summary: recent advances in understanding …...16 summary: recent advances in understanding...
TRANSCRIPT
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Title: The impacts of fluctuating light on crop performance 1
Short title: Impacts of fluctuating light on crops 2
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Rebecca A. Slattery1,2, Berkley J. Walker3, Andreas P. M. Weber3, Donald R. Ort1,2,4,* 4
1Global Change and Photosynthesis Research Unit, Agricultural Research Service, United States 5
Department of Agriculture, Urbana, IL, USA 6
2Carl R. Woese Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL, 7
USA 8
3Institute of Plant Biochemistry, Cluster of Excellence on Plant Sciences (CEPLAS), Heinrich-Heine-9
University Düsseldorf, Düsseldorf, Germany 10
4Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL, USA 11
*Address correspondence to [email protected] 12
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Author contributions: All authors contributed to the intellectual input and writing of the manuscript. 14
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Summary: Recent advances in understanding photosynthetic responses to dynamic light environments 16 reveal opportunities to improve crop plant photosynthetic efficiency. 17
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Funding: This work was supported by the USDA/ARS (RAS, DRO), the Bill and Melinda Gates 19
Foundation grant (OPP1060461) titled ‘RIPE-Realizing Increased Photosynthetic Efficiency for 20
Sustainable Increases in Crop Yield’ (DRO), a postdoctoral research fellowship awarded by the 21
Alexander von Humboldt Foundation (BJW), the Cluster of Excellence on Plant Science (CEPLAS, EXC 22
1028; APMW), and the Bundesministerium für Bildung und Forschung (BMBF) grant 031B0205A 23
(APMW). 24
Plant Physiology Preview. Published on November 30, 2017, as DOI:10.1104/pp.17.01234
Copyright 2017 by the American Society of Plant Biologists
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Abstract 25
Rapidly changing light conditions can reduce carbon gain and productivity in field crops because 26
photosynthetic responses to light fluctuations are not instantaneous. Plant responses to fluctuating light 27
occur across levels of organizational complexity from entire canopies to the biochemistry of a single 28
reaction and across orders of magnitude of time. Although light availability and variation at the top of the 29
canopy are largely dependent on the solar angle and degree of cloudiness, lower crop canopies rely more 30
heavily on light in the form of sunflecks, the quantity of which depends mostly on canopy structure but 31
may also be affected by wind. The ability of leaf photosynthesis to respond rapidly to these variations in 32
light intensity is restricted by the relatively slow opening/closing of stomata, activation/deactivation of C3 33
cycle enzymes, and upregulation/downregulation of photoprotective processes. The metabolic complexity 34
of C4 photosynthesis creates the apparently contradictory possibilities that C4 photosynthesis may be both 35
more and less resilient than C3 to dynamic light regimes, depending on the frequency at which these light 36
fluctuations occur. We review the current understanding of the underlying mechanisms of these 37
limitations to photosynthesis in fluctuating light that have shown promise in improving response times of 38
photosynthesis-related processes to changes in light intensity. 39
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Introduction 41
As sessile organisms, plants must respond to dynamically and rapidly changing environmental 42
conditions. Light intensity is the most dynamic condition to which plants must respond and can change at 43
time scales from a season to less than a second (Assmann and Wang, 2001). The maximum amount of 44
light incident at any point in time for a given area of ground is determined seasonally (Fig. 1A), but 45
within seasonal variation, there is also variation as a function of the time of day from complete darkness 46
to full sunlight (Fig. 1B). Clouds can substantially reduce incident light, and their intermittency 47
introduces dynamic intensity changes (Fig. 1B). Atmospheric aerosols, which are a complex and dynamic 48
mixture of solid and liquid particles from natural and anthropogenic sources, can also interact with the 49
Earth's radiation budget and climate directly by reflecting light back into space and indirectly by changing 50
how the clouds reflect and absorb sunlight. These sources of variation in incident light upon a canopy can 51
have a large impact on canopy photosynthesis because the upper canopy drives ~75% of crop canopy 52
carbon gain (Long et al., 2006). While the amount of sunlight reaching lower leaves within a canopy 53
depends on many factors, sunflecks provide the majority of light in the lower canopy (Fig. 1C). Despite 54
the transient nature of sunflecks, they can provide up to 90% of the available light in lower layers of 55
dense canopies, highlighting the importance of harnessing these rapid fluctuations of light for canopy 56
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productivity (Chazdon, 1988; Knapp and Smith, 1989; Pearcy, 1990; Chazdon and Pearcy, 1991; Way 57
and Pearcy, 2012). 58
The importance of within-canopy photosynthetic performance on crop photosynthetic 59
performance has been long recognized (e.g., Ort and Baker, 1988), and there is a growing realization of 60
the contribution from within-canopy fluctuating light on crop productivity (Murchie et al., 2008; Lawson 61
et al., 2012). For example, concurrent to many reviews focusing on the effects of sunflecks on forest 62
understories (Chazdon, 1988; Chazdon and Pearcy, 1991; Pearcy and Pfitsch, 1995; Way and Pearcy, 63
2012), there have been advances in examining the same illumination dynamics in major crop species such 64
as soybean (Pearcy et al., 1990; Pearcy et al., 1997), rice (Nishimura et al., 1998; Nishimura et al., 2000), 65
and maize (Wang et al., 2008) using experimental and modeling approaches. Given the importance of 66
dynamic light in crop canopies, various light-sensitive factors have been identified as targets to improve 67
crop photosynthesis and productivity in fluctuating light conditions. For example, changes in canopy and 68
leaf morphology can improve sunfleck abundance and distribution throughout canopy layers, while 69
stomatal opening/closing kinetics determine carbon supply and water use during these transient periods of 70
light. Delays in C3 cycle enzyme activation reduce the efficiency with which available CO2 is used during 71
transitions to high light, and the slow relaxation of photoprotection lowers light use efficiency upon 72
transitions to low light. The coordination of C3 and C4 cycles in C4 crops presents an additional layer of 73
complexity during dynamic light conditions. Although large metabolite pools may aid C4 photosynthesis 74
in acclimating rapidly to fluctuating light, some evidence suggests the contrary. Thus, the mechanisms of 75
