Blackwell Science, LtdOxford, UKPCEPlant, Cell and Environment0016-8025Blackwell Science Ltd 20032004
272155165
Original Article
Transformation with foreign Rubisco for productivity
X.-G. Zhu
et al.
Plant, Cell and Environment (2004) 27, 155–165
OPINION
Would transformation of C3 crop plants with foreign Rubisco
increase productivity? A computational analysis
extrapolating from kinetic properties to canopy
photosynthesis
X.-G. ZHU1, A. R. PORTIS JR1,2,3,4 & S. P. LONG1,2,3
1
Physiological and Molecular Plant Biology, 2Department of Plant Biology and 3Department of Crop Sciences, University of
Illinois, Urbana, IL 61801, USA and 4Photosynthesis Research Unit, Agricultural Research Service, United States Department
of Agriculture, Urbana, IL 61801, USA
ABSTRACT
Genetic modification of Rubisco to increase the specificity
for CO2 relative to O2 (t) would decrease photorespiration
and in principle should increase crop productivity. When
the kinetic properties of Rubisco from different photosynthetic organisms are compared, it appears that forms with
high t have low maximum catalytic rates of carboxylation
per active site (kcc). If it is assumed that an inverse relationship between kcc and t exists, as implied from measurements, and that an increased concentration of Rubisco per
unit leaf area is not possible, will increasing t result in
increased leaf and canopy photosynthesis? A steady-state
biochemical model for leaf photosynthesis was coupled to
a canopy biophysical microclimate model and used to
explore this question. C3 photosynthetic CO2 uptake rate
(A) is either limited by the maximum rate of Rubisco activity (Vcmax) or by the rate of regeneration of ribulose-1,5bisphosphate, in turn determined by the rate of whole chain
electron transport (J). Thus, if J is limiting, an increase in
t will increase net CO2 uptake because more products of
the electron transport chain will be partitioned away from
photorespiration into photosynthesis. The effect of an
increase in t on Rubisco-limited photosynthesis depends on
both kcc and the concentration of CO2 ([CO2]). Assuming
a strict inverse relationship between kcc and t, the simulations showed that a decrease, not an increase, in t increases
Rubisco-limited photosynthesis at the current atmospheric
[CO2], but the increase is observed only in high light. In
crop canopies, significant amounts of both light-limited and
light-saturated photosynthesis contribute to total crop carbon gain. For canopies, the present average t found in C3
terrestrial plants is supra-optimal for the present atmospheric [CO2] of 370 mmol mol-1, but would be optimal for
Correspondence: Dr Stephen P. Long, Department of Crop Sciences, 190 ERML, 1201 W. Gregory Dr, Urbana, IL 61801, USA.
Fax: +1 217 244 7563; e-mail: [email protected]
© 2004 Blackwell Publishing Ltd
a CO2 concentration of around 200 mmol mol-1, a value
close to the average of the last 400 000 years. Replacing the
average Rubisco of terrestrial C3 plants with one having a
lower and optimal t would increase canopy carbon gain by
3%. Because there are significant deviations from the strict
inverse relationship between kcc and t, the canopy model
was also used to compare the rates of canopy photosynthesis for several Rubiscos with well-defined kinetic constants.
These simulations suggest that very substantial increases
(> 25%) in crop carbon gain could result if specific Rubiscos having either a higher t or higher kcc were successfully
expressed in C3 plants.
Key-words: Amaranthus edulis; Griffithsia monilis; Phaeodactylum tricornatum; Calvin cycle; global atmospheric
change; model; photorespiration; ribulose bisphosphate
carboxylase/ oxygenase; substrate specificity.
INTRODUCTION
Ribulose bisphosphate carboxylase/oxygenase (Rubisco;
EC 4.1.1.39) is a bifunctional enzyme. It catalyses the addition of CO2 to ribulose-1,5-bisphosphate (RuBP) through
the photosynthetic carbon reduction cycle (PCRC) to produce two molecules of 3-photosphoglycerate (3-PGA),
which are then metabolized to triose phosphate. It also
catalyses the addition of O2 to RuBP to produce one molecule of 3-PGA and one molecule of 2-phosphoglycolate.
The 2-phosphoglycolate is metabolized through the photosynthetic carbon oxidation pathway (PCOP) releasing one
CO2 for every two oxygenations. PCOP serves to recycle
75% of the carbon entering 2-phosphoglycolate through
RuBP oxygenation. CO2 and O2 compete for the same
active sites of Rubisco. Although PCOP provides precursors for various metabolic pathways, these are dispensable
since plants can be grown successfully at saturating CO2
concentrations ([CO2]), where little or no photorespiration
occurs (Spreitzer 1999). Photorespiration in C3 crops is esti155
156 X.-G. Zhu et al.
mated to decrease productivity by over 30% at the current
atmospheric [CO2] (Ogren 1984; Long 1998). Although the
benefit of decreasing photorespiration as a means to
increase crop yields was once questioned (Evans 1993),
many experiments that have grown C3 field crops under
elevated CO2 provide clear evidence that very substantial
gains in yield may be achieved by inhibiting photorespiration (Kimball 1983; Long 1998). Competitive inhibition of
carboxylation by O2, the release of CO2 by the PCOP pathway and the energetic cost of recycling the carbon diverted
to this pathway all contribute to the reduction in C3 crop
productivity (e.g. Long 1991). Theoretically, the inhibition
of PCOP reactions downstream of Rubisco, but before the
glycine decarboxylase, will decrease photorespiratory CO2
release. However, any restriction here will prevent recycling of carbon back to the PCRC. Decreasing the activity
of glycine decarboxylase also proved to be an inappropriate
solution for decreasing photorespiratory CO2 release. Photosynthesis in Arabidopsis with deficient glycine decarboxylase activity was irreversibly inhibited in normal air
(Somerville & Ogren 1982; Somerville & Somerville 1983).
Similarly the host-selective toxin victorin, which severely
inhibits glycine decarboxylase activity, causes symptoms
typical of Victoria blight disease on susceptible plants
(Douce & Heldt 2000). Therefore, the only feasible means
to eliminate photorespiration and increase net photosynthesis for field-grown crops is via modification of Rubisco
to prevent the initial reaction of the pathway, RuBP oxygenation. Decreasing photorespiratory CO2 loss by increasing the specificity of Rubisco for CO2 relative to O2 is an
obvious target.
The specificity of Rubisco for CO2 relative to O2 (t), was
first defined by Jordan & Ogren (1981) as:
t=
k cc K om
k oc K cm
where kcc and koc are the numbers of carboxylations and
oxygenations, respectively, that one active site of Rubisco
may catalyse per second. kcm and K∞m are the Michaelis–
Menten constants of Rubisco for CO2 and O2, respectively.
Although Rubisco is a highly conserved protein within C3
terrestrial plants, t shows great variation when all groups
of photosynthetic organisms are considered (Jordan &
Ogren 1981; Tabita 1999). When measured at 25 ∞C, terrestrial C3 plants show an average t of about 92.5; cyanobacteria and green algae generally have a lower t of about 50–
60; whereas a few marine red algae may have a t above 100
(Tabita 1999). The existence of different t in Rubisco from
different organisms, especially the existence of high t in
marine non-green algae, raises the possibility of transforming C3 crop plants to express these forms of Rubisco with
a higher t and to decrease photorespiration. So far, efforts
to increase Rubisco-limited photosynthetic rate by increasing t via directed mutagenesis have had little success
(Chene, Day & Fersht 1992; Romanova, Cheng & Mcfadden 1997; Madgwick, Parmar & Parry 1998; Ramage, Read
& Tabita 1998). Most mutants exhibit a lower t than the
wild type from which they were obtained (Bainbridge et al.
