-walled cells
containing chloroplasts (fig. 3.3). PEP carboxylase has
a greater affinity for CO
2
than Rubisco; moreover, O
2
levels in the bundle sheath are low, and CO
2
concen-
trations are near what is required to saturate Rubisco,
whose oxygenating action (causing photorespiration)
is practically eliminated. But structural differences
(Kranz anatomy) are not obligatory: Voznesenskaya et al.
(2001) discovered that a species of Chenopodiaceae
(Borszczowia aralocaspica) uses C
4
pathway through spa-
tial compartmentation of enzymes and separation of two
types of chloroplasts.
AC
4
pathway needs more energy than the Calvin cycle
alone. Additional ATP is required to energize the regen-
eration of pyruvate to PEP, but in the absence of photo-
respiration its overall net conversion efficiencies are
considerably higher. Maximum daily growth rates of C
3
and C
4
species are, respectively, 34-39 g/m
2
and 50-
54 g/m
2
, a 40% difference that is especially significant in
food production (Monteith 1978; Edwards and Walker
1983). Daily maxima averaged over the whole growing
season show still greater difference: with 22 g/m
2
,C
4
plants are about 70% ahead of C
3
species that fix 13 g/
m
2
.C
4
species also do not have any light saturation (C
3
plants saturate at irradiances around 300 W/m
2
), and
their optimum photosynthetic temperature is 30
C-
45
C (15
C-25
CinC
3
plants). The C
4
pathway thus
appears to be an adaptation to hot climates and aridity,