these processes are currently being studied in more detail under fluctuating light conditions and are the 76
focus of this article. 77
78
Seeing the sun through the trees 79
The degree of variation in light intensity incident on a given leaf is strongly dependent on the 80
structure of the canopy around it, especially within the dense canopy structures typical of mature crop 81
stands. Light variation within the canopy is affected by leaf shading by other leaves, which varies rapidly 82
(Pearcy et al., 1990) and depends on a host of environmental, physiological, and morphological factors. 83
As compared to forests, crop canopies have steeper angles of light penetration due to their short, dense 84
canopies, resulting in fewer partially shaded regions. Thus, a large proportion of within-canopy 85
fluctuating illumination comes from high-intensity, lower-canopy sunflecks, especially in crops, such as 86
soybean, as compared to forest canopies (Pearcy et al., 1990). Although the total number of sunflecks 87
may decrease as canopies become denser, sunflecks still contribute a large percentage of total light 88
intercepted by the canopy, even with relatively dense leaf area index (LAI) values of >5. Leaf 89
arrangement affects the abundance of sunflecks in lower canopies. The quantity of sunflecks decreases 90
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with greater leaf clumping in modeled, artificial, and experimentally manipulated tree and crop canopies 91
under fixed LAI (Chen and Black, 1992). 92
Leaf angle can also determine sunfleck frequency depending on what proportion of incoming 93
light is diffuse. For example, leaf angle and orientation in rice influences the dynamic nature of incident 94
light under sunny but not overcast diffuse light conditions (Nishimura et al., 1998). An increased 95
proportion of diffuse light allows penetration of light to deeper layers of a crop canopy, which improves 96
light use efficiency and photosynthesis at the canopy level (Sinclair and Muchow, 1999; Roderick et al., 97
2001; Li et al., 2014). Diffuse light also reduces the temporal variation in light intensity, thus reducing the 98
relative contribution of sunflecks to canopy photosynthesis and dampening the effects of fluctuating light 99
on canopies (Pearcy and Pfitsch, 1995; Way and Pearcy, 2012; Li et al., 2016). The relative proportion of 100
light incident as either sunflecks or diffuse light varies depending on canopy structure and depth. In layers 101
of a soybean canopy where sunflecks occur, sunflecks can account for 20-93% of the total available light 102
on cloudless days with the remainder being diffuse (Pearcy et al., 1990). However, there is evidence that 103
radiation incident on earth’s surface is becoming more diffuse as a result of anthropogenic aerosol 104
emission and changes in weather patterns, resulting in global dimming of global surface radiation and 105
possible changes to primary productivity (Mercado et al., 2009; Wild, 2009). Therefore, while diffuse 106
light is important to overall canopy photosynthesis, it is not a focus of this review on fluctuating light in 107
crop canopies other than to highlight its general importance and mention that it serves to dampen the 108
impact of sunflecks. 109
It has long been recognized that wind plays an integral role in driving within-canopy light 110
variation, but its effects are difficult to model given the static and averaging nature of previous canopy 111
models when representing plant structure and carbon assimilation. Recent advances in ray tracing 112
algorithms facilitated by improved imaging and computational abilities have enabled models with a 113
greater ability to account for the actual structure of a crop canopy and more precise localization of light 114
regimes (Song et al., 2013; Burgess et al., 2015). These models have been leveraged to produce a 115
representation of the impact of wind speed, variability, and direction on inter-canopy radiation regimes 116
and net assimilation of a wheat canopy (Burgess et al., 2016). This work reveals that an increase in light 117
penetration into the canopy due to wind-driven canopy movement can increase total canopy carbon gain 118
by up to 17% and raises interesting possibilities to the improvement of canopy photosynthesis through 119
canopy structural modifications. 120
Individual leaf morphology, diurnal response, and ultrastructure can also impact the response of 121
crop plants to variable light regimes. For example, long-term exposure to either high or low light affects 122
aspects of leaf development and morphology, such as size, thickness, and shape, which impacts light 123
absorption (Boardman, 1977). Diurnal changes in light can affect leaf movement and orientation, 124
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allowing leaves to track the movements of the sun across the sky or avoid direct sunlight in times of stress 125
(Raven, 1989; Ehleringer and Forseth, 1990). In a matter of minutes following an alteration in light 126
intensity, chloroplasts rearrange within cells from the upper surfaces of the cell in low light to the sides of 127
the cell in high light (Wada, 2013). These effects and responses are important considerations to how crop 128
plants respond to seasonal, diurnal, and transient changes in light intensity at the leaf and canopy level 129
with biochemical considerations constituting the focus of the remaining discussion. 130
131
The physiological politics of stomata 132
Stomata present the initial limitation in both C3 and C4 plants by controlling the entry of CO2 into 133
the leaf and therefore the substrate supply for photosynthesis. Stomata are also responsible for balancing 134
CO2 uptake for photosynthesis with water loss from the leaf through transpiration. Stomatal closure 135
conserves water when there is no need for CO2 entry into the leaf for photosynthesis, but stomata must 136
open to provide the CO2 necessary for the carbon reduction cycle (Lawson and Blatt, 2014). The opening 137
and closing of stomata are regulated by guard cells, the cells that surround the stomatal pore and regulate 138
the pore aperture. Blue and red light-mediated processes result in an influx of solute and therefore water 139
into the guard cells, causing an increase in turgor that opens the stomata. When solute concentrations 140
decline, guard cells lose water, resulting in stomata closure. Other external signals are also related to 141
stomata aperture control, including CO2 concentration, humidity, temperature, and abscisic acid 142
concentrations (Lawson, 2009). 143
While stomatal conductance and photosynthesis are generally well coordinated in steady-state 144
conditions (Wong et al., 1979; Farquhar and Sharkey, 1982), the lag in photosynthetic response during 145
transitions from low to high light (Fig. 2A) is often due to insufficient supply of CO2 needed for the 146
carbon cycle. This is because the opening of stomata occurs at a much slower rate than the initial 147