1995; Parry et al. 2003). Comparison of the kinetic properties of Rubisco from different photosynthetic bacteria,
green algae and land plants suggests an inverse relationship
between t and kcc (Bainbridge et al. 1995; Spreitzer & Salvucci 2002). This correlation suggests t might only be
increased via genetic engineering of the protein at the
expense of a decrease in kcc and vice versa; namely higher
specificity is at the cost of slower catalysis. Slower catalysis
could be overcome by expressing more Rubisco in the photosynthetic cell; however, Rubisco already represents about
50% of leaf soluble protein in C3 crop leaves, and calculations of volumes suggest there may not be physical capacity
to add more (Pyke & Leech 1987). What is the implication
for photosynthesis at the leaf and canopy level if we assume
that t can be increased only at the expense of kcc? It has
been pointed out that t, by itself, does not necessarily confer higher Rubisco-limited photosynthesis if kcc is too low
or kcm is too high (Whitney et al. 2001; Spreitzer & Salvucci
2002). Furthermore, extrapolating conclusions based on
leaf photosynthesis models to the crop canopy level is complicated by the fact that net carbon uptake in a canopy will
be the result of a combination of RuBP- and Rubiscolimited photosynthesis. These two respond differently to
changes in kcc and t. Can we assess the impact of these
opposite effects at the canopy level?
The widely validated C3 biochemical model of Farquhar,
von Caemmerer & Berry (1980) predicts the steady-state
rate of leaf photosynthetic CO2 uptake (A) for given conditions of light, temperature and [CO2]. It assumes that for
any given set of conditions A is limited by either the maximum activity of Rubisco (Vcmax) or the rate of regeneration
of RuBP, which is in turn limited by the rate of whole chain
electron transport (J). Changes in t and kcc affect RuBPlimited photosynthesis and Rubisco-limited photosynthesis
differently. An increase in t will increase RuBP-limited
photosynthesis because a lower oxygenase activity diverts
less of the limiting flow of electrons to PCOP. Rubiscolimited photosynthesis however, depends on changes in
both kcc and t. Increasing t without a change in kcc will
increase Rubisco-limited photosynthesis because there is
less inhibition of carboxylation by oxygen and less CO2
released by the PCOP cycle. An increase in t together with
a decrease in kcc following the inverse relationship will
influence Rubisco-limited photosynthesis in a complex
manner depending on the relative changes in kcc and t. At
low light, A is limited by J, but at high light, it is more likely
to be limited by Vcmax. For a crop canopy with multiple
layers of leaves, total CO2 uptake results from a combination of both RuBP-limited and Rubisco-limited photosynthesis. Because photorespiratory loss decreases with rising
CO2, the benefit of increasing t at the expense of kcc also
depends on the atmospheric [CO2]. By integrating a steadystate biochemical model of leaf photosynthesis (Farquhar
et al. 1980) into a canopy microclimate model (Norman
1980; Forseth & Norman 1993), these combined effects
were assessed. The objectives of this study were to determine how increasing t affects crop photosynthesis for current and past [CO2], assuming that t and kcc have a fixed
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165
Transformation with foreign Rubisco for productivity 157
inverse relationship and that Rubisco content per unit leaf
area cannot be increased beyond the level found in sun
leaves of non-stressed mature C3 crops. Because some naturally occurring Rubiscos appear to deviate substantially
from the inverse relationship between t and kcc, we also
assessed the hypothetical value to C3 crop canopy photosynthesis of substituting the average form of Rubisco found
in terrestrial C3 plants with forms from other photosynthetic organisms.
THEORY, MODEL AND METHOD
Dependence of kcc on t
To quantify the apparent inter-relationship between t and
kcc, literature estimates for these values at 25 ∞C from different photosynthetic organisms (Jordan & Ogren 1981,
1984; Jordan & Chollet 1985; Parry, Keys & Gutteridge
1989; Read & Tabita 1994; Bainbridge et al. 1995; Horken
& Tabita 1999) were compiled (Fig. 1). Where values were
obtained at temperatures other than 25 ∞C, estimates were
corrected to 25 ∞C following Bernacchi et al. (2001). Ignoring mutant forms of Rubisco, the least-square best-fit
inverse relationship of t and kcc was determined (Fig. 1).
This relationship was assumed in the subsequent simulations unless otherwise noted.
(2000). Equations 3–6 were used to predict the leaf photosynthetic rate of CO2 uptake. The value of intercellular
[CO2] (Ci) is assumed to be 70% of ambient [CO2] (Ca)
(Eqn 7). The value of intercellular [O2] is assumed to be the
same as ambient [O2] (Eqn 8). Equations 9 and 10 were
used to predict the potential rate of electron transport governing the RuBP-limited rate of photosynthesis (Evans &
Farquhar 1991). The reference point for these simulations
were the amounts and properties of Rubisco reported for
non-stressed mature C3 crop leaves. The kinetic constants
(K∞m and kcm) were those of Bernacchi et al. (2001). t and
kcc for the average C3 leaf were set to 92.5 and 2.5, respectively, which was obtained by calculating the t corresponding to the average kcc of terrestrial C3 plants following the
inverse relationship shown in Fig. 1 (Farquhar et al. 1980;
Jordan & Ogren 1981; Bainbridge et al. 1995). Variation in
t around this C3 crop average was simulated by changing
K∞m and Kcm simultaneously with each contributing to half
of the change in t. If there is m% increase in t, the following
equations were used to obtain the changes in Kcm and K∞m.
( K o m ) = 1 + m / 100 - 1
( K c m ) =
1 - 1 + m / 100
1 + m / 100
The value of kcc was predicted from t as defined in Fig. 1.
Variation in kcc was simulated via Vcmax as
Leaf photosynthesis
Vcmax = Mkcc
Prediction of the leaf-level photosynthetic rate was based
on the mechanistic steady-state biochemical model of Farquhar et al. (1980); which has been widely used and validated across a large range of terrestrial C3 plants (von
Caemmerer & Farquhar 1981; Long 1985; Harley 1992).
The equations (see Appendix I) are from the models of
Farquhar et al. (1980) and as modified by von Caemmerer
where M is the concentration of Rubisco active sites per
unit leaf area (assumed to be: 26 mmol m-2), calculated from
the average Vcmax for 109 studies of C3 plants species)
(Wullschleger 1993). A fixed ratio of koc to kcc of 0.3 was
assumed at 25 ∞C (Bernacchi et al. 2001). Leaf temperature
was kept constant at 25 ∞C throughout the simulations
except for data presented in Fig. 4d.
Figure 1. The specificity (t) versus catalytic
rate per active site (kcc) for Rubisco from
different photosynthetic organisms. Values
are for wild type (closed symbols) and
mutated forms (open symbols) from Jordan
& Ogren (1981, 1984), Jordan & Chollet
(1985), Parry et al. (1989), Read & Tabita
(1994), Bainbridge et al. (1995), Horken &
Tabita (1999) (, C3 plants; , C4 plants; ▲,
green algae; ▼, non-green algae; , prokaryotes; diamond with white spot, mutant). The
least-square, best-fit inverse relationship
(——) between t and kcc for the wild-type
forms of Rubiscos was an exponential:
1
k cc
Ê e5.16 ˆ 0.69 2
=Á
(r = 0.89) . The arrows
˜
Ë t ¯
point to the t (92.5) and kcc (2.5) of a hypothetical ‘average’ C3 crop Rubisco, used for
the subsequent simulations.
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165
158 X.-G. Zhu et al.
Canopy photosynthesis
We used the sunlit–shaded model in WIMOVAC, a model
that integrates the widely used biochemical model of Farquhar et al. (1980) and the biophysical model of canopy
microclimate of Norman (1980), as described in detail previously (Humphries & Long 1995). In summary, the sunlitshaded model treats the leaves of the canopy as two
dynamic populations, namely those sunlit and those
shaded, following Forseth & Norman (1993). This division
of leaves into sunlit and shaded classes was shown to provide a substantial improvement in prediction over models
which simply assumed an exponential decline in light
through homogeneously lit canopy layers (Norman 1980).
The method for calculating canopy photosynthesis was as
described previously by Long (1991) except the leaf and
canopy temperatures were maintained at a constant 25 ∞C
(Eqns 11–23). The simulation was for the 200th day of the
year at 44∞ N, since this would correspond to the middle of
reproductive growth for many temperate C3 annual crops.
The canopy was assumed to have a leaf area index of three
(LAI = 3) and random angles of orientation and inclination, namely a ratio of the horizontal : vertical projected
leaf area of unity, which are good approximations to many
C3 crop canopies (Forseth & Norman 1993). The values of
K∞m, Kcm and Vcmax were as given in the leaf photosynthesis
model. The Rubisco content of the leaves was assumed to
be constant throughout the simulations.
Sensitivity analysis
A sensitivity analysis was conducted by sequentially varying six major parameters affecting canopy photosynthesis:
LAI, the maximal rate of electron transport (Jmax); the ratio
of the horizontal : vertical projected leaf area of canopy (c),
atmospheric transmittance (a), the ratio of [Ci]/[Ca], and
leaf temperature (Tl). The response of K∞m, Kcm, kc, ko to Tl
follows Bernacchi et al. (2001). The response of Jmax to Tl
follows Bernacchi, Pimental & Long (2003).
RESULTS
Leaf photosynthesis
Figure 1 shows the inverse interrelationship between kcc
and t which was most effectively described by Eqn 1
(Appendix I). For each t, the corresponding kcc was calculated based on this inverse relationship. Asat is the lightsaturated rate of CO2 uptake under each [CO2]. At each
[CO2], Asat was calculated for a range of t (from 40 to 160).
The optimal t for any given [CO2] was assumed to be that
at which Asat was a maximum. Figure 2 shows that the optimal t for Asat declines exponentially with [CO2]. The t of
92.5, which is the average t for terrestrial C3 plants, would
be optimal, with respect to Asat, for a [CO2] of about
150 mmol mol-1 and supra-optimal at the current atmospheric [CO2] of 370 mmol mol-1 (Fig. 2). Furthermore, simulations showed that at the current [CO2], a decrease in t
from the current 92.5 (kcc = 2.5) to 65 (kcc = 4.1) would
increase Asat by 12%.
Canopy photosynthesis
Photosynthesis of leaves in real canopies can be either light
saturated or light limited. Light-limited photosynthesis is
RuBP-limited whereas light-saturated photosynthesis is
commonly Rubisco-limited, particularly at lower [CO2].
The effect of varying t at different light levels on single
leaves is shown in Fig. 3. Leaves with a simulated 10%
higher t exhibited higher light-limited photosynthesis and
lower light-saturated photosynthesis compared with those
with unaltered t. Similarly, leaves containing a Rubisco
Figure 2. Assuming a fixed number of
active sites per unit leaf area and the
dependence of kcc on t described in Fig. 1,
the line shows, for any given atmospheric
CO2 concentration, the t that will give the
highest light-saturated rate of leaf photosynthetic CO2 uptake (Asat). The average t
for terrestrial C3 crop plants (92.5) is indicated (t1) together with the interpolated
atmospheric [CO2] at which it would yield
the maximum Asat (C1). Point t2 is the specificity that would yield the highest Asat at
the current [CO2] of the atmosphere (C2).
At C2, decrease in t from current average
(t1) to the optimum for current CO2 concentration (t2) can increase light-saturated
leaf photosynthetic carbon uptake by
12%.
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165
Transformation with foreign Rubisco for productivity 159
Figure 3. The relative effects of a 10% increase and of a 10%
decrease in t on net photosynthetic CO2 uptake as a function of
photon flux. Calculations assume the dependence of kcc on t
described in Fig. 1. The control was the average t (92.5) and average kcc (2.5 CO2 per active site per second) for terrestrial C3 vascular plants. ——, control; ········, 10% higher t; –- –, 10% lower t.
with 10% lower t (and therefore higher kcc according to
Fig. 1) exhibited higher light-saturated photosynthesis and
lower light-limited photosynthesis compared with the control (Fig. 3).
The opposing effects of increasing t on light-saturated
and light-limited photosynthesis and the fact that variation
in [CO2] does not affect these two kinds of photosynthesis
equally resulted in a different optimal t for canopy photosynthesis at each [CO2]. Assuming Jmax as 250 mmol m-2 s-1,
simulations showed that for a canopy of LAI = 3 at 44∞ N
latitude on the 200th day of the year, the current t is higher
than that which would be optimal for achieving maximal
canopy photosynthesis at the current atmospheric [CO2]
(Fig. 4a). The optimal t (t2, 78; in Fig. 4a) for maximizing
the daily integral of canopy CO2 uptake (Ac¢) for current
[CO2] (Point C2 in Fig. 4a), fell significantly below the current average of 92.5 (t1). The t of 92.5 would be optimal for
an atmospheric [CO2] of about 220 mmol mol-1 (Fig. 4a).
The LAI of canopies constantly change during growth and
development of crop canopies. Since LAI influences the
light conditions inside the canopy (Eqns 12, 13 and 19 in
Appendix I), canopies with different LAI will have different optimal t under each [CO2]. To test the validity of the
conclusions obtained using an LAI of 3, we used an LAI of
2 to represent a canopy at the early stage of development
and an LAI of 6 to represent a closed canopy. Simulations
showed that alteration of LAI does not change the trend,
namely optimal t decreases with [CO2] (Fig. 4a).
Similar to LAI, the ratio of the horizontal : vertical projected leaf area (c) also influences light conditions inside
the canopy (Eqn 14 in Appendix I). Sensitivity analysis was
conducted using c-values of 2, 1 and 0.5. Canopies with c
of 2 have low inclination angles (the angle between leaf axis
and horizontal plane); canopies with c of 0.5 have high
inclination angles; whereas canopies with c of 1 represent
canopies with leaves with an equal distribution between all
angles of inclination. Figure 4b shows that c has little effect
on optimal t and its decrease with [CO2].
Atmospheric transmittance (a) influences the photon
flux density above the canopy and the ratio of diffuse to
direct light, and therefore affects the light conditions at the
canopy surface and below (Eqn 17). Decreasing a
decreases light incident on all leaves. The a of 0.5 corresponds to a cloudy day; while a of 0.95 corresponds to a
clear sky. Nevertheless, the general trend in optimal t with
[CO2] is unchanged (Fig. 4c).
Temperature influences RuBP-limited photosynthesis
and Rubisco-limited photosynthesis differently (Bernacchi
et al. 2001, 2003). Therefore, the optimal t for a given [CO2]
differs with temperature. Crop canopies in temperate zones
often experience temperatures between 20 to 35 ∞C. Over
this range, the pattern of decline in optimal t with [CO2]
shows little variation (Fig. 4d).
For a given ambient [CO2], variation in the ratio of [Ci]/
[Ca] will alter [Ci] and therefore affects proportions of
Rubisco-limited photosynthesis and RuBP-limited photosynthesis. Therefore, optimal t under a given [CO2] differs
with the ratio of [Ci]/[Ca]. The ratio of [Ci]/[Ca] varies within
20% of its ‘typical’ value. Within this range, the pattern of
the decline in optimal t with [CO2] shows little variation
(Fig. 4e).
Leaves and species can differ with respect to their capacity for RuBP regeneration. Jmax of 250 and 180 mmol m-2
s-1 both show that optimal t decreases with [CO2] (Fig. 4f).
If Jmax was assumed as 150 mmol m-2 s-1, the optimal t first
decreases, but then increases in contrast to the trend of
continuous decrease of optimal t under higher Jmax with
increase in [CO2] (Fig. 4f). Therefore, the general trend
revealed throughout Fig. 4a–e was altered for low Jmax.
Compared with the optimal t of 78 for current [CO2] with
a Jmax of 250 mmol m-2 s-1, the optimal t for current [CO2]
with Jmax = 180 mmol m-2 s-1 is slightly higher at 83 (kcc = 2.9)
(Fig. 4f and Table 1). The gain in canopy photosynthesis
that would be achieved by using a Rubisco with an optimal
t for this Jmax in the canopy is also smaller (Table 1, Fig. 4f).
This difference is explained by the fact that at the higher
Jmax (Jmax = 250 mmol m-2 s-1), 61% of Ac¢ is attributed to
Rubisco-limited photosynthesis, but only 52% for the lower
Jmax (Table 1).
Figure 1 indicates that for a few species, there is considerable deviation from the strict inverse relationship
between t and kcc assumed in the simulations presented
above. Other potentially important observations with
respect to the potential for increasing crop photosynthesis
are that (a) among higher plants, kcc and kcm vary far more
than t, especially in comparisons of C3 and C4 species
(Yeoh, Badger & Watson 1981; Seeman, Badger & Berry
1984) and (b) non-green algal Rubiscos possess very high t
that may not be compromised by a very low kcc (Whitney
et al. 2001). Table 2 shows the predicted effects of hypothetically substituting the ‘average’ C3 terrestrial Rubisco with
Rubiscos from other species. Compared with the average
C3 crop Rubisco used in the previous simulations, the
tobacco parameters result in an 11.4% increase in Ac¢. Thus,
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165
160 X.-G. Zhu et al.
Figure 4. As for Fig. 2, but showing the t
that results in the greatest daily total canopy carbon gain (Ac¢) at any given [CO2].
The prediction uses sun–earth geometry to
determine solar angles and incident photon
flux, to predict light at different points
within the canopy. The control is for the
200th day of the year, and assumes a day
with clear sky, a constant leaf temperature
of 25 ∞C, a maximum rate of electron transport supporting ribulose-1:5 bisphosphate
regeneration (Jmax) of 250 mmol m-2 s-1, a
leaf area index 3 and a random inclination
of foliage. As in Fig. 2, the atmospheric CO2
concentration (C1) at which the average
specificity of C3 crops (t1) would be optimal
for maximizing Ac¢ and the specificity (t2)
that would be optimal for maximizing Ac¢ in
the current atmosphere (C2) are also indicated in (a). For each individual simulation,
control parameters were used except for
the tested parameters that were varied. The
six parameters varied in the simulations are:
(a), LAI; (b), the ratio of
horizontal : vertical projected leaf area (c);
(c), atmospheric transmittance (a); (d),
canopy temperature; (e), the ratio of [Ci]/
[Ca]; (f ), Jmax. In (c), the values of t at different temperatures were all converted to t
at 25 ∞C for easy comparison. The percentage increase of canopy carbon uptake by
decreasing t from t1 to t2 at C2 is 3.1% in
(a).
Table 1. Specificity (t) found to be optimal for a canopy of LAI = 3 and random inclination of foliage elements on a clear summer day at
latitude 44∞N assuming two different capacities for whole chain electron transport (Jmax)
Jmax (mmol m-2 s-1)
180
Predicted optimal t for current atmospheric [CO2]
Predicted percent increase in Ac¢ that would be achieved by substituting the optimal
t under current [CO2]
Predicted increase in Ac¢ with the optimal t under current [CO2]
The percentage of Ac¢ that is Rubisco-limited for the corresponding optimal Rubisco
The predicted optimal atmospheric [CO2] (mmol mol-1) for current t
83
1.7%
15 mmol m-2 d-1
52%
200
250
78
3.1%
30 mmol m-2 d-1
61%
220
The increases in the daily integral of canopy photosynthesis that would result from substituting such an optimal Rubisco into the canopy,
relative to the current average form (t = 92.5). The proportion of the daily integral of canopy carbon gain (Ac¢) that would be obtained
under Rubisco-limited conditions is also given. The atmospheric [CO2] at which the current Rubisco would yield the highest Ac¢ for the two
values of Jmax is also calculated.
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165
Transformation with foreign Rubisco for productivity 161
Table 2. Estimates of the daily canopy carbon gain (Ac¢), as in Table 1 and Fig. 4, assuming the hypothetical replacement of the ‘average’
form of Rubisco from C3 crop species with Rubiscos from other species.
Species
A c¢
(mmol m-2 d-1)
A c¢
(%)
Asat
(mmol m-2 s-1)
Percentage*
Current ‘average’ C3 crop (kcc = 2.5, t = 92.5)
Tobacco (kcc = 3.4, t = 82)
Zea mays (kcc = 5.6, t = 78)
Amaranthus edulis (kcc = 7.3, t = 82)
Griffithsia monilis (kcc = 2.6, t = 167)
Phaeodactylum tricornutum (kcc = 3.4, t = 113)
1040
1170
1180
1250
1430
965
100
111.4
111.9
117.1
127
92.3
14.9
19.1
19.8
28.3
21.5
12.5
59.2
54.8
41.9
0
61.6
60.9
Reported values for kcc and Kcm of these species (Jordan & Ogren 1984; Seeman et al. 1984; Whitney et al. 2001) were used to calculate K∞m
using Eqn 2a (Appendix I) and assuming a fixed ratio of koc to kcc of 0.3 at 25 ∞C (Bernacchi et al. 2001). The Farquhar et al. (1980) model
uses intercellular [CO2] in vivo. Since Kcm was reported as the [CO2] around Rubisco in solution, mesophyll conductance was used to adjust
this reported value to Kcm expressed as intercellular CO2 concentration. Percentage* represents the percentage of Ac¢ attributed to Rubiscolimited photosynthesis.
the lower than average t (82) is more than compensated for
by the higher kcc of the tobacco (3.4) Rubisco. Of the two
C4 Rubiscos examined, Rubisco from Amaranthus edulis
substantially increased Ac¢ compared to crop C3 Rubisco
forms due to its high kcc, with maintenance of a sufficiently
low Kcm. Rubisco from a non-green algae, Griffithsia monilis, which has a higher t but maintains a similar kcc and kcm
to tobacco (Whitney et al. 2001), would increase Ac¢ by 27%
(Table 2).
DISCUSSION
Within the strict assumptions made, decreasing t to the
apparent optimum would increase leaf photosynthesis by
12% and canopy photosynthesis by considerably less. The
increase in canopy photosynthesis is less than in leaf photosynthesis because a significant portion of canopy photosynthesis is RuBP-limited and therefore unaffected by an
increase in kcc. Importantly, increasing specificity would
result in less, not more, net photosynthesis at both the leaf
and canopy level at current [CO2]. These conclusions rest
on several critical assumptions; namely that t and kcc are
inversely related (Eqn 1, Fig. 1) and that the parameters
chosen to describe leaf, canopy and environmental conditions are representative. Several surveys have shown a clear
trend in which t appears inversely related to kcc (Bainbridge
et al. 1995; Spreitzer & Salvucci 2002) (Fig. 1) as assumed
here. As noted earlier, mutagenesis has so far either
decreased t and kcc at the same time, or increased t at the
expense of kcc, and vice versa (Fig. 1). The repeated evolution of C4 photosynthesis from C3 despite the complexities
of Kranz structure and partitioning of enzymes and transporters to avoid photorespiration suggest that land plants
may have met an evolutionary barrier, that is, the ‘best’
achievable structure of higher plant Rubisco for high CO2
affinity with an adequate kcc might have already been
obtained (Long 1998). The rationale is that if it were possible to significantly increase t without decreasing kcc, evolution would have already found a route, rather than
evolving the complex Kranz structure and using the C4
pathway for concentrating CO2. If we assume that the relationship described by Fig. 1 is generally invariable,
improved crop photosynthesis could still be achieved by
determining the optimal t for the given canopy structure,
and replacing the current Rubisco with a form that lies
close to the predicted optimum for current atmospheric
[CO2].
The simulations assume 26 mmol Rubisco active sites per
square metre leaf area. This value was held constant in all
the simulations. Rubisco content varies between species,
leaves within a canopy, and even mature leaves transferred
from low to high grown irradiances (Sims & Pearcy 1992;
Frak et al. 2001; Oguchi, Hikosaka & Hirose 2003). However, less Rubisco in shaded leaves may reflect the fact that
photosynthesis will be RuBP-limited in low light. Theoretically, decreasing kcc with increasing t could be compensated for by simply increasing the amount of Rubisco. A
survey across 109 studies of C3 species gave a mean Vcmax
of 64 mmol m-2 s-1 (Wullschleger 1993), which would give,
assuming a mean kcc of 2.5, a Rubisco concentration of
26 mmol m-2 as used here. As noted earlier Rubisco typically accounts for about 50% of leaf soluble protein, and
calculations of chloroplast and photosynthetic cell volumes
suggest there may be little or no physical capacity to add
more (Pyke & Leech 1987). At such a high concentration,
Rubisco is already expensive in terms of the amount of
nitrogen and energy it represents. Increasing Rubisco to
compensate for a lower kcc may result in a proportional
decrease in efficiency of nitrogen use, an undesirable trait
in crop plants.
The lower the Jmax/Vcmax, the lower the proportion of
Rubisco-limited photosynthesis to RuBP-limited photosynthesis. The lower the Jmax/Vcmax in a leaf or canopy, the less
important kcc is, since a smaller proportion of plant carbon
is fixed by Rubisco-limited photosynthesis. Jmax at
250 mmol m-2 s-1 used in these simulations is high, but in
line with values reported for C3 crops grown under high
nitrogen and high light (Farage, McKee & Long 1998;
Bunce 2000). Decreasing Jmax by 28% (from 250 to
180 mmol m-2 s-1) roughly halved the advantage of selecting
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165
162 X.-G. Zhu et al.
a Rubisco optimized for canopy carbon gain (Table 1). The
benefit of replacing current Rubisco with a form with a
higher kcc and lower t will in part depend on the amount of
Rubisco relative to the capacity for regeneration of RuBP.
Although a close positive correlation between these two
capacities is apparent in surveys of C3 species (Wullschleger
1993), over two-fold variation in the ratio of these two
capacities is found within crops (Bunce 2000) and linkage
can also be varied by altering rbcS expression (Harrison
et al. 2001).
It is remarkable that even with the large changes in LAI,
c, a, temperature and the ratio of [Ci]/[Ca], the trend that
optimal t for maximum canopy carbon uptake decreases
with [CO2] does not change (Fig. 4a–e). This pattern only
broke down when Jmax was lowered by 40% to 150 mmol
m-2 s-1. Here the optimal t for canopy carbon uptake first
decreases then increases with [CO2] (Fig. 4f). This is
because a higher proportion of canopy photosynthesis is
RuBP-limited photosynthesis under lower Jmax, and the
gain due to high t outweighs the loss due to lower kcc.
However, as noted earlier for well fertilized C3 crops a Jmax
of 150 mmol m-2 s-1 would be low.
If Rubisco was modified to the t that would maximize
Ac¢, then the increase in Ac¢ is greater in canopies with
properties favouring Rubisco-limited photosynthesis (e.g.
higher Jmax, lower Vcmax, lower LAI, lower c, higher atmospheric transmittance, and lower ratio of [Ci]/[Ca]) compared with those favouring RuBP-limited photosynthesis.
Modifying t to its optimum would be particularly beneficial
to plants in semi-arid regions. These plants generally show
low stomata conductance (Gomes, Meilke & de Almeida
2002) and low LAI. Low stomata conductance leads to a
low [Ci] and low LAI leads to a high-light microclimate in
plant canopies. As a consequence of these two factors, a
high proportion of canopy photosynthesis would be
Rubisco-limited photosynthesis in semi-arid regions.
Improved ability to assimilate CO2 at low [Ci] would allow
increased efficiency of water use, which can be a competitive
advantage in these regions. Optimizing t and kcc can
increase Ac¢ by up to 12% when all leaves are light-saturated
and Rubisco-limited, for example, during early crop growth
prior to canopy closure, lowering to 3% at canopy closure.
Accepting the inverse relationship between kcc and t and
assuming the parameters describing canopy and environment used in this study are representative, our simulations
revealed that a decrease, not an increase in t will increase
Ac¢ at current [CO2]. However naturally occurring forms of
Rubisco might not strictly follow this assumed inverse relationship. For example, Rubiscos from some non-green algae
have been reported with a significantly greater t than that
of C3 crops, yet with a similar or higher kcc (Whitney et al.
2001) and C4 forms of Rubisco generally appear to have
adapted to the high CO2 environment by increasing kcc at
the expense of Kcm with little change in t (Sage 2002). If it
were possible to substitute some of these forms into C3
crops, much larger increases in canopy photosynthesis could
be achieved for the same quantity of Rubisco (Table 2).
So far transferring sequences coding for the non-green
algal forms of Rubisco to the higher-plant plastome has not
succeeded in providing a functional Rubisco (Whitney et al.
2001) because of problems in either folding or assembly of
these quite evolutionarily diverse forms. However, successful folding and assembly of Rubisco transferred from other
higher plants has been achieved (Kanevski et al. 1999;
Whitney & Andrews 2001). Our results suggest that replacing the average Rubisco of C3 crops or the Rubisco found
in tobacco with the Rubisco of C4 plants, such as Amaranthus edulis, could result in a substantial increase in canopy
photosynthesis for the same amount of Rubisco. If forms
from non-green algae are successfully expressed in crop
leaves, even larger increases become possible. Whitney
et al. (2001) has already shown through modelling that the
Rubisco from Griffithsia could increase leaf photosynthesis
by 4%. Here we extend this by showing that it would
increase Ac¢ by 22 and 27% compared to tobacco Rubisco
and the hypothetical average C3 Rubisco, respectively
(Table 2). The potential increase in canopy photosynthesis
for other C3 plant replacements will depend on the relative
kinetic parameters of the Rubiscos being considered, but
can now be readily ascertained by the modelling procedures we have followed.
Our simulation suggests that Rubisco of current C3 plants
would be optimal for an atmospheric [CO2] of about
200 mmol mol-1 (Table 1, Fig. 4a–f). This falls in the range
of atmospheric [CO2] over the last 450 thousand years,
which fluctuated between 180 and 290 mmol mol-1 as
detected from the Vostok ice core (Barnola et al. 1999).
Rubisco appeared early in the history of life more than
three billion years ago when [CO2] was orders of magnitude
higher than current [CO2] and when [O2] was low. In that
environment, RuBP oxygenation would have been a rare
event (Sage 1999) and not a selective factor in the evolution
of Rubisco. The increase in photorespiratory potential triggered by the increase in the atmospheric [O2] : [CO2] over
the past 50 million years created high evolutionary pressure
for dealing with these disadvantages (Ehleringer et al.
1991). The current Rubisco found in C3 crops might be a
result of evolutionary optimization to the low [CO2] and
high [O2] over the past 450 thousand years after millions of
years of evolution or selection. From the simulations made
here, the current Rubisco is not operating at its optimal t
and kcc, which is possibly due to the unprecedented rapid
increase in [CO2] since the Industrial Revolution, which
may have far exceeded the speed of Rubisco evolution.
In conclusion, this study examined the rationale of current efforts to increase photosynthetic rate by increasing t
using genetic modification of existing land-plant Rubisco
and/or replacing them with other naturally occurring
Rubisco with significantly different kinetic parameters. If
increasing t can only be achieved at the expense of a
decrease in kcc, a decrease, not an increase in t will increase
both leaf photosynthesis and daily canopy carbon gain for
the current atmospheric [CO2] if the leaf or canopy has a
sufficiently high Jmax. In shade environments or leaves with
a low Jmax/Vcmax, increasing t can increase the total canopy
carbon gain for current [CO2]. The optimal t reflects a
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165
Transformation with foreign Rubisco for productivity 163
balance between the Rubisco-limited photosynthesis and
RuBP-limited photosynthesis. These simulations show that
even if engineering a Rubisco with improved t without loss
of kcc is elusive, crop carbon gain could be increased substantially by substituting the existing average C3 crop
Rubisco with those from other photosynthetic organisms.
Specifically, substituting the average existing C3 crop
Rubisco with the Griffithsia monilis Rubisco (a non-green
algae) could increase carbon gain by more than 25% without any increase in the amount of Rubisco per unit leaf
area. Substituting with the Rubisco from Amaranthus edulis could increase carbon gain by 17%. It is hard to conceive
of other genetic transformations that could result in a
greater increase in potential C3 crop yields.
REFERENCES
Bainbridge G., Madgwick P.J., Parmar S., Mitchell R., Paul M.,
Pitts J., Keys A.J. & Parry M.A.J. (1995) Engineering Rubisco
to change its catalytic properties. Journal of Experimental Botany 46, 1269–1276.
Barnola J.M., Raynaud D., Lorius C. & Barkov N.I. (1999) Historical CO2 record from the Vostok ice core. In Trends: a Compendium of Data on Global Change. Carbon Dioxide
Information Analysis Center, Oak Ridge National Laboratory.
Department of Energy, Oak Ridge, TN, USA.
Bernacchi C.J., Pimentel C. & Long S.P. (2003) In vivo temperature response functions of parameters required to model RuBPlimited photosynthesis. Plant, Cell and Environment 26, 1419–
1430.
Bernacchi C.J., Singsaas E.L., Pimentel C., Portis J.A.R. & Long
S.P. (2001) Improved temperature response functions for models of Rubisco-limited photosynthesis. Plant, Cell and Environment 24, 253–259.
Bunce J.A. (2000) Contrasting effects of carbon dioxide and irradiance on the acclimation of photosynthesis in developing soybean leaves. Photosynthetica 38, 83–89.
von Caemmerer S. (2000) Modelling C3 photosynthesis. In Biochemical Models of Leaf Photosynthesis. Techniques in Plant
Sciences, 2, pp. 165. CSIRO Publishing, Collingwood, Victoria,
Australia.
von Caemmerer S. & Farquhar G.D. (1981) Some relationships
between the biochemistry of photosynthesis and the gas
exchange of leaves. Planta 153, 376–387.
Chene P., Day A.G. & Fersht A.R. (1992) Mutation of asparagine
111 of Rubisco from Rhodospirillum rubrum alters the carboxylase/oxygenase specificity. Journal of Molecular Biology 225,
891–896.
Douce R. & Heldt H.W. (2000) Photorespiration. In Photosynthesis: Physiology and Metabolism (eds R.C. Leegood, T.D. Sharkey & S. von Caemmerer), pp. 115–136. Kluwer Academic
Publishers, Dordrecht, The Netherlands.
Ehleringer J.R., Sage R.F., Flanagan L.B. & Pearcy R.W. (1991)
Climate change and the evolution of C4 photosynthesis. Trends
in Ecology and Evolution 6, 95–99.
Evans L.T. (1993) Crop Evolution, Adaptation and Yield. Cambridge University Press, Cambridge, UK.
Evans J.H. & Farquhar G.D. (1991) Modelling canopy photosynthesis from the biochemistry of C3 chloroplast. In Modelling
Crop Photosynthesis-from Biochemistry to Canopy (eds K.J.
Boote & R.S. Loomis), pp. 1–15. Crop Science Society of America, Madison, WI, USA.
Farage P.K., McKee I.F. & Long S.P. (1998) Does a low nitrogen
supply necessarily lead to acclimation of photosynthesis to elevated CO2? Plant Physiology 118, 573–580.
Farquhar G.D., von Caemmerer S. & Berry J.A. (1980) A biochemical model of photosynthetic CO2 assimilation in leaves of
C3 species. Planta 149, 78–90.
Forseth I.N. & Norman J.M. (1993) Modelling of solar irradiance,
leaf energy budget, and canopy photosynthesis. In Techniques
in Photosynthesis and Productivity Research for a Changing
Environment (eds D.O. Hall, J.M.O. Scurlock, R.C. BolharNordenkampf, R.C. Leegood & S.P. Long). Chapman & Hall,
London, UK.
Frak E., Le Roux X., Millard P., Dreyer E., Jaouen G., Saint-Joanis
B. & Wendler R. (2001) Changes in total leaf nitrogen and
partitioning, of leaf nitrogen drive photosynthetic acclimation to
light in fully developed walnut leaves. Plant, Cell and Environment 24, 1279–1288.
Gomes F.P., Mielke M.S. & de Almeida A.A.F. (2002) Leaf gas
exchange of green dwarf coconut (Cocos nucifera L. var. nana)
in two contrasting environments of the Brazilian north-east
region. Journal of Horticultural Science and Biotechnology 77,
766–772.
Harley P.C. (1992) Modeling photosynthesis of cotton in elevated
CO2. Plant, Cell and Environment 15, 271–282.
Harrison E.P., Olcer H., Lloyd J.C., Long S.P. & Raines C.A.
(2001) Small decreases in SBPase cause a linear decline in the
apparent RuBP regeneration rate, but do not affect Rubisco
carboxylation capacity. Journal of Experimental Botany 52,
1779–1784.
Horken K.M. & Tabita F.R. (1999) Closely related form I ribulose
bisphosphate carboxylase/oxygenase molecules that possess different CO2/O2 substrate specificities. Archives of Biochemistry
and Biophysics 361, 183–194.
Humphries S.W. & Long S.P. (1995) WIMOVAC: a software package for modelling the dynamics of plant leaf and canopy photosynthesis. Computer Application in the Biosciences 11, 361–371.
Jordan D.B. & Chollet R. (1985) Subunit dissociation and reconstituation of ribulose-1,5-bisphosphate carboxylase from Chromatium vinosum. Archives of Biochemistry and Biophysics 236,
487–496.
Jordan D.B. & Ogren W.L. (1981) Species variation in the specificity of ribulose biphosphate carboxylase/oxygenase. Nature
291, 513–515.
Jordan D.B. & Ogren W.L. (1984) The carbon dioxide/oxygen
specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase.
Planta 161, 308–313.
Kanevski I., Maliga P., Rhoades D.F. & Gutteridge S. (1999) Plastome engineering of ribulose-1,5-bisphosphate carboxylase/oxygenase in tobacco to form a sunflower large subunit and tobacco
small subunit hybrid. Plant Physiology 119, 133–141.
Kimball B.A. (1983) Carbon dioxide and agricultural yield – an
assemblage and analysis of 430 prior observations. Agronnmy
Journal 75, 779–788.
Long S.P. (1985) Leaf gas exchange. In Photosynthetic Mechanistic
Mechanisms and the Environment (eds J. Barber & N.R. Baker),
pp. 453–500. Elsevier, London, UK.
Long S.P. (1991) Modification of the response of photosynthetic
productivity to rising temperature by atmospheric CO2 concentrations: has its importance been underestimated? Plant, Cell
and Environment 14, 729–739.
Long S.P. (1998) Rubisco, the key to improved crop production
for a world population of more than eight billion people? In
Feeding the World Population- Rank Prize Symposium I (eds
J.C. Waterlow & R. Riley), pp. 124–136. Oxford University
Press, Oxford, UK.
Madgwick P.J., Parmar S. & Parry M.A.J. (1998) Effect of mutations of residue 340 in the large subunit polypeptide of rubisco
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165
164 X.-G. Zhu et al.
from Anacystic nidulans. European Journal of Biochemistry 253,
476–479.
Norman J.M. (1980) Interfacing leaf and canopy light interception
models. In Predicting Photosynthesis for Ecosystem Models (eds
J.D. Hesketh & J.W. Jones) Vol. 2, pp. 49–67. CRC Press, Boca
Raton, FL, USA.
Ogren W.L. (1984) Photorespiration, pathway, regulation & modification. Annual Review of Plant Physiology 35, 415–442.
Oguchi R., Hikosaka K. & Hirose T. (2003) Does the photosynthetic light-acclimation need change in leaf anatomy? Plant, Cell
and Environment 26, 505–512.
Parry M.A.J., Andralojc P.J., Mitchell R.A.C., Madgwick P.J. &
Keys A.J. (2003) Manipulation of Rubisco: the amount, activity,
function and regulation. Journal of Experimental Botany 54,
1321–1333.
Parry M.A.J., Keys A.J. & Gutteridge S. (1989) Variation in the
specificity factor of C3 higher plant Rubiscos determined by the
total consumption of RuBP. Journal of Experimental Botany 40,
317–320.
Pyke K.A. & Leech R.M. (1987) The control of chloroplast number in wheat mesophyll cells. Planta 170, 416–420.
Ramage R.T., Read B.A. & Tabita F.R. (1998) Alteration of the
alpha helix region of cyanobacterial ribulose 1,5-bisphosphate
carboxylase/oxygenase to reflect sequences found in high substrate specificity enzymes. Archives of Biochemistry and Biophysics 349, 81–88.
Read B.A. & Tabita F.R. (1994) High substrate specificity factor
ribulose bisphosphate carboxylase/oxygenase from eukaryotic
marine algae and properties of recombinant cyanobacterial
rubisco containing ‘algal’ residue modifications. Archives of Biochemistry and Biophysics 312, 210–218.
Romanova A.K., Cheng Z.Q. & McFadden B.A. (1997) Activity
and carboxylation specificity factor of mutant ribulose 1,5-bisphosphate carboxylase/oxygenase from Anacystis nidulans. Biochemistry and Molecular Biology International 42, 299–307.
Sage R.F. (1999) Why C4 photosynthesis? In C4 Plant Biology (eds
R.F. Sage & R.K. Monson), pp. 3–14. Academic Press, San
Diego, CA, USA.
Sage R.F. (2002) Variation in the k (cat) of Rubisco in C-3 and C4 plants and some implications for photosynthetic performance
at high and low temperature. Journal of Experimental Botany
53, 609–620.
Seeman J.R., Badger M.R. & Berry J.A. (1984) Variations in the
specificity activity of ribulose-1,5-bisphosphate carboxylase
between species utilizing differing photosynthetic pathways.
Plant Physiology 74, 791–794.
Sims D.A. & Pearcy R.W. (1992) Response of leaf anatomy and
photosynthetic capacity in Alocasia-macrorrhiza (Araceae) to a
transfer from low to high light. American Journal of Botany 79,
449–455.
Somerville C.R. & Ogren W.L. (1982) Mutants of the cruciferous
plant Arabidopsis thaliana lacking glycine decarboxylase activity. Biochemistry Journal 202, 373–380.
Somerville S.C. & Somerville C.R. (1983) Effects of oxygen and
carbon dioxyde on photorespiratory flux determined from glyucine accumulation in a mutant of Arabidopsis thaliana. Journal
of Experimental Botany 34, 415–424.
Spreitzer R.J. (1999) Questions about the complexity of chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase. Photosynthesis Research 60, 29–42.
Spreitzer R.J. & Salvucci M.E. (2002) RUBISCO: structure, regulatory interactions, and possibilities for a better enzyme. Annual
Review of Plant Biology 53, 449–475.
Tabita F.R. (1999) Microbial ribulose 1,5-bisphosphate carboxylase/oxygenase: a different perspective. Photosynthesis Research
60, 1–28.
Whitney S.M. & Andrews T.J. (2001) The gene for the ribulose1,5-bisphosphate carboxylase/oxygenase (Rubisco) small
subunit relocated to the plastid genome of tobacco directs the
synthesis of small subunits that assemble into Rubisco. Plant
Cell 13, 193–205.
Whitney S.M., Baldett P., Hudson G.S. & Andrews T.J. (2001)
Form I Rubiscos from non-green algae are expressed abundantly but not assembled in tobacco chloroplasts. Plant Journal
26, 535–547.
Wullschleger S.D. (1993) Biochemical limitations to carbon assimilation in C(3) plants – a retrospective analysis of the A/Ci curves
from 109 species. Journal of Experimental Botany 44, 907–920.
Yeoh H.-H., Badger M.R. & Watson L. (1981) Variations in kinetic
properties of ribulose-1,5-bisphosphate carboxylases among
plants. Plant Physiology 67, 1151–1155.
APPENDIX I
Equations used to simulate leaf and canopy net photosynthetic carbon uptake
1
k
c
c
t=
Ê e5.16 ˆ 0.69 2
=Á
(r = 0.89)
˜
Ë t ¯
(1)
k cc K o m
k oc K c m
(2a)
Vcmax = kcc M
(2b)
( K
(2c)
o
m
)=
1 + m / 100 - 1
1 - 1 + m / 100
, where m/100 represents the per1 + m / 100
(2d)
centage change in t
0.5Oi
(3)
* =
t
*ˆ
(4)
A = Ê1 min(Wc , Wj ) - Rd
Ë
Ci ¯
( K c m ) =
Wc =
VcmaxCi
Ci + K c m (1 + Oi / K o m )
(5)
Wj =
JCi
4.5Ci + 10.5 *
(6)
Ci = 0.7 Ca
(7)
Oi = Oa
(8)
2
0.5
J = {I2 + Jmax - [(I2 + Jmax) - 4QI2Jmax] }/2Q
(9)
I2 = I0(1 - s)(1 - r)/2
(10)
Ac = ƒ[Isun, Tl, Ci, Oi].Fsun + ƒ[Ishade, Tl, Ci, Oi].Fshade
(11)
where ƒ indicates A as a function of these variables as
described in Eqns 1–9.
Fsun = [1 - e( - kF/cosq)]cosq/k
(12)
Fshade = F - Fsun
(13)
k=
( x2 + tan 2 q )
0.5
cosq
x + 1.744( x + 1.882)-0.733
cos q = sinW sin d + cosW cos d cos(15[t - tsn])
(14)
(15)
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165
Transformation with foreign Rubisco for productivity 165
d = - 23.5 cos[360(Dj + 10)/365]
I dir = I s
◊a [( P Po ) cosq ]
I diff = 0.5I s (1 - a [( P Po )
(16)
(17)
cosq ]
) cosq
0.7
I shade = I diff ◊e( -0.5 F ) + I scat
Iscat = 0.07Idir(1.1 - 0.1F)e-cosq
(18)
(19)
(20)
Isun = Idir cos l/cos q + Ishade
-1
l = cos k
(21)
(22)
t =24 h
A¢c =
Ú
Ac◊dt
(23)
0
Received 17 September 2003; received in revised form 29 September
2003; accepted for publication 7 October 2003
APPENDIX II
Definition of symbols. Values in parenthesis are those used in simulations, unless stated otherwise
Term
Units
Definition
A
Ac
A c¢
Asat
Ca
Ci
Dj
F
Fshade
Fsun
gs
I
Io
Idiff
Idir
Is
Iscat
Ishade
Isun
I2
J
Jmax
k
k cc
k oc
K cm
K∞m
M
Oa
Oi
P
Po
r
Rd
s
t
tsn
Tl
Vcmax
Wc
Wj
x
G*
t
a
Q
mmol m-2 s-1
mmol m-2 s-1
mmol m-2 d-1
mmol m-2 s-1
mmol mol-1
mmol mol-1
day of year
m2 m-2
m2 m-2
m2 m-2
mmol mol-1
mmol m-2 s-1
mmol m-2 s-1
mmol m-2 s-1
mmol m-2 s-1
mol m-2 s-1
mmol m-2 s-1
mmol m-2 s-1
mmol m-2 s-1
mmol m-2 s-1
mmol m-2 s-1
mmol m-2 s-1
dimensionless
s-1
s-1
mmol mol-1
mmol mol-1
mol m-2
mmol mol-1
mmol mol-1
kPa
kPa
dimensionless
mmol m-2 s-1
dimensionless
hour
hour
∞C
mmol m-2 s-1
mmol m-2 s-1
mmol m-2 s-1
dimensionless
mmol mol-1
dimensionless
dimensionless
dimensionless
d
W
q
l
degree
∞
∞
∞
Photosynthetic CO2 uptake rate
Canopy carbon uptake per metre square ground area per second
Ac integrated over the course of one day
Light saturated rate of leaf photosynthetic rate under certain [CO2]
Atmospheric CO2 concentration
Intercellular CO2 concentration
The ith day in a year (200)
Total leaf area index, i.e. the ratio of leaf area per unit ground area (3)
F that is shaded at any point in time
F that is sunlit at any point in time
Stomatal conductance
Photon flux density
Incident photon flux density
Photon flux density of diffuse radiation
Photon flux density of direct radiation
Solar constant, i.e. the photon flux density in a plane perpendicular to the solar beam above the atmosphere (2600)
Photon flux density of scattered radiation within the canopy
Mean I for shaded leaves within a canopy
Mean I for sunlit leaves within a canopy
Photon flux density absorbed by PSII
Potential rate of whole chain electron transport through PSII for a given I2
Light saturated J (250 or 180)
Foliar absorption coefficient
Maximum rate of carboxylation per active site of Rubisco (2.5 for the control)
Maximum rate of oxygenation per active site of Rubisco.
Rubisco Michaelis–Menten constant for CO2 (460 for the control)
Rubisco Michaelis–Menten constant for O2 (330 for the control)
The concentration of Rubisco active sites on a leaf area basis (26)
Atmospheric O2 concentration (210)
Intercellular O2 concentration (210)
Atmospheric pressure
Standard atmospheric pressure at sea level (101.324)
Percentage of light that is reflected and transmitted (23%)
Dark respiration rate (0)
Spectral imbalance (0.25), indicating the percentage of light energy that can not be used in photochemistry
Time of day
Time of solar noon (12)
Leaf temperature (25)
Maximum rate of carboxylation at RuBP and CO2 saturation
Rubisco-limited rate of carboxylation
RubP-limited rate of carboxylation
The ratio of horizontal : vertical projected area of a canopy (1)
CO2 compensation point in the absence of dark respiration
The specificity of Rubisco for CO2 relative to O2 (92.5 for the control)
Atmospheric transmittance (0.85)
Convexity factor for the nonrectangular hyperbolic response of electron transport through photosystem II to
photon flux (0.7)
Solar declination
Latitude (44∞N)
Solar zenith angle
Angle between leaf surface and the direct beam solar radiation
© 2004 Blackwell Publishing Ltd, Plant, Cell and Environment, 27, 155–165