Modifying photorespiration to optimize crop performance

Abstract

The initial reaction of CO2 fixation has a shared specificity with oxygen, resulting in the formation of 2-phosphoglycolate, an inhibitory sugar. 2-phosphoglycolate is detoxified by photorespiration, but this recycling requires energy and releases carbon, substantially reducing rates of net CO2 fixation in many crop plants. Many strategies are in development to modify native photorespiration and more efficiently detoxify intermediates of photorespiration to boost subsequent net carbon assimilation and crop productivity. Current approaches include increasing the activities of enzymes present in native photorespiration to minimize the pool sizes of inhibitory intermediates or replacing native photorespiration with novel pathways that process 2-phosphoglycolate with improved carbon-conserving, or even fixing, series of reactions. There are also developments in improving photorespiration under elevated temperatures, which occur under typical growing conditions but are outside the scope of many lab-based studies. We finally discuss the potential for improving photorespiration under non-steady-state conditions, which are a hallmark of field crop production systems.

Full text

Understanding and
improving crop
photosynthesis
Edited by Dr Robert Sharwood
Western Sydney University, Australia
BURLEIGH DODDS SERIES IN AGRICULTURAL SCIENCE
E-CHAPTER FROM THIS BOOK

http://dx.doi.org/10.19103/AS.2022.0119.12
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to
optimize crop performance
XinyuFu, KailaSmith, LukeGregory, LudmilaRoze and BerkleyWalker, Michigan State
University, USA
1 Introduction
2 Photorespiration: the good, the bad and the inevitable
3 Recent efforts to improve photorespiration
4 How can photorespiration beat the heat?
5 Photorespiration under non-steady-state conditions: could this improve
carbon and nitrogen budgets?
6 Conclusion
7 Where to look for further information
8 References
1 Introduction
Photorespiration is a major component of plant central metabolism in most
crop plants and is responsible for large inefciencies in photosynthesis.
Photorespiration recycles the intermediates produced when rubisco, the
initial enzyme of carbon xation, catalyzes a reaction with oxygen instead of
CO2. Photorespiratory recycling is metabolically expensive, requiring 30–40%
of plant energy under ambient light conditions and reducing net carbon
assimilation by ~20% under typical growing conditions (Walker et al., 2016b,
Sharkey, 1988). Photorespiration imposes these limitations in all crop plants
that have not evolved mechanisms to concentrate CO2 concentrations around
rubisco to minimize oxygenation reactions using four-carbon intermediates (C4
photosynthesis). Plants lacking the ability to concentrate CO2 (and therefore
with high rates of photorespiration) are classied as C3 plants and include rice,
wheat, potato, soybean, and most vegetables and fruits consumed around the
world.
Our goals for this chapter are to introduce the core features of
photorespiration and highlight the current state of the art in approaches
to minimize the effects of this critical process in crop plants. There are many
Modifying photorespiration to optimize crop performance Modifying photorespiration to optimize crop performance
Chapter taken from: Sharwood, R. (ed.), Understanding and improving crop photosynthesis, pp. 203–222,
Burleigh Dodds Science Publishing, Cambridge, UK, 2023, (ISBN: 978 1 80146 129 0; www.bdspublishing.com)

Modifying photorespiration to optimize crop performance2
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
excellent reviews highlighting various approaches to minimizing energy and
carbon loss due to photorespiration, and here we extend these with some
unique approaches to consider for ‘next generation’ improvements to this vital
process (South et al., 2018, Betti et al., 2016, Kubis and Bar-Even, 2019). We also
contend that future engineering efforts require a careful re-thinking of the root
cause and design of successful photorespiratory bypasses and that untapped
potential may lie in approaching photorespiration under non-steady-state
conditions.
2 Photorespiration: the good, the bad and the inevitable
The basic biochemistry, physiology, and genes of photorespiration are well
known and have been the subject of many excellent reviews (Fig. 1, Bauwe
et al., 2010, Walker et al., 2016b, Busch, 2020, Ogren, 1984). The need for
photorespiration was clear following the discovery that rubisco catalyzed a
reaction between ribulose 1,5-bisphosphate (RuBP) and oxygen to produce
the inhibitory molecule 2-phosphoglycolate (2-PG) in addition to the typical
C3 cycle intermediate 3-phosphoglycerate (3-PGA) (Ogren and Bowes,
1971). 2-PG is inhibitory to the C3 cycle enzymes triosephosphate isomerase
and sedoheptulose-1,7-bisphosphate phosphatase and represents a waste
of previously xed carbon, making it essential for the plant to recycle 2-PG
(Anderson, 1971, Flügel et al., 2017). Resolving the biochemistry that recycles
2-PG back into intermediates of the C3 cycle was one of the rst major successes
of forward genetics using Arabidopsis thaliana as a model organism (Somerville
and Ogren, 1979, 1980a,b, 1981, Somerville, 2001). This core biochemistry is
now well-resolved (save some interesting exceptions discussed below) but
understanding the implications of photorespiration to plant productivity is less
clear.
Often photorespiration is referred to in a negative sense, due to its high
demands for energy and massive carbon loss, but viewed from a different
angle photorespiratory metabolism is benecial. While it is true that decreasing
photorespiration by growing plants under elevated CO2 often improves
growth and yield (Ainsworth and Rogers, 2007, Ainsworth and Long, 2021),
it is also true that disrupting ux through photorespiration by knocking out
key genes is lethal or severely detrimental to growth under ambient CO2 and
oxygen concentrations (Foyer et al., 2009, Timm and Bauwe, 2013). Thus,
photorespiration serves a metabolically expensive, yet vital, plant function
stemming from rubisco oxygenation. In this sense, a clear distinction should
be made between the source of the problem (rubisco oxygenation) and the
particular solution that has evolved to handle the problem (photorespiration).
In addition to a vital role in detoxifying 2-PG, photorespiration is linked
to benecial processes like nitrate assimilation. Rates of nitrate assimilation

© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to optimize crop performance 3
decrease as rates of photorespiration relative to carbon xation decrease in
both wheat and Arabidopsis (Bloom et al., 2010, Bloom, 2006, Rachmilevitch
et al., 2004). The mechanisms linking photorespiration to nitrate assimilation
are not clear but could relate to increased cytosolic reductant concentrations
Figure 1 Simplied biochemical pathway of photorespiration illustrating the major
metabolites and coordination between the chloroplast, peroxisome, and mitochondrion.
Enzymatic reactions are shown in black, and putative non-enzymatic reactions are shown
in red. The number in parenthesis shows the stoichiometry of each reaction, and the gene
names are shown in the white ovals. Rbc, rubisco; PGLP, 2-phosphoglycolate phosphatase;
GOX, glycolate oxidase; CAT, catalase; GGAT, glycine:glyoxylate aminotransferase; SHMT,
serine hydroxymethyltransferase; GDC, glycine decarboxylase; SGAT, serine:glyoxylase
aminotransferase; HPR, hydroxypyruvate reductase; GLYK, glycerate kinase.

Modifying photorespiration to optimize crop performance4
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
available to reduce nitrate to nitrite due to increased shuttling of reducing
equivalents, the surplus of reducing equivalents driven by the higher ATP to
reductant demand of photorespiration, or possibly the removal of amino acids
from photorespiration as discussed further below (Walker et al., 2020, Zhao
et al., 2021). The link between photorespiration and nitrate assimilation may
become more important in crop systems as increasing CO2 concentrations
reduce photorespiratory rates, which may decrease rates of nitrate assimilation
and restrict crop growth. Future research should focus on conrming the exact
nature of this mechanism and if this describes an initial response to altered
photorespiratory rates or if this reduction in nitrate assimilation is long-term.
This knowledge is vital to understand how plant nitrogen status may respond to
future climates and possibly re-engineer this response to maintain or increase
rates of nitrate assimilation under current or future CO2 concentrations.
Photorespiration is also proposed to play a role in light energy dissipation
when CO2 is limiting to photosynthesis such as under drought stress and
stomatal closure. Under this hypothesis, reducing equivalents produced in
the light reactions are dissipated by the energy demand of photorespiration
to prevent over-reduction of the photosynthetic apparatus (Pieruschka et al.,
2006, Osmond et al., 1997).
The above discussion highlights that while photorespiration is responsible
for high rates of energy and carbon loss, it performs a vital plant function. It
also highlights the nuance between rubisco oxygenation and downstream
photorespiration. For this chapter, we will ignore the important efforts to minimize
rates of photorespiration discussed elsewhere in this volume (e.g. transferring
C4 traits into C3 crops or improving rubisco specicity for CO2) and treat rubisco
oxygenation as inevitable. In this, we recognize that approaches to minimize
rubisco oxygenation contain great promise but are outside of the purview of
this chapter focused on the reactions downstream of rubisco oxygenation. We
also do not consider the unique adaptation of C2 photosynthesis and efforts to
engineer this modication to photorespiration, where CO2 release is localized
into the bundle sheath for enhanced re-xation (Lundgren, 2020, Khoshravesh
et al., 2016). In the next sections, we will further break down the biochemistry of
photorespiration to illustrate where the inefciencies are and how they might
be exploited to engineer higher rates of net photosynthesis, as well as discuss
the future of photorespiration with climate change.
2.1 Where are carbon and energy lost during
photorespiration?
Photorespiration requires massive amounts of previously reduced carbon
and plant energy, but where do these losses specically occur? Carbon loss
is generally assumed to occur entirely during glycine decarboxylation, where

© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to optimize crop performance 5
two, two-carbon glycine molecules are converted to a single, three-carbon,
serine with the loss of a single CO2 and production of one NADH (Fig. 1). In
this sense, photorespiration is 75% efcient at maintaining the carbon originally
present in the initial 2-PG. The energy demand of photorespiration is more
complex but revolves around the net energy demands of photorespiration and
subsequent energy needed to regenerate RuBP back into the C3 cycle (Edwards
and Walker, 1983, von Caemmerer, 2000). The direct energy-requiring steps of
photorespiration are the energy used to convert the 3-PGA from the oxygenation
reaction to GAP, the ATP required to rex the released NH4
+ into organic forms,
and the ATP required to convert glycerate into 3-PGA and that 3-PGA into GAP.
A reductant is required to rex the released NH4
+ and reduce hydroxypyruvate,
but this cost is assumed to be offset by the NADH generated from the glycine
decarboxylase complex. When these energy demands are combined with the
needs to process 3-PGA into RuBP, photorespiration requires net 3.5 ATP and
2 NADPH equivalents.
These energy and carbon requirements assume that carbon is cycled
entirely from 2-PG and 3-PGA to RuBP. If the photorespiratory ux is partial,
these losses are decreased. For example, there is clear evidence that serine and,
to a lesser extent, glycine involved in photorespiration leave the cycle (Busch
et al., 2018, Harley and Sharkey, 1991). The exit of serine and glycine decrease
the energy needed to process 2-PG, and in the case of glycine, the amount of
CO2 lost per rubisco oxygenation. Indeed the most efcient detoxication of
2-PG might occur in a transient in mutants of serine hydroxymethyltransferase
(a component of the glycine decarboxylase complex) (Somerville and Ogren,
1981). These mutants are unable to decarboxylate glycine and survive under
photorespiratory conditions by accumulating high glycine pools. This strategy
avoids glycine decarboxylation and the energy costs of NH4
+ re-xation and
RuBP regeneration. This transient build-up of glycine is not sustainable since
it requires continual de novo assimilation of NH4
+ and results in continual
production of glycine, but it illustrates the possible benets of non-steady-state
efciency gains in photorespiration.
2.2 How will future climates affect photorespiration?
Rates of rubisco oxygenation directly respond to the two main effects of climate
change, increasing CO2 and temperature. On the one hand, elevated CO2
decreases rates of rubisco oxygenation relative to carboxylation, decreasing
necessary rates of photorespiration. This primary effect results generally in
improved crop productivity under elevated CO2, especially under conditions
where CO2 availability is limited (Ainsworth and Rogers, 2007, Ainsworth and
Long, 2021). CO2 availability is limited under lower-moisture conditions when
stomata close to conserve water and restrict CO2 diffusion into the intercellular

Modifying photorespiration to optimize crop performance6
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
airspace. On the other hand, increasing temperatures increase relative rates
of photorespiration since the specicity of rubisco for CO2 decreases as
temperature increases and the solubility ratio of CO2/O2 decreases (Brooks
and Farquhar, 1985). The net impact of climate change on photorespiration
will depend then on the absolute degree of increases in temperature and CO2.
Under currently predicted climate change scenarios, there is a consensus that
photorespiration will be reduced under future climates but still play a large role
in plant metabolism and energetics. For example, when several combinations
of CO2 and temperature scenarios presented in the 2007 Intergovernmental
Panel on Climate Change were used to parameterize an earth-systems
model of photosynthesis, photorespiration universally decreased, but the
magnitude of this decrease varied based on the specic scenario (IPCC,
2007, Walker et al., 2016b). In other words, the increased CO2 concentrations
reduced photorespiration more than the increases in temperature increased
photorespiration, but we note that this depends on the specic physiological
response of the plant to the modied atmosphere (Dusenge et al., 2019).
3 Recent efforts to improve photorespiration
Because photorespiration requires a large portion of energy and reduces net
carbon assimilation, it has been identied as a clear target for crop improvement
(Betti et al., 2016). The benets are potentially massive; simulations estimate
that ~150 trillion calories are lost annually to photorespiration and that a 5%
reduction in photorespiratory loss would be worth ~$500 million in the US
Midwest alone (Walker et al., 2016b). These benets extend not only to food
production but also to the sustainability of our food systems. For example, 30
times more energy is used by photorespiration as compared to the energy
required to produce the fertilizer applied during the cultivation of potatoes
(Roney and Walker, 2021). While improvements to photorespiration and
photosynthesis, in general, do not directly offset carbon-producing human
energy supplements, they provide an opportunity to grow more food using the
same inputs, effectively lowering the carbon footprint per calorie. Recent efforts
to improve photorespiration can be classied generally into efforts to speed
ux through photorespiration or to create novel ‘photorespiratory bypasses’
that funnel 2-PG through alternative synthetic pathways (Kubis and Bar-Even,
2019, South et al., 2018).
3.1 Keeping traffic moving to prevent inhibition
Increasing ux capacity through photorespiration increases net photosynthetic
rates in Arabidopsis and tobacco (Strategy shown in Fig. 2). Specically, when
glycine decarboxylase activity is increased, carbon xation and growth rates

© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to optimize crop performance 7
increase substantially (Timm et al., 2012a, López-Calcagno et al., 2019). This
increase in net photosynthesis likely occurs when inhibitory intermediates of
photorespiration, like 2-PG, are processed more quickly and are therefore less
inhibitory to the C3 cycle (discussed in more detail in the next section). With
less inhibition, the C3 cycle and rubisco carboxylation is maintained at a higher
rate resulting in higher rates of net CO2 xation and growth. Interestingly,
minimizing the pool sizes of inhibitory intermediates of photorespiration may
Figure 2 Conceptual approaches for increasing the efciency of photorespiration.
Featured are the ve basic approaches to improving photorespiratory ux and efciency,
as discussed in more detail in the text with the pathway title as named in the main text.
Organelles are unlabeled but follow the same pattern and color as in Figure 1.

Modifying photorespiration to optimize crop performance8
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
also help explain at least some of the apparent benets of photorespiratory
bypass lines.
3.2 Current photorespiratory bypass approaches
Many efforts to improve photorespiration involve engineering a novel set of
reactions that can detoxify glycolate via alternative pathways (Fig. 2). These
alternative pathways are referred to generally as ‘photorespiratory bypasses’
and have resulted in higher productivity and photosynthesis in a variety of model
and crop species (Kebeish et al., 2007, Nölke et al., 2014, Maier et al., 2012,
Dalal et al., 2015, South et al., 2019). Some pathways rely on the condensation
and subsequent reduction of glyoxylate to form glycerate, which can enter
the C3 cycle following 2-PG to 3-PGA (Kebeish et al., 2007, Dalal et al., 2015).
These ‘glyoxylate condensation’ bypasses release the same amount of carbon
as native photorespiration but require less energy and do not require NH4
+
release and re-assimilation. This carbon release also is moved to the chloroplast,
which increases the chance that the CO2 is re-assimilated and not lost to the
intercellular airspace (Xin et al., 2014). Therefore, for glyoxylate condensation
bypasses to work from a carbon balance perspective, photorespiration must
natively leak substantial amounts of CO2 into the intercellular airspace. It is not
clear how much CO2 is lost to the intercellular airspace when it is released in the
mitochondria from native photorespiration, with some evidence suggesting
high rates of release while others argue for low or modest rates of release and
effective recapture (Busch et al., 2013, Walker and Ort, 2015, Walker et al.,
2016a, Berghuijs et al., 2017, Tholen and Zhu, 2011, Tholen et al., 2012). These
pathways also have various energy demands in the form of ATP or reducing
equivalents which can make them more or less efcient from an energetics
perspective (Xin et al., 2014).
In other classes of photorespiratory bypasses, carbon from glyoxylate is
fully oxidized and decarboxylated in the chloroplast resulting in increased net
CO2 assimilation (South et al., 2019, Maier et al., 2012). The benets of these
‘full oxidation’ bypasses are not clear from a carbon balance perspective since
100% of the carbon produced as 2-PG would be lost as CO2 and require
additional energy to be re-xed, even if that re-xation was more efcient when
the carbon release occurs in the chloroplast. These schemes do not produce a
simulated benet from a carbon balance perspective and are actually predicted
to reduce rates of net xation when considered on a carbon balance basis (Xin
et al., 2014).
One hypothesis is that glyoxylate condensation and/or full oxidation
bypasses target the same mechanism of improvement as improvements to ux
capacity outlined above. Perhaps these bypasses all serve to minimize pools
of inhibitory intermediates of photorespiration and maintain higher rates of C3

© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to optimize crop performance 9
cycle activity. The higher rates of C3 cycle activity could be large enough to offset
the carbon-negative aspects of cycles exhibited in plants that appear to fully
oxidize glyoxylate. Interestingly, partial expression of glyoxylate condensation
pathways also increases net assimilation and growth, suggesting that other
mechanisms (not simply increasing carbon recapture) explain the increases in
net photosynthesis (Kebeish et al., 2007, Nölke et al., 2014). It is also argued
that glyoxylate concentration might be benecial to net carbon xation and that
pathways that increase glyoxylate concentration in the chloroplast may even
suppress rates of rubisco oxygenation (Oliver and Zelitch, 1977, Oliver, 1980,
Kubis and Bar-Even, 2019). Clearly, more research is needed to understand the
exact mechanism by which these seemingly carbon-losing pathways result in
higher rates of net xation and growth, allowing us to target these mechanisms
more fundamentally for crop improvement.
3.3 Can a photorespiratory bypass be carbon neutral or even
carbon fixing?
‘Carbon-conserving’ photorespiratory bypasses schemes seek to minimize the
issues of CO2 release and recapture entirely by maintaining all carbon present
originally in 2-PG. One proposed pathway involves the reduction of 2-PG and
subsequent condensation with dihydroxyacetone phosphate to produce the
C3 cycle precursor for xylulose 5-phosphate (Ort et al., 2015). However, it has
been noted that this pathway contains highly reversible reactions and the
production of the inhibitory molecule xylulose bisphosphate (Kubis and Bar-
Even, 2019, Yokota, 1991, Zhu and Jensen, 1991, Parry et al., 2008). Several
carbon-conserving photorespiratory bypass schemes are predicted to be more
favorable and involve the reduction of glycolate and ultimate aldol condensation
to enter the C3 cycle (Bar-Even, 2018, Trudeau et al., 2018). These schemes
required the engineering of novel enzymes and resulted in the demonstration of
in vitro photorespiratory schemes that do not release CO2 (Trudeau et al., 2018).
Some proposed carbon-conserving bypass lines go a step further, actually
xing carbon as part of their activities. One implementation of a ‘carbon-
xing’ bypass showed no phenotype in cyanobacteria (Shih et al., 2014), while
another resulted in elevated bicarbonate assimilation also in cyanobacteria (Yu
et al., 2018). Rates of photorespiration are low in cyanobacteria, due to their
functional carbon concentrating mechanisms. Therefore, it will be interesting to
see how these bypasses perform in C3 plants, which have orders of magnitude
more ux through photorespiration.
4 How can photorespiration beat the heat?
Crop systems currently experience relatively high temperatures compared
to laboratory conditions, and high-temperature episodes will only increase

Modifying photorespiration to optimize crop performance10
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
under future climate conditions. For example, growing temperatures in the US
Midwest regularly reach 35°C, very different from the ‘standard’ 25°C often used
to evaluate plant performance in the lab. The most recent Intergovernmental
Panel on Climate Change estimates a global increase of 1.4–5.7°C by 2100,
indicating that the ability of photorespiration to handle ux under elevated
temperatures will become more important in the future climates (Masson-
Delmotte et al., 2021). The next section will focus on specic reactions of
photorespiration that are promising targets to improve the temperature
response of net photosynthesis, an area of obvious interest to increasing crop
productivity under present and future conditions.
4.1 2-phosphoglycolate-glycolate metabolism
Insufcient 2-PG phosphatase activities at elevated temperatures may
restrict ux through photorespiration and result in the build-up of 2-PG and
inhibition of the C3 cycle and net CO2 assimilation (Anderson, 1971, Kelly
and Latzko, 1976). Additionally, 2-PG pools are high in phosphate, possibly
decreasing available free phosphate from the sugar-phosphates of the C3
cycle and also limiting the rate of net CO2 assimilation (Timm et al., 2019,
Sharkey, 1985, Harley and Sharkey, 1991, Yang et al., 2016). Arabidopsis lines
overexpressing 2-PG phosphatase have higher rates of net photosynthesis
at elevated temperatures compared to wild type lines, presumably because
of the faster degradation of 2-phophoglycolate (Flügel et al., 2017, Timm
et al., 2019). Therefore, maintaining high capacity for 2-PG cycling into
downstream photorespiration may be important for minimizing inhibitory
consequences at elevated temperatures and maintaining higher rates of net
photosynthesis.
4.2 Glycolate oxidase
Elevated glycolate oxidase activities may also improve the photosynthetic
efficiency of crops by maintaining carbon flux through photorespiration at
elevated temperatures. If a higher level of peroxisomal glycolate oxidase is
not available to degrade glycolate, large pools of glycolate can accumulate,
which have been linked to the inhibition of rubisco (González-Moro et al.,
1997, Wendler et al., 1992). Indeed, glycolate oxidase suppression lines
show a photorespiratory phenotype, most likely from a severe reduction
in assimilation rate (Xu et al., 2009, Cui et al., 2016, Lu et al., 2014).
However, rice lines overexpressing glycolate oxidase maintain a higher
photosynthetic rate under high temperatures compared to WT lines (Cui
et al., 2016).

© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to optimize crop performance 11
4.3 Exploiting the role of catalase in minimizing wasteful
inefficiencies
It has long been hypothesized that excess hydrogen peroxide (H2O2) in the
peroxisome can decrease the efciency of photorespiration through non-
enzymatic decarboxylation reactions. H2O2 is generated from the oxidation
of glycolate and is broken down in the peroxisome by catalase. If catalase
activity is insufcient, H2O2 can react with the photorespiratory intermediates
glyoxylate and/or hydroxypyruvate and release excess CO2, reducing the
carbon recycling efciency of photorespiration (Cousins et al., 2008, 2011,
Keech et al., 2012, Grodzinski and Butt, 1976, Bao etal. 2021). Mutant analysis
of photorespiratory genes indicates that under laboratory conditions, the
enzymatic decarboxylation of photorespiratory intermediates predominates,
but it is unclear if the efciency of photorespiration is maintained under
stress conditions. For example, combined measurements of gas exchange
and rubisco biochemistry indicate that the stoichiometric amount of CO2
released from photorespiration may increase under elevated temperatures
in many model and crop species, consistent with increases in non-enzymatic
decarboxylation reactions (Walker and Cousins, 2013, Walker et al., 2017). It is
therefore possible that catalase activity is insufcient under some conditions to
decrease rates of non-enzymatic decarboxylation, offering a potential route for
increasing photorespiratory efciency.
5 Photorespiration under non-steady-state conditions:
could this improve carbon and nitrogen budgets?
The above approaches focus on improving photorespiration under steady-
state conditions, but what approaches might target non-steady-state
conditions? Environmental conditions are incredibly dynamic under actual
cultivation conditions (Slattery et al., 2018), but the dynamics and efciency
of photorespiratory metabolism during non-steady-state conditions are
unresolved. For example, photorespiratory metabolism is mostly studied under
steady-state conditions, either after acclimation to various environmental
conditions or genetic perturbations in the photorespiratory pathway (e.g.
Wingler et al., 1999, Abadie et al., 2021, Timm et al., 2012b). Shifting plants
from low- to high photorespiratory conditions often leads to a signicant
accumulation of photorespiratory intermediates (Abadie et al., 2016, Eisenhut
et al., 2017). However, the dynamics of the changes in concentration of
photorespiratory intermediates are rarely studied due to the challenges of
fast sampling on a short time scale and a possible under-appreciation of how
much time plants spend under a non-steady state. Therefore, to harness the
potential of photorespiration to improve crop productivity, a potential avenue

Modifying photorespiration to optimize crop performance12
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
is to understand how changes in photorespiratory metabolism impact non-
steady-state photosynthesis and how non-steady-state photorespiration will
contribute to gross carbon gain and nitrogen assimilation under environmental
uctuations in nature.
Although it is technically challenging to capture the metabolic changes
of photorespiratory intermediates under non-steady-state conditions, the CO2
release from photorespiratory glycine directly impacts net CO2 assimilation and
can be non-invasively tracked within seconds using gas exchange analyzers
(Walker et al., 2018). A sample gas exchange measurements captured the
dynamic response of net CO2 assimilation to changes in photorespiratory
uxes manipulated by O2 mole fractions using a custom gas mixing system
(Fig. 3). The delay in assimilation reaching a lower steady-state rate during
the transient (shaded area) may be attributed to reduced CO2 release from
glycine decarboxylase. Specically, when glycine accumulates under transient
conditions, CO2 release from the glycine decarboxylase reaction will be delayed
until the glycine pool sizes stabilize and metabolism reaches a steady-state rate.
Alternatively, excess CO2 release may occur as glycine pool sizes decline
to a lower concentration. This excess release of CO2 from photorespiration (as
compared to steady-state rates of release) occurs during transients from high
light to low light or darkness. This effect is often called the ‘post-illumination burst’
and results in more reduced rates of net assimilation (or more negative rates of
Figure 3Dynamics of net CO2 assimilation rate transitioning from 2% O2 to 40% O2.
The delay in reaching the new steady-state assimilation rate is shown by the shaded
region and is possibly due in part to accumulation in glycine pools. Measurements were
performed on the youngest-fully expanded leaf of Nicotiana tabacum using an LI-Cor
6800 and a custom gas mixing system to control O2 and N2.

© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to optimize crop performance 13
respiration in the dark) during these transitions (Bulley and Tregunna, 1971, Vines
et al., 1983). The post-illumination burst most likely stems from the glycine pool
that was previously accumulated before the light-dark transitions (Rawsthorne
and Hylton, 1991). While the constant accumulation of glycine is not sustainable
in the steady state, perhaps it is possible to relax glycine pools during non-
steady-state conditions by re-routing carbon to other metabolic pathways that
do not release CO2 temporarily. Further development in transient approaches
is needed to understand the efciencies of photorespiration under non-steady-
state conditions and how these efciencies may be targeted for improvement.
The transient increase in the pools of photorespiratory intermediates could
serve as a carbon and nitrogen reservoir supporting other metabolic processes
that share intermediates with the photorespiratory pathway. A large portion of
the photorespiratory intermediates (e.g. glycine, serine, and glycerate) exist as
inactive pools, evidenced by the incomplete labeling of these pools at 1h of
feeding with 13CO2 (Szecowka et al., 2013, Ma et al., 2014, Xu et al., 2021) and
their presence in the vacuole (Szecowka et al., 2013). If these inactive pools
of photorespiratory intermediates are remobilized during transient conditions
to maintain high photorespiratory uxes, there will be a substantial gain of
carbon without compromising the photosynthetically xed carbon. Besides
fueling glycerate production and turnover of the Calvin-Benson cycle, the extra
photorespiration-derived glycine and serine can be exported to sink tissues
and used for protein synthesis in source leaves (Madore and Grodzinski, 1984,
Cegelski and Schaefer, 2005).
6 Conclusion
Optimizing photosynthesis by modifying photorespiration is an exciting avenue
for crop improvement, but much more work remains. We are encouraged
by recent work placing these modications in crop plants and look forward
to additional validation under eld conditions. Testing strategies under eld
conditions is especially important to ensure that improvements are robust
and in practice contribute to increased crop yield. We are optimistic about
the future of these general approaches and look forward to advances in the
coming years.
7 Where to look for further information
1 For a comprehensive and updated picture of photorespiration and its
connection to nitrogen metabolism (Busch, 2020).
2 International Congress on Photosynthesis Research, which is held every
4 years and often includes many talks on photorespiration. Hosted by
the International Society for Photosynthesis Research.

Modifying photorespiration to optimize crop performance14
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
3 CO2 assimilation in Plants from Genome to Biome, a Gordon Research
Conference, which is held every 3 years and includes intimate coverage
of the latest advances in photorespiratory research.
8 References
Abadie, C., Boex-Fontvieille, E. R. A., Carroll, A. J. and Tcherkez, G. 2016. In vivo
stoichiometry of photorespiratory metabolism. Nature Plants 2, 15220.
Abadie, C., Lalande, J., Limami, A. M. and Tcherkez, G. 2021. Non-targeted 13C metabolite
analysis demonstrates broad re-orchestration of leaf metabolism when gas exchange
conditions vary. Plant, Cell and Environment 44(2), 445–457.
Ainsworth, E. A. and Long, S. P. 2021. 30 years of free-air carbon dioxide enrichment
(FACE): what have we learned about future crop productivity and its potential for
adaptation? Global Change Biology 27(1), 27–49.
Ainsworth, E. A. and Rogers, A. 2007. The response of photosynthesis and stomatal
conductance to rising [CO2]: mechanisms and environmental interactions. Plant, Cell
and Environment 30(3), 258–270.
Anderson, L. E. 1971. Chloroplast and cytoplasmic enzymes II. Pea leaf triose phosphate
isomerases. Biochimica et Biophysica Acta 235(1), 237–244.
Bao, H., Morency, M., Rianti, W., Saeheng, S., Roje, S., Weber, A. P. M. and Walker, B. J. 2021.
Catalase protects against nonenzymatic decarboxylations during photorespiration.
Arabidopsis thaliana.Plant Direct 5(12), e366. https://doi .org /10 .1002 /pld3 .366.
Bar-Even, A. 2018. Daring metabolic designs for enhanced plant carbon xation. Plant
Science 273, 71–83.
Bauwe, H., Hagemann, M. and Fernie, A. R. 2010. Photorespiration: players, partners and
origin. Trends in Plant Science 15(6), 330–336.
Berghuijs, H. N. C., Yin, X., Ho, Q. T., Retta, M. A., Verboven, P., Nicolaï, B. M. and Struik, P.
C. 2017. Localization of (photo) respiration and CO2 re-assimilation in tomato leaves
investigated with a reaction-diffusion model. PLoS ONE 12(9), e0183746.
Betti, M., Bauwe, H., Busch, F. A., Fernie, A. R., Keech, O., Levey, M., Ort, D. R., Parry,
M. A. J., Sage, R., Timm, S., Walker, B. and Weber, A. P. M. 2016. Manipulating
photorespiration to increase plant productivity: recent advances and perspectives
for crop improvement. Journal of Experimental Botany 67(10), 2977–2988.
Bloom, A. J. 2006. Rising carbon dioxide concentrations and the future of crop production.
Journal of the Science of Food and Agriculture 86(9), 1289–1291.
Bloom, A. J., Burger, M., Asensio, J. S. R. and Cousins, A. B. 2010. Carbon dioxide
enrichment inhibits nitrate assimilation in wheat and Arabidopsis. Science 328(5980),
899–903.
Brooks, A. and Farquhar, G. D. 1985. Effect of temperature on the CO2/O2 specicity of
ribulose-1,5-bisphosphate carboxylase/oxygenase and the rate of respiration in the
light. Planta 165(3), 397–406.
Bulley, N. R. and Tregunna, E. B. 1971. Photorespiration and the postillumination CO2
burst. Canadian Journal of Botany 49(8), 1277–1284.
Busch, F. A. 2020. Photorespiration in the context of Rubisco biochemistry, CO2 diffusion
and metabolism. The Plant Journal 101(4), 919–939.
Busch, F. A., Sage, R. F. and Farquhar, G. D. 2018. Plants increase CO2 uptake by assimilating
nitrogen via the photorespiratory pathway. Nature Plants 4(1), 46–54.

© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to optimize crop performance 15
Busch, F. A., Sage, T. L., Cousins, A. B. and Sage, R. F. 2013. C3 plants enhance rates of
photosynthesis by reassimilating photorespired and respired CO2. Plant, Cell and
Environment 36(1), 200–212.
Cegelski, L. and Schaefer, J. 2005. Glycine metabolism in intact leaves by in vivo 13C and
15N labeling. Journal of Biological Chemistry 280(47), 39238–39245.
Cousins, A. B., Pracharoenwattana, I., Zhou, W., Smith, S. M. and Badger, M. R. 2008.
Peroxisomal malate dehydrogenase is not essential for photorespiration
in Arabidopsis but its absence causes an increase in the stoichiometry of
photorespiratory CO2 release. Plant Physiology 148(2), 786–795.
Cousins, A. B., Walker, B. J., Pracharoenwattana, I., Smith, S. M. and Badger, M. R. 2011.
Peroxisomal hydroxypyruvate reductase is not essential for photorespiration
in Arabidopsis but its absence causes an increase in the stoichiometry of
photorespiratory CO2 release. Photosynthesis Research 108(2–3), 91–100.
Cui, L.-L., Lu, Y.-S., Li, Y., Yang, C. and Peng, X.-X. 2016. Overexpression of glycolate
oxidase confers improved photosynthesis under high light and high temperature in
rice. Frontiers in Plant Science 7, 1165.
Dalal, J., Lopez, H., Vasani, N. B., Hu, Z., Swift, J. E., Yalamanchili, R., Dvora, M., Lin, X.,
Xie, D., Qu, R. and Sederoff, H. W. 2015. A photorespiratory bypass increases plant
growth and seed yield in biofuel crop Camelina sativa. Biotechnology for Biofuels
8, 175.
Dusenge, M. E., Duarte, A. G. and Way, D. A. 2019. Plant carbon metabolism and climate
change: elevated CO2 and temperature impacts on photosynthesis, photorespiration
and respiration. New Phytologist 221(1), 32–49.
Edwards, G. E. and Walker, D. A. 1983. C3, C4: Mechanisms, and Cellular and Environmental
Regulation, of Photosynthesis. London: Oxford.
Eisenhut, M., Bräutigam, A., Timm, S., Florian, A., Tohge, T., Fernie, A. R., Bauwe, H.
and Weber, A. P. M. 2017. Photorespiration is crucial for dynamic response of
photosynthetic metabolism and stomatal movement to altered CO2 availability.
Molecular Plant 10(1), 47–61.
Flügel, F., Timm, S., Arrivault, S., Florian, A., Stitt, M., Fernie, A. R. and Bauwe, H. 2017.
The photorespiratory metabolite 2-phosphoglycolate regulates photosynthesis and
starch accumulation in Arabidopsis. The Plant Cell 29(10), 2537–2551.
Foyer, C. H., Bloom, A. J., Queval, G. and Noctor, G. 2009. Photorespiratory metabolism:
genes, mutants, energetics, and redox signaling. Annual Review of Plant Biology 60,
455–484.
González-Moro, B., Lacuesta, M., Becerril, J. M., Gonzalez-Murua, C. and Muñoz-Rueda,
A. 1997. Glycolate accumulation causes a decrease of photosynthesis by inhibiting
Rubisco activity in maize. Journal of Plant Physiology 150(4), 388–394.
Grodzinski, B. and Butt, V. S. 1976. Hydrogen peroxide production and the release of
carbon dioxide during glycollate oxidation in leaf peroxisomes. Planta 128(3),
225–231.
Harley, P. C. and Sharkey, T. D. 1991. An improved model of C3 photosynthesis at high CO2:
reversed O2 sensitivity explained by lack of glycerate reentry into the chloroplast.
Photosynthesis Research 27(3), 169–178.
IPCC 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working
Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate
Change, Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B.,

Modifying photorespiration to optimize crop performance16
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Tignor, M. and Miller, H. L. (Eds.). Cambridge, UK and New York, NY, USA: Cambridge
University Press.
Kebeish, R., Niessen, M., Thiruveedhi, K., Bari, R., Hirsch, H.-J., Rosenkranz, R., Stabler, N.,
Schonfeld, B., Kreuzaler, F. and Peterhansel, C. 2007. Chloroplastic photorespiratory
bypass increases photosynthesis and biomass production in Arabidopsis thaliana.
Nature Biotechnology 25(5), 593–599.
Keech, O., Zhou, W., Fenske, R., Colas-Des-Francs-Small, C., Bussell, J. D., Badger, M. R.
and Smith, S. M. 2012. The genetic dissection of a short-term response to low CO2
supports the possibility for peroxide-mediated decarboxylation of photorespiratory
intermediates in the peroxisome. Molecular Plant 5(6), 1413–1416.
Kelly, G. J. and Latzko, E. 1976. Inhibition of spinach-leaf phosphofructokinase by
2-phosphoglycollate. FEBS Letters 68(1), 55–58.
Khoshravesh, R., Stinson, C. R., Stata, M., Busch, F. A., Sage, R. F., Ludwig, M. and Sage,
T. L. 2016. C3–C4 intermediacy in grasses: organelle enrichment and distribution,
glycine decarboxylase expression, and the rise of C2 photosynthesis. Journal of
Experimental Botany 67(10), 3065–3078.
Kubis, A. and Bar-Even, A. 2019. Synthetic biology approaches for improving
photosynthesis. Journal of Experimental Botany 70(5), 1425–1433.
López-Calcagno, P. E., Fisk, S., Brown, K. L., Bull, S. E., South, P. F. and Raines, C. A. 2019.
Overexpressing the H-protein of the glycine cleavage system increases biomass
yield in glasshouse and eld-grown transgenic tobacco plants. Plant Biotechnology
Journal 17(1), 141–151.
Lu, Y., Li, Y., Yang, Q., Zhang, Z., Chen, Y., Zhang, S. and Peng, X.-X. 2014. Suppression
of glycolate oxidase causes glyoxylate accumulation that inhibits photosynthesis
through deactivating Rubisco in rice. Physiologia Plantarum 150(3), 463–476.
Lundgren, M. R. 2020. C2 photosynthesis: a promising route towards crop improvement?
New Phytologist 228(6), 1734–1740.
Ma, F., Jazmin, L. J., Young, J. D. and Allen, D. K. 2014. Isotopically nonstationary 13C
ux analysis of changes in Arabidopsis thaliana leaf metabolism due to high light
acclimation. Proceedings of the National Academy of Sciences of the United States
of America 111(47), 16967–16972.
Madore, M. and Grodzinski, B. 1984. Effect of oxygen concentration on 14C-photoassimilate
transport from leaves of Salvia splendens L. Plant Physiology 76(3), 782–786.
Maier, A., Fahnenstich, H., von Caemmerer, S., Engqvist, M. K., Weber, A. P., Flügge, U.-I.
and Maurino, V. G. 2012. Transgenic introduction of a glycolate oxidative cycle into
A. thaliana chloroplasts leads to growth improvement. Frontiers in Plant Science 3,
38.
Masson-Delmotte, V., Zhai, P., Pirani, A., Connors, S. L., Péan, C., Berger, S., Caud, N.,
Chen, Y., Goldfarb, L., Gomis, M. I., Huang, M., Leitzell, K., Lonnoy, E., Matthews, J.
B. R., Maycock, T. K. and Watereld, T., Yelekçi, O, Yu, R. and Zhou, B. (Eds), 2021.
IPCC, 2021: summary for policymakers. In: Climate Change 2021: The Physical
Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the
Intergovernmental Panel on Climate Change. Cambridge University Press.
Nölke, G., Houdelet, M., Kreuzaler, F., Peterhänsel, C. and Schillberg, S. 2014. The
expression of a recombinant glycolate dehydrogenase polyprotein in potato
(Solanum tuberosum) plastids strongly enhances photosynthesis and tuber yield.
Plant Biotechnology Journal 12(6), 734–742.

© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to optimize crop performance 17
Ogren, W. L. 1984. Photorespiration: pathways, regulation, and modication. Annual
Review of Plant Physiology 35(1), 415–442.
Ogren, W. L. and Bowes, G. 1971. Ribulose diphosphate carboxylase regulates soybean
photorespiration. Nature: New Biology 230(13), 159–160.
Oliver, D. J. 1980. The effect of glyoxylate on photosynthesis and photorespiration by
isolated soybean mesophyll cells. Plant Physiology 65(5), 888–892.
Oliver, D. J. and Zelitch, I. 1977. Increasing photosynthesis by inhibiting photorespiration
with glyoxylate. Science 196(4297), 1450–1451.
Ort, D. R., Merchant, S. S., Alric, J., Barkan, A., Blankenship, R. E., Bock, R., Croce, R.,
Hanson, M. R., Hibberd, J. M., Long, S. P., Moore, T. A., Moroney, J., Niyogi, K. K., Parry,
M. A. J., Peralta-Yahya, P. P., Prince, R. C., Redding, K. E., Spalding, M. H., Van Wijk, K.
J., Vermaas, W. F. J., von Caemmerer, S., Weber, A. P. M., Yeates, T. O., Yuan, J. S. and
Zhu, X. G. 2015. Redesigning photosynthesis to sustainably meet global food and
bioenergy demand. Proceedings of the National Academy of Sciences of the United
States of America 112(28), 8529–8536.
Osmond, B., Badger, M., Maxwell, K., Björkman, O. and Leegood, R. 1997. Too many
photons: photorespiration, photoinhibition and photooxidation. Trends in Plant
Science 2(4), 119–121.
Parry, M. A. J., Keys, A. J., Madgwick, P. J., Carmo-Silva, A. E. and Andralojc, P. J. 2008. Rubisco
regulation: a role for inhibitors. Journal of Experimental Botany 59(7), 1569–1580.
Pieruschka, R., Schurr, U., Jensen, M., Wolff, W. F. and Jahnke, S. 2006. Lateral diffusion of
CO2 from shaded to illuminated leaf parts affects photosynthesis inside homobaric
leaves. New Phytologist 169(4), 779–788.
Rachmilevitch, S., Cousins, A. B. and Bloom, A. J. 2004. Nitrate assimilation in plant shoots
depends on photorespiration. Proceedings of the National Academy of Sciences of
the United States of America 101(31), 11506–11510.
Rawsthorne, S. and Hylton, C. M. 1991. The relationship between the post-illumination
CO2 burst and glycine metabolism in leaves of C3 and C3–C4 intermediate species of
Moricandia. Planta 186(1), 122–126.
Roney, H. E. and Walker, B. J. 2021. Photon to plate: A holistic view of photosynthetic and
anthropogenic energy uxes. Food and Energy Security 00, e348.https://doi .org /10
.1002 /fes3 .348.
Sharkey, T. D. 1985. O2-insensitive photosynthesis in C3 plants: its occurrence and a
possible explanation. Plant Physiology 78(1), 71–75.
Sharkey, T. D. 1988. Estimating the rate of photorespiration in leaves. Physiologia
Plantarum 73(1), 147–152.
Shih, P. M., Zarzycki, J., Niyogi, K. K. and Kerfeld, C. A. 2014. Introduction of a synthetic
CO2-xing photorespiratory bypass into a cyanobacterium. Journal of Biological
Chemistry 289(14), 9493–9500.
Slattery, R. A., Walker, B. J., Weber, A. P. M. and Ort, D. R. 2018. The impacts of uctuating
light on crop performance. Plant Physiology 176(2), 990–1003.
Somerville, C. R. 2001. An early Arabidopsis demonstration. Resolving a few issues
concerning photorespiration. Plant Physiology 125(1), 20–24.
Somerville, C. R. and Ogren, W. L. 1979. A phosphoglycolate phosphatase-decient
mutant of Arabidopsis. Nature 280(5725), 833–836.
Somerville, C. R. and Ogren, W. L. 1980a. Inhibition of photosynthesis in Arabidopsis
mutants lacking leaf glutamate synthase activity. Nature 286(5770), 257–259.

Modifying photorespiration to optimize crop performance18
© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Somerville, C. R. and Ogren, W. L. 1980b. Photorespiration mutants of Arabidopsis
thaliana decient in serine-glyoxylate aminotransferase activity. Proceedings of the
National Academy of Sciences of the United States of America 77(5), 2684–2687.
Somerville, C. R. and Ogren, W. L. 1981. Photorespiration-decient mutants of
Arabidopsis thaliana lacking mitochondrial serine transhydroxymethylase activity.
Plant Physiology 67(4), 666–671.
South, P. F., Cavanagh, A. P., Liu, H. W. and Ort, D. R. 2019. Synthetic glycolate metabolism
pathways stimulate crop growth and productivity in the eld. Science 363(6422),
609–616.
South, P. F., Cavanagh, A. P., Lopez-Calcagno, P. E., Raines, C. A. and Ort, D. R. 2018.
Optimizing photorespiration for improved crop productivity. Journal of Integrative
Plant Biology 60(12), 1217–1230.
Szecowka, M., Heise, R., Tohge, T., Nunes-Nesi, A., Vosloh, D., Huege, J., Feil, R., Lunn,
J., Nikoloski, Z., Stitt, M., Fernie, A. R. and Arrivault, S. 2013. Metabolic uxes in an
illuminated Arabidopsis rosette. The Plant Cell 25(2), 694–714.
Tholen, D., Ethier, G., Genty, B., Pepin, S. and Zhu, X.-G. 2012. Variable mesophyll
conductance revisited: theoretical background and experimental implications.
Plant, Cell and Environment 35(12), 2087–2103.
Tholen, D. and Zhu, X. G. 2011. The mechanistic basis of internal conductance: a theoretical
analysis of mesophyll cell photosynthesis and CO2 diffusion. Plant Physiology 156(1),
90–105.
Timm, S. and Bauwe, H. 2013. The variety of photorespiratory phenotypes – employing
the current status for future research directions on photorespiration. Plant Biology
15(4), 737–747.
Timm, S., Florian, A., Arrivault, S., Stitt, M., Fernie, A. R. and Bauwe, H. 2012a. Glycine
decarboxylase controls photosynthesis and plant growth. FEBS Letters 586(20),
3692–3697.
Timm, S., Mielewczik, M., Florian, A., Frankenbach, S., Dreissen, A., Hocken, N., Fernie, A.
R., Walter, A. and Bauwe, H. 2012b. High-to-low CO2 acclimation reveals plasticity of
the photorespiratory pathway and indicates regulatory links to cellular metabolism
of Arabidopsis. PLoS ONE 7(8), e42809.
Timm, S., Woitschach, F., Heise, C., Hagemann, M. and Bauwe, H. 2019. Faster removal of
2-phosphoglycolate through photorespiration improves abiotic stress tolerance of
Arabidopsis. Plants 8(12), 563.
Trudeau, D. L., Edlich-Muth, C., Zarzycki, J., Scheffen, M., Goldsmith, M., Khersonsky, O.,
Avizemer, Z., Fleishman, S. J., Cotton, C. A. R., Erb, T. J., Tawk, D. S. and Bar-Even,
A. 2018. Design and in vitro realization of carbon-conserving photorespiration.
Proceedings of the National Academy of Sciences of the United States of America
115(49), E11455–E11464.
Vines, H. M., Tu, Z. P., Armitage, A. M., Chen, S. S. and Black, C. C. Jr. 1983. Environmental
responses of the post-lower illumination CO2 burst as related to leaf photorespiration
1. Plant Physiology 73(1), 25–30.
von Caemmerer, S. 2000. Biochemical Models of Leaf Photosynthesis. Collingwood:
CSIRO.
Walker, B. J., Busch, F. A., Driever, S. M., Kromdijk, J. and Lawson, T. 2018. Survey of tools
for measuring in vivo photosynthesis. In: Covshoff, S. (Ed.) Photosynthesis: Methods
and Protocols, 3–24. New York, NY: Springer New York.

© Burleigh Dodds Science Publishing Limited, 2023. All rights reserved.
Modifying photorespiration to optimize crop performance 19
Walker, B. J. and Cousins, A. B. 2013. Inuence of temperature on measurements of the
CO2 compensation point: differences between the Laisk and O2-exchange methods.
Journal of Experimental Botany 64(7), 1893–1905.
Walker, B. J., Kramer, D. M., Fisher, N. and Fu, X. 2020. Flexibility in the energy balancing
network of photosynthesis enables safe operation under changing environmental
conditions. Plants 9(3), 301.
Walker, B. J., Orr, D. J., Carmo-Silva, E., Parry, M. A. J., Bernacchi, C. J. and Ort, D. R. 2017.
Uncertainty in measurements of the photorespiratory CO2 compensation point and its
impact on models of leaf photosynthesis. Photosynthesis Research 132(3), 245–255.
Walker, B. J. and Ort, D. R. 2015. Improved method for measuring the apparent CO2
photocompensation point resolves the impact of multiple internal conductances to
CO2 to net gas exchange. Plant, Cell and Environment 38(11), 2462–2474.
Walker, B. J., Skabelund, D. C., Busch, F. A. and Ort, D. R. 2016a. An improved approach
for measuring the impact of multiple CO2 conductances on the apparent
photorespiratory CO2 compensation point through slope-intercept regression.
Plant, Cell and Environment 39(6), 1198–1203.
Walker, B. J., Vanloocke, A., Bernacchi, C. J. and Ort, D. R. 2016b. The costs of photo-
respiration to food production now and in the future. Annual Review of Plant Biology
67, 107–129.
Wendler, C., Putzer, A. and Wild, A. 1992. Effect of glufosinate (phosphinothricin) and
inhibitors of photorespiration on photosynthesis and ribulose-1,5-bisphosphate
carboxylase activity. Journal of Plant Physiology 139(6), 666–671.
Wingler, A., Lea, P. J. and Leegood, R. C. 1999. Photorespiratory metabolism of glyoxylate
and formate in glycine-accumulating mutants of barley and Amaranthus edulis.
Planta 207(4), 518–526.
Xin, C., Tholen, D. and Zhu, X.-G. 2014. The benets of photorespiratory bypasses: how
can they work? Plant Physiology 167(2), 574–585.
Xu, H., Zhang, J., Zeng, J., Jiang, L., Liu, E., Peng, C., He, Z. and Peng, X. 2009. Inducible
antisense suppression of glycolate oxidase reveals its strong regulation over
photosynthesis in rice. Journal of Experimental Botany 60(6), 1799–1809.
Xu, Y., Fu, X., Sharkey, T. D., Shachar-Hill, Y. and Walker, A. B. J. 2021. The metabolic
origins of non-photorespiratory CO2 release during photosynthesis: a metabolic ux
analysis. Plant Physiology 186(1), 297–314.
Yang, J. T., Preiser, A. L., Li, Z., Weise, S. E. and Sharkey, T. D. 2016. Triose phosphate
use limitation of photosynthesis: short-term and long-term effects. Planta 243(3),
687–698.
Yokota, A. 1991. Carboxylation and detoxication of xylulose bisphosphate by spinach
ribulose bisphosphate carboxylase/oxygenase. Plant and Cell Physiology 32,
755–762.
Yu, H., Li, X., Duchoud, F., Chuang, D. S. and Liao, J. C. 2018. Augmenting the Calvin–
Benson–Bassham cycle by a synthetic malyl-CoA-glycerate carbon xation pathway.
Nature Communications 9(1), 2008.
Zhao, H.-L., Chang, T.-G., Xiao, Y. and Zhu, X.-G. 2021. Potential metabolic mechanisms for
inhibited chloroplast nitrogen assimilation under high CO2. Plant Physiology 187(3),
1812–1833.
Zhu, G. and Jensen, R. G. 1991. Xylulose 1,5-bisphosphate synthesized by ribulose
1,5-bisphosphate carboxylase/oxygenase during catalysis binds to decarbamylated
enzyme 1. Plant Physiology 97(4), 1348–1353.