upregulation of photosynthetic electron transport (Fig. 2B). In times of rapidly changing light conditions, 148
this limitation can represent a substantial inefficiency in photosynthesis and therefore crop productivity. A 149
recent study by McAusland et al. (2016) measured a 10-15% limitation to photosynthesis across several 150
C3 and C4 crop species during the time it took leaves to reach steady-state conditions upon transfer from 151
low light to high light. However, the limitation to carbon assimilation by stomatal conductance during 152
transitions from low to high light can vary depending on the initial stomatal conductance in low light. If 153
stomatal conductance is greater in low light, which might occur if water is not limiting (Lawson et al., 154
2012; Way and Pearcy, 2012) and was the case in the McAusland et al. (2016) study, stomatal limitation 155
is likely less during the transition, whereas lower stomatal conductance in low light, which might be 156
evident in water-limited conditions (Knapp and Smith, 1989), likely leads to much higher stomatal 157
limitation to photosynthetic recovery. It should be noted that in fluctuating light, greater stomatal 158
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conductance at low light could also alleviate temperature stress via evaporative cooling upon sudden 159
exposure to high light intensity (Schymanski et al., 2013) but at the expense of water use efficiency. 160
A sluggish stomatal response when leaves are rapidly shaded by a cloud or overhead foliage can 161
also cause excessive water losses due to a lack of coordination between assimilatory demand for CO2 and 162
stomatal conductance. Thus, when stomata do not close rapidly enough, water is lost disproportionately 163
relative to carbon gained, which reduces water use efficiency during the transition (Lawson and Blatt, 164
2014; McAusland et al., 2016). Upon low to high light transitions in some species, stomata conductance 165
continues to climb, even after maximum rates of photosynthesis have been reached; thus, these species 166
‘overshoot’ the rate of stomatal conductance that achieves maximum photosynthesis, which also reduces 167
water use efficiency (McAusland et al., 2016). Some C3 crops bred for high photosynthesis rates display 168
lower water use efficiency in fluctuating light (McAusland et al., 2016), likely because selection for yield 169
in non-water-limiting conditions does not penalize accessions with inferior water use efficiencies (Fischer 170
et al., 1998; Koester et al., 2016). Given that crop productivity and yield is often water-limited and is 171
likely to become more so (Ort and Long, 2014), improving stomatal closure kinetics and regulation in 172
response to fluctuating light regimes could substantially improve canopy water use efficiency and yield. 173
Careful coordination of stomatal kinetics and photosynthesis are needed to both ensure CO2 174
availability for photosynthesis and limit water loss, especially for C3 crops. While some studies suggest 175
many small stomata could achieve this end (Drake et al., 2013; Raven, 2014), other research suggests that 176
stomata type and biochemistry may be more influential. A smaller change in turgor can have larger 177
effects on stomatal aperture in dumbbell- versus elliptical-shaped guard cells, leading to more rapid 178
opening/closing kinetics (Hetherington and Woodward, 2003; McAusland et al., 2016). In addition, equal 179
and opposite transfer of turgor from subsidiary epidermal cells to guard cells during opening and closing, 180
rather than constant pressure in the subsidiary cells, not only enhances stomatal aperture by allowing 181
displacement of the subsidiary cell but also increases the rate of stomatal movement (Franks and 182
Farquhar, 2007; Raissig et al., 2017). The rate of solute transport into and out of guard cells also presents 183
potential targets for modification to obtain more rapid opening upon transitions to high or low light 184
(Lawson and Blatt, 2014; Wang et al., 2014b), but the intuitive solution of increasing the ratio of 185
individual ion channels per surface area often has the opposite effect (Wang et al., 2014b; Wang et al., 186
2014c). However, modeling suggests that manipulating the gating of these ion channels may have more 187
favorable consequences on stomatal kinetics (Wang et al., 2014b). 188
Despite the complexity and lack of established approaches to manipulate stomatal kinetics, recent 189
studies have shown that variation exists within and among certain species that could guide future 190
engineering efforts or enable optimization by selective breeding. McAusland et al. (2016) showed 191
substantial variation in stomatal responses across 13 crop species, and measurements conducted by Qu et 192
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al. (2016) showed variation in stomatal kinetics among 204 rice accessions. Non-crop species may also 193
provide clues for ways in which to improve stomatal kinetics. For example, unlike the delayed increase in 194
stomatal conductance compared to photosynthesis seen in several crops (McAusland et al., 2016), the 195
stomatal response to increased light of the C4 shade tolerant Microstegium vimineum is faster than the 196
photosynthetic response following low-light acclimation (Horton and Neufeld, 1998). Other traits may be 197
important to study in crops, such as the degree of anisohydry, or the tendency of stomata to remain open 198
for CO2 entry into the leaf despite a decline in leaf water content, across species, as stomatal kinetics are 199
more rapid in tree species with more anisohydric tendencies (Meinzer et al., 2017). In addition, variation 200
could be explained by differences in the speed of signaling components driving the mechanistic changes. 201
Identifying the sources of variation in stomatal kinetics will therefore be crucial for mechanistic 202
understanding and further engineering efforts to increase the rate and coordination of stomatal opening 203
and closing with light intensity. 204
Mesophyll conductance presents an additional limitation to carbon supply to the chloroplast 205
during fluctuating light. Mesophyll conductance varies within minutes when leaves are exposed to 206
changes in light, temperature, and CO2 concentration (Flexas et al., 2007; Flexas et al., 2008; Tholen et 207
al., 2008; Evans and von Caemmerer, 2013), and more recent research has shown reductions in mesophyll 208
conductance when plants are grown in fluctuating light conditions (Huang et al., 2015; Vialet-Chabrand et 209
al., 2017). Although the mechanisms for these reductions are not yet understood, aquaporin levels, 210
carbonic anhydrase concentration, and leaf and cell anatomy may contribute to variation in mesophyll 211
conductance (Flexas et al., 2012). In a recent study, growth in fluctuating light resulted in thinner palisade 212
and spongy mesophyll layers and altered cell shape as compared to steady-light conditions (Vialet-213
Chabrand et al., 2017), but a direct relationship to overall mesophyll conductance was not investigated. 214
Thus, elucidating the mechanisms of mesophyll conductance is also necessary for engineering to ensure 215
sufficient carbon supply to photosynthesis during fluctuating light conditions. 216
217
The control of carbon reduction enzymes in the face of stop-and-go traffic 218
While stomatal and mesophyll conductance kinetics determine the supply of CO2 for 219
photosynthesis during dynamic light conditions, the activation rate of C3 cycle enzymes determines how 220
rapidly available CO2 is reduced to form sugars. Upon the transition from low light to high light, such as 221
occurs when a shaded leaf is exposed to a sunfleck, electron transport responds almost instantaneously. 222
However, the relatively slower induction of the carbon reduction cycle (Fig. 2C) limits photosynthesis, 223
thereby limiting total carbon gain by the leaf and canopy. In tobacco, the time needed to reach maximum 224
stomatal conductance in response to light (~40 min; McAusland et al., 2016) is greater than the time 225
needed to reach maximum Rubisco activation (~7 min; Hammond et al., 1998) and is likely more limiting 226
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in some situations. However, in conditions favoring higher initial stomatal conductance, such as the brief 227
periods between high-frequency sunflecks when biochemical enzymes relax more quickly than stomatal 228
conductance, or in high CO2 concentrations, such as those present in the lower canopy, C3 cycle enzyme 229
regulation likely limits photosynthesis to a greater degree than stomatal conductance (Sassenrath-Cole 230
and Pearcy, 1992; Tomimatsu and Tang, 2012; Graham et al., 2017). 231
While gene expression and protein synthesis of enzymes involved in carbon reduction determine 232
enzyme concentrations and therefore activity at longer timescales, reversible posttranslational 233
modifications are responsible for the activation/deactivation of these enzymes during the more rapid 234
transitions between light and dark conditions or even due to more modest changes in light intensity. The 235
key enzymes controlling the C3 cycle during light intensity transitions include ribulose-1,5-bisphosphate 236
carboxylate oxygenase (Rubisco), Rubisco activase (Rca), glyceraldehyde-3-phosphate dehydrogenase 237
(GAPDH), fructose-1,6-bisphosphatase (FBPase), sedoheptulose-1,7-bisphosphatase (SBPase), and 238
phosphoribulokinase (PRK). 239
Before Rubisco can catalyze the carboxylation reaction of photosynthesis, the enzyme must 240
undergo a series of reactions to reach its activated state. A lysine residue in Rubisco must be 241
carbamylated by CO2 and then stabilized by Mg2+ before it combines ribulose-1,5-bisphosphate (RuBP) 242
with CO2 in photosynthesis (or O2 in photorespiration). However, the activation process can be inhibited 243
by binding of sugar phosphates, such as RuBP, to Rubisco’s active site, which prevents carbamylation. 244
Thus, a separate process is needed to free the Rubisco active site for activation and is achieved through 245
the activity of Rca. Rca hydrolyzes ATP to remove sugar phosphates from Rubisco, thus allowing 246
Rubisco to proceed through the reactions necessary for full carbamylation and activation (Wang and 247
Portis, 1992; Portis, 2003). But what activates Rca? In many species, there are two forms of Rca: a longer 248
-isoform, which contains two cysteines that when reduced by thioredoxin render the enzyme less 249
sensitive to inhibition by ADP and when oxidized render the enzyme more strongly inhibited by ADP, 250
and a shorter -isoform, which lacks this redox regulatory component. Therefore, as light levels increase, 251
the ratio of ATP/ADP and the redox poise of thioredoxin increase levels of activating Rca in species 252
containing the -isoform. However, this may not be the only mechanism for enabling Rca to function as 253
some species contain only the shorter -isoform version of Rca, which is not directly redox regulated but 254
responds to ATP/ADP ratios in a species-dependent manner and is also able to activate Rubisco in light 255
(Portis, 2003; Portis et al., 2008). 256
Rubisco and its activation by Rca are important potential targets for crop improvement, especially 257
in regards to photosynthesis during changes in light environment (Parry et al., 2013; Carmo-Silva et al., 258
2015). While Rca quantity is not usually considered limiting to Rubisco activation unless reduced by 259
more than 60%, these results only refer to steady-state conditions (Carmo-Silva et al., 2015). Under 260
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dynamic conditions, tobacco expressing antisense Rca constructs shows a linear response between Rca 261
content and the initial slope of carbon fixation during transition from low to high light under a broad 262
range of Rca contents, indicating that Rca indeed limits the induction of carbon fixation (Hammond et al., 263
1998). However, the reduction in carbon assimilation upon transition from high to low light is not 264
sensitive to Rca content (Hammond et al., 1998). A modeling study by Mott and Woodrow (2000) shows 265
a greater allocation of resources to Rca at the expense of Rubisco benefits the induction time of 266
photosynthesis in fluctuating light conditions. This greater investment in Rca is a typical characteristic of 267
shade leaves (von Caemmerer and Quick, 2000), which ensures more efficient use of available light in the 268
form of sunflecks (Pearcy, 1990). Experimental overexpression of Rca in rice increases Rubisco 269
activation and the rate of photosynthesis induction upon transition from low to high light (Fukayama et 270
al., 2012; Yamori et al., 2012). However, overexpression of Rca in non-fluctuating light conditions leads 271
to a decrease in total Rubisco content and a small reduction in net carbon assimilation (Fukayama et al., 272
2012), confirming that Rca is only limiting in fluctuating light conditions (Yamori et al., 2012). 273
Modifying the regulation of Rca may present a more attractive option to increasing the rate of 274
photosynthetic induction upon light transitions. In an Arabidopsis thaliana transformant containing only 275
the non-redox-regulated Rca -isoform, which is insensitive to physiologically relevant ATP/ADP ratios, 276
Rca remains activated in low light, resulting in an instantaneous activation of Rubisco upon the transition 277
to high light. This leads to faster rates of photosynthetic induction, which is correlated with large 278
increases in biomass in fluctuating light compared to non-fluctuating light conditions that are not seen in 279
the wild type (Carmo-Silva and Salvucci, 2013). The presence of two Rca isoforms with different 280
regulatory properties raises the question of why Rca is regulated at all if constant activation is beneficial 281
for carbon assimilation. The answer may lie, at least in part, in the observation that some inhibitors of 282
Rubisco, like carboxy-D-arabinitol 1-phosphate (CA1P), bind Rubisco during dark periods and may help 283
protect against proteolysis (Andralojc et al., 1994; Khan et al., 1999). Therefore, given the cost of Rubisco 284
biosynthesis, efforts to improve net assimilation through constant Rubisco activation should examine the 285
impact on Rubisco turnover as well. In addition, inactivation of Rca in the dark would limit unnecessary 286
ATP hydrolysis to maintain Rca activation under non-carbon fixing conditions. 287
As with the -isoform of Rca, numerous other enzymes of the C3 cycle are at least partially 288
regulated by the ferredoxin-thioredoxin system, including the key enzymes involved in the carbon 289
reduction cycle: GADPH, FBPase, SBPase, and PRK. Enzyme activation by the chloroplast ferredoxin-290
thioredoxin system can take minutes for full activation when leaves are transferred from low to high light, 291
thereby creating a lag in reaching full photosynthetic capacity (Sassenrath-Cole et al., 1994; Sassenrath-292
Cole and Pearcy, 1994). While there are several isoforms and subtypes of thioredoxins present in plant 293
cells, recent studies have identified numerous thioredoxin proteins responsible for the activation of C3 294
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cycle enzymes in the light. Specific f-type thioredoxin proteins have been identified in the direct 295
activation of C3 cycle enzymes (Thormählen et al., 2013; Yoshida et al., 2015; Naranjo et al., 2016). In 296
addition, m-type thioredoxins have been shown to have no effect on photosynthesis in steady-state 297
conditions but alter photosynthetic efficiency in fluctuating light and are therefore essential in rapidly 298
changing light conditions (Thormählen et al., 2016). 299
Additional systems have been shown to contribute to the regulation of carbon reduction enzymes 300
in fluctuating light. The NADPH-thioredoxin reductase C (NTRC) pathway, which likely uses NADPH 301
indirectly as an electron donor, plays a distinct but cooperative role with the ferredoxin-thioredoxin 302
system in modulating chloroplast functions and in regulating FBPase and SBPase activity (Yoshida and 303
Hisabori, 2016). The NTRC pathway also plays a key role in maintaining photosynthetic efficiency in 304
fluctuating light by controlling the NADPH redox status of the stroma (Thormählen et al., 2016). 305
Dissociation from large protein complexes is also involved in C3 cycle enzyme activation. Although PRK 306
and GADPH are directly reduced by thioredoxins (Marri et al., 2009), they are not fully activated until 307
dissociated from the complex they form with a small protein, CP12. The level of dissociation correlates 308
well with light levels, thus allowing a more rapid activation/deactivation response during dynamic light 309
conditions than redox regulation alone (Howard et al., 2008; Marri et al., 2009). However, the presence of 310
this complex is not universal across species (Howard et al., 2011), suggesting the regulation of these 311
enzymes may be even more intricate and diverse. Reversible phosphorylation may also contribute to 312
enzyme regulation, as Rca phosphorylation seems to occur in dark but not light conditions (Kim et al., 313
2016). While the effects of phosphorylation on Rca activity are not known yet, this suggests a potential 314
method of regulation for carbon reduction enzymes in addition to the thioredoxin-mediated regulation 315
discussed above. Other post-translational regulatory mechanisms directly affect Rubisco (Houtz et al., 316
2008), such as acetylation (Gao et al., 2016), but their effects in dynamic light have yet to be examined. 317
As more of the key players are identified and the mechanisms by which they regulate carbon metabolism 318
in rapidly changing light conditions are elucidated, they will likely be used to enhance the responsiveness 319
of carbon reduction enzymes in fluctuating light. 320
321
Is photoprotection overprotective? 322
The photosynthetic response to light is linear at low light intensities but becomes saturated as 323
light increases. In many crops, light saturation occurs by 25% of full sunlight in absence of other stresses 324
(Long et al., 2006). When leaves are experiencing additional stress or light increases more rapidly than 325
photochemical capacity (e.g., slow responses of stomata and C3 cycle enzymes as detailed above), light 326
saturation may occur at even lower light levels (Fig. 2D). As a result, leaves at the top of the canopy 327
experience excessive amounts of light during the majority of daylight hours (Ort, 2001). When absorbed 328
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light is in excess of photosynthetic capacity, it has the potential to damage photosynthetic proteins and 329
membranes. Thus, plants have evolved various photoprotective mechanisms, the most universal and 330
important of which is called non-photochemical quenching (NPQ), which enables chloroplasts to safely 331
dissipate excess light as heat (Niyogi, 1999). 332
By engaging NPQ in high light conditions, leaves achieve protection from photodamage without 333
any cost to carbon gain. Photoprotection is beneficial to leaves at high light because it prevents 334
irreversible damage to photosystem II that must be repaired through protein synthesis. However, when 335
NPQ remains engaged under non-saturating light conditions, it limits carbon gain by lowering the 336
maximum quantum yield of photosystem II and thereby the quantum efficiency of CO2 assimilation 337
(Long et al., 1994). While NPQ inductions can occur within a few seconds upon the transition to high 338
light, the relaxation process during the transition from high light to low light is much slower and can take 339
minutes to hours to occur. As a result, when plants are in a photoprotected state and light levels decline 340
rapidly to non-saturating levels, some of the available light that could be safely harnessed for 341
photochemical processes is dissipated as heat and therefore wasted (Fig. 2D). For crops grown in the 342
field, fluctuations in light, such as when a leaf shades another leaf due the continuous diurnal change in 343
solar azimuth or when a passing cloud occludes the sun, present opportunities for this inefficiency to 344
significantly limit leaf and canopy carbon gain. In fact, it has been estimated that the slow relaxation of 345
NPQ can limit daily canopy carbon uptake of crops grown in the field by up to 32% (Zhu et al., 2004). 346
NPQ is comprised of several processes involved with protecting the photosynthetic machinery 347
from excess light. The most rapid of these processes is energy-dependent quenching, or qE, which 348
engages on the scale of seconds to minutes (Müller et al., 2001). qE is initiated when protons accumulate 349
in the lumen, thereby reducing the lumen pH. PsbS, a protein associated with photosystem II, senses the 350
low pH and signals a conformation change in the LHCII protein (Li et al., 2000; Sacharz et al., 2017). 351
This conformational change occurs in concert with the xanthophyll cycle through which violaxanthin is 352
converted to zeaxanthin by violaxanthin de-epoxidase in high light (Demmig-Adams, 1990). Quenching 353
via state transitions, or qT, also diverts excitation energy away from photosystem II, engages on a slightly 354
longer time scale (5-10 min) than qE, but accounts for a very small portion of NPQ in higher plants 355
(Horton et al., 1996). In addition to enhancing qE, zeaxanthin pool size is associated with qZ, or 356
zeaxanthin-dependent quenching (Nilkens et al., 2010), which arises on a slower time scale (10-15 357
minutes) than qE and qT. qI is the slowest onset process, requiring up to hours to appear, and is often 358
associated with accumulation of photodamage to photosystem II (Müller et al., 2001). 359
Due to its complexity, manipulations of single facets of NPQ have not been sufficient for 360
improving the relaxation kinetics of NPQ while maintaining full capacity for CO2 assimilation at high 361
light. Overexpression of PsbS increases induction and relaxation rates and increases the maximum 362
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capacity of qE (Li et al., 2002a; Zia et al., 2011; Hubbart et al., 2012), representing a potential benefit to 363
plant biomass under stressful light conditions (Li et al., 2002b) but at a cost to CO2 assimilation under less 364
stressful conditions as qE remains higher than needed (Hubbart et al., 2012). While an increase in 365
zeaxanthin concentration can improve stress resistance (Johnson et al., 2007), it slows the kinetics of 366
NPQ, demonstrating that zeaxanthin concentration is less important for induction/relaxation rates than the 367
ratio of zeaxanthin to violaxanthin, or the de-epoxidation state of the xanthophyll cycle (Johnson et al., 368
2008). A lingering pool of zeaxanthin, which increases the de-epoxidation state, may serve as a ‘memory’ 369
of previous high light conditions (Murchie et al., 2008) that would delay the relaxation rate of NPQ in 370
‘anticipation’ of another high-light event. Overexpression of an ion antiporter helps increase the lumen 371
pH more quickly upon transfer of leaves to low light, thus speeding up the relaxation of qE (Armbruster 372
et al., 2014). However, overexpression also leads to photoinhibitory effects at high light, thus requiring 373
more sophisticated regulation of the protein for increasing relaxation kinetics. To overcome the limits of 374
altering single processes within NPQ, a recent study by Kromdijk et al. (2016) used a multi-target 375
approach to increase NPQ kinetics by transforming tobacco to overexpress PsbS, violaxanthin de-376
epoxidase, and zeaxanthin epoxidase. The overexpression of violaxanthin de-epoxidase and zeaxanthin 377
epoxidase increased the kinetics of the xanthophyll cycle, which led to a lower de-epoxidation state, while 378
overexpression of Psbs maintained the amplitude of qE in high light when the de-epoxidation state was 379
low. When grown in chambers under fluctuating light, the transformants show increased photosynthetic 380
efficiency and reduced average NPQ compared to the wild type. Moreover, this more complex approach 381
of simultaneously altering the expression of multiple enzymes related to NPQ leads to a 15% increase in 382
plant biomass when grown in field conditions under natural light fluctuations (Kromdijk et al., 2016). 383
Since the mechanisms of NPQ are conserved across most plant species, altering NPQ kinetics will likely 384
lead to increased yields in many other crops. 385
386
C4 photosynthesis: adding complexity under fluctuating light 387
While the processes described above may apply to both C3 and C4 plants, the unique metabolism 388
of C4 plants may further impact their response to the fluctuating light of field conditions. Plants 389
performing C4 photosynthesis are among the most important crop species globally due to the high 390
efficiency with which they can capture and reduce CO2 with decreased water use and nitrogen investment 391
in Rubisco. C4 crops, such as maize, sugarcane, millet, and sorghum, achieve higher carbon fixation rates 392
by first carboxylating phosphoenolpyruvate (PEP) with atmospheric CO2 to form oxaloacetate via PEP 393
carboxylase (PEPc; Fig. 3A). The carbon in oxaloacetate is then either reduced with NADPH into malate 394
or converted into aspartate before moving into the bundle sheath cell where either substrate is 395
decarboxylated. Thus, the concentration of CO2 around Rubisco increases, which minimizes 396
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13
photorespiration in the coupled C3 cycle. To maintain the cycle, reduced carbon must be transported from 397
the bundle sheath cells back into the mesophyll cells to replace the carbon transported by the original C4 398
carbonic acid. This return transport can occur directly via pyruvate produced following malate 399
decarboxylation, via alanine produced from pyruvate through alanine aminotransferase, and/or via the 400
shuttling of 3-phosphoglycerate from the C3 cycle in the bundle sheath cells. PEP is then regenerated 401
from pyruvate at the cost of ATP, which imposes an additional energetic cost to C4 metabolism. C4 402
species have traditionally been classified by the different decarboxylating enzymes employed, i.e. NADP-403
ME, NAD-ME, and PEP-CK, but there is a growing consensus that C4 plants like maize use multiple 404
decarboxylation pathways in parallel (Pick et al., 2011; Arrivault et al., 2017). However, there are few 405
examples of a crop plant exclusively using PEP-CK as a decarboxylation enzyme, which suggests PEP-406
CK predominantly functions as a supplemental pathway supporting NADP-ME and NAD-ME 407
decarboxylation (Wang et al., 2014a). Thus, only the NADP-ME and NAD-ME pathways are presented 408
here (Fig. 3A). 409
The efficiency of C4 photosynthesis depends on the coordination of C4 carbonic acid production 410
within the mesophyll cells with C3 carbon reduction within the bundle sheath cells. There is ample 411
theoretical and experimental evidence that indicates this coordination is impacted to some degree by 412
variations in light regimes. For example, during high to low light transitions, accumulated malate and/or 413
aspartate are still available for decarboxylation in the bundle sheath cell, but there is insufficient 414
photophosphorylation to generate the ATP necessary for C3 carbon fixation. This could result in a 415
transient over-pumping of CO2 into the bundle sheath cells and subsequent leaking of the CO2 back into 416
the mesophyll cells (Fig. 3B). This leakiness is energetically costly since PEP must still be regenerated at 417
the cost of ATP without the usual fixation of carbon (von Caemmerer and Furbank, 1999; Sage and 418
McKown, 2006). Transient increases in leakiness are reported in many C4 plants in response to low light 419
shifts (Cousins et al., 2006; Kubásek et al., 2007; Cousins et al., 2008; Tazoe et al., 2008; Pengelly et al., 420
2010), especially under light values lower than 100 µmol m-2 s-1 (Kromdijk et al., 2010; Bellasio and 421
Griffiths, 2014a), although it should be noted that the 13CO2 discrimination assumptions used to interpret 422
earlier reports can overstate the impact of transient light to leakiness (Ubierna et al., 2011; Ubierna et al., 423
2013; Kromdijk et al., 2014). While leakiness in the C4 carbon pump results in inefficient carbon fixation 424
and transient decreases in the quantum efficiency of C4 carbon fixation, these losses appear small. But 425
what about low to high light transitions? 426
The coordination of C4 and C3 cycles is complicated during the transition from low to high light 427
due to the reliance of C4 acid transport into the bundle sheath cell on concentration gradients (Fig. 3C). 428
Transport of C4 acids into bundle sheath cells is thought to occur mainly via diffusion along concentration 429
gradients between the mesophyll and bundle sheath cells, which need to be a minimum of 5-10 mM to 430
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14
facilitate rapid transport in C4 metabolism (Hatch and Osmond, 1976). Gradients between the two cell 431
types have been determined experimentally for malate in maize and range between 6 and 88 mM 432
(Leegood, 1985; Stitt and Heldt, 1985; Arrivault et al., 2017), indicating that large active C4 acid pools 433
are required. Therefore, during a low to high light transition, these large C4 acid pools must accumulate to 434
optimal values before the C4 cycle and C3 cycle are synchronized. Before optimal pool sizes are 435
established for a given light intensity, sub-optimal CO2 concentrations near Rubisco would increase rates 436
of oxygenation, leading to increased photorespiration and thus incurring the double costs of C4 carbon 437
pumping and C3 photorespiration (Fig. 3C; de Veau and Burris, 1989; von Caemmerer and Furbank, 438
2003; Sage and McKown, 2006; Bellasio and Griffiths, 2014b). The transport mechanisms returning C3 439
acids into the mesophyll following decarboxylation in the bundle sheath cell are less clear. Alanine and 440
triose-phosphate seem to require modest gradients between the mesophyll and bundle sheath cell to 441
facilitate return, but pyruvate shows small to even reverse gradients between the two cell types (Arrivault 442
et al., 2017). 443
444
C4 metabolic pool sizes in fluctuating light: a tale of two hypotheses 445
The reliance of malate and aspartate on gradient-driven transport and the pool sizes that enable 446
this transport lead to two potentially contradictory hypotheses concerning the resilience of C4 447
photosynthesis during low to high light transitions. One hypothesis, as suggested above, is that C4 448
photosynthesis is more negatively impacted by fluctuating light than C3 photosynthesis due to transient 449
incoordination between the C3 and C4 cycles occurring while transport gradients either collapse or re-450
establish. An alternative hypothesis suggests that the large pool sizes are beneficial from a 451
photoprotective role. Under this hypothesis, C4 metabolism can store excess harvested ATP and NADPH 452
within the large pools of carbon shuttling intermediates without having to resort to NPQ during transient 453
increases in light energy capture, resulting in a higher quantum yield of photosynthesis and effectively 454
using C4 intermediates transiently as alternative electron acceptors or donors for the coupled C3 cycle 455
(Stitt and Zhu, 2014). The evidence for these contrasting hypotheses is discussed below. 456
There are several lines of experimental evidence indicating C4 photosynthesis is less resilient than 457
C3 photosynthesis to rapid fluctuations in light. For example, C4 species relax their photosynthetic 458
capacity more rapidly under decreasing light and take longer to reach high rates of photosynthesis under 459
return to high light, resulting in a decreased ability of C4 plants to rapidly re-induce photosynthesis in 460
response to sunflecks (Chazdon and Pearcy, 1986; Horton and Neufeld, 1998; Sage and McKown, 2006). 461
This is consistent with a requirement to first re-establish large malate or aspartate pools upon illumination 462
to achieve optimal transport of carbon into the bundle sheath. While this may explain why examples of 463
understory or shade-tolerant C4 plants are few, it also suggests that the shaded regions within C4 crop 464
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15
canopies would be less able to take advantage of rapidly fluctuating light regimes and frequent sunflecks 465
that can occur in lower C4 canopies (Tang et al., 1988). This relative difference is also seen in the slower 466
response of maize leaves to simulated sunflecks as compared to soybean (Pons and Pearcy, 1992; Krall 467
and Pearcy, 1993). A direct comparison of C3 and C4 responses to fluctuating light has also shown that 468
plant biomass is reduced to a greater degree in C4 plants (58% reduction) as compared to C3 plants (30-469
51% reduction) when grown under dynamic light conditions (Kubásek et al., 2013). Additionally, C4 470
photosynthetic rates decrease during low-light periods in fluctuating light environments compared to 471
steady-state light conditions, whereas C3 photosynthetic rates increase, and dynamic light regimes 472
decrease the photochemical efficiency of C4 plants more than C3 plants due in part to C4 leakiness, which 473
also increases under dynamic light conditions (Kubásek et al., 2013). 474
Given the above observations, can the large metabolite pools involved in C4 photosynthesis act to 475
buffer energy supply and demand during fluctuating light in a way that confers any advantage to these 476
crop species? While there are many potential advantages to the newly recognized flexibility of C4 477
photosynthesis to utilize various metabolites with different energy balances (Stitt and Zhu, 2014), we 478
focus here on malate transport, since we have the most data concerning its presence, specifically to 479
estimate the total electron buffering capacity of malate and its effective time scale. The total active malate 480
pool was recently reported as ~5 mol g-1 fresh weight (Arrivault et al., 2017), which translates to 1,500 481
mol m-2 assuming a fresh weight specific leaf area of 300 g m-2. Since each malate carries the reductive 482
power of two electrons and four electrons are required to reduce one CO2 molecule, this malate pool 483
contains enough reductive power to reduce ~750 mol CO2 m-2, or enough reductant to support the 484
electron demands of carbon fixation occurring at a rate of 50mol m-2 s-1 for 15 seconds. Thus, the active 485
pool of malate could contain a sufficient supply of reductant to buffer against transitions from high to low 486
light, perhaps explaining why greater leakiness is not observed during these transitions as discussed 487
above. Of course, the estimate of 15 seconds is an upper boundary for the ability of the active malate pool 488
to supply energy to the C3 cycle during high to low light transitions since malate transport would slow as 489
pool sizes within the mesophyll and bundle sheath cell come into equilibrium and C3 carbon fixation 490
becomes limited by ATP availability. 491
Using the same logic, we can estimate the potential of C4 metabolite pools to buffer against rapid 492
shifts from low to high light by acting as electron sinks by determining the immediate capacity for 493
carboxylation and reduction. The total active pool size of metabolites upstream of malate (only PEP, 494
pyruvate, alanine and 3-PGA; oxaloacetate was not determined) presented in Arrivault et al., (2017) is 3.6 495
mol g-1 or ~1,000mol m-2. This represents the capacity to provide alternative electron acceptors for 496
~2,000 electrons while the C3 cycle activates and malate pools establish, or the capacity to provide 10 497
seconds of alternative electron acceptors assuming an electron transport rate of 200 e- m-2 s-1 during a low 498
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16
to high light transition without having to initiate NPQ in the bundle sheath. Again, this represents an 499
upper boundary to the possible buffering capacity since all upstream metabolites would not be 500
immediately transported into the bundle sheath and converted to oxaloacetate. Naturally, both calculations 501
above depend upon the light conditions of Arrivault et al. (2017), which were moderate (~500 mol 502
photons m-2 s-1). However, these calculations serve as initial estimates of the impact gradient formation 503
and pool sizes can have on C4 photosynthesis during light transitions. 504
Given the above calculations, the buffering capacity of C4 photosynthesis during light 505
fluctuations is expected to be limited to, at maximum, the first 10-15 seconds of a transition between light 506
intensities. This is a much faster timescale than the minute timescales examined in 13CO2 discrimination-507
based measurements of leakiness (Cousins et al., 2006; Kubásek et al., 2007; Cousins et al., 2008; Tazoe 508
et al., 2008; Pengelly et al., 2010) or growth analysis (Kubásek et al., 2013) discussed above but on the 509
order of sunfleck light availability (Pons and Pearcy, 1992; Krall and Pearcy, 1993). We therefore argue 510
that C4 photosynthesis may be made both more and less resilient to dynamic light regimes, depending on 511
the frequency at which these light fluctuations occur. We propose that a more systematic examination of 512
the response of C4 photosynthesis to a range of dynamic light frequencies would help resolve the 513
interaction between C4 photosynthesis and dynamic light. 514
515
Conclusions 516
To date, the majority of studies on crop photosynthesis have been investigated in steady-state 517
conditions, but more recent research has been trending toward exploring the responses of photosynthesis, 518
especially in field-grown crops, to dynamic conditions. With this shift in emphasis has come the 519
realization of the importance of the efficiency of photosynthesis in fluctuating light to the overall 520
efficiency of canopy photosynthesis. The studies incorporating fluctuating light conditions have shown a 521
lagging response of photosynthesis to both increasing and decreasing light intensity, presenting a 522
significant limitation to crop productivity that at the same time reveals opportunities for improving 523
performance. Going forward, it is clearly important to incorporate measurements of non-steady state 524
photosynthesis in the experimental design of field experiments and into system models of crop growth 525
and performance. This may be even more important when assessing photosynthetic performance under 526
both dynamic light and future climate conditions. The effects of elevated CO2 concentration on carbon 527
gain in fluctuating light have been studied in limited species and show conflicting results (Tomimatsu and 528
Tang, 2016; Kaiser et al., 2017), and the interaction of elevated CO2 with other factors, such as increased 529
temperature and altered precipitation patterns, remain to be studied. Therefore, research in artificial light 530
environments must either attempt to simulate dynamic light conditions similar to those experienced in the 531
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17
field or acknowledge the potential biases that may be present in experimental results from steady-state 532
light conditions (Vialet-Chabrand et al., 2017). 533
534 Figure Legends 535
Figure 1. Dynamics of light intensity above and within crop canopies. A) Maximum solar energy incident 536
upon a canopy over the course of a year at 50°N. B) Light intensity at the top of a canopy on a clear 537
sunny day (black line) and on a day with intermittent cloud cover (gray line). C) Light reaching a mid-538
canopy leaf on a clear sunny day. Figures are based on B) SURFRAD data from Bondville, IL, USA 539
(40°N latitude) and C) Zhu et al. (2004). 540
541
Figure 2. Schematic showing the relative induction and relaxation rates of photosynthesis-related 542
processes during changes in light intensity. Relative rates of A) photosynthesis, B) stomatal conductance, 543
C) C3 cycle enzymes, and D) non-photochemical quenching are shown as a function of time during 544
transitions from low light (gray background) to high light (white background) and high light to low light. 545
Curves are based on data from McAusland et al. (2016), Sassenrath-Cole and Pearcy (1994), and 546
Kromdijk et al. (2016). 547
548
Figure 3. Schematic showing relevant components of C4 biochemistry and the theoretical impacts of light 549
fluctuations on C4 and C3 cycle coordination. A) A simplified diagram shows two types of C4 pathways, 550
NADP-ME and NAD-ME. The PEP-CK pathway is absent. B) A high to low light transition results in a 551
transient over pumping of CO2 into and subsequent CO2 leakage out of the bundle sheath. C) A low to 552
high light transition results in higher rates of Rubisco oxygenation. The relative substrate size represents 553
the concentration gradient between the mesophyll and bundle sheath cells based on measured data (see 554
text). Metabolites are black, enzymes are blue, and cofactors are gray. Abbreviations: 555
phosphoenolpyruvate (PEP), oxaloacetate (OA), aspartate (Asp), malate (M), pyruvate (Pyr), alanine 556
(Ala), PEP carboxylase (PEPC), NADP-malate dehydrogenase (NADP-MDH), Asp aminotransferase 557
(AspAT), NAD malic enzyme (NAD-ME), Ala aminotransferase (AlaAT), and Pyr phosphate dikinase. 558
559 560
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875
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Figure 1. Dynamics of light intensity above and within crop canopies. A) Maximum solar energy incident upon a canopy over the course of a year at 50°N. B) Light intensity at the top of a canopy on a clear
sunny day (black line) and on a day with intermittent cloud cover (gray line). C) Light reaching a mid-
canopy leaf on a clear sunny day. Figures are based on B) SURFRAD data from Bondville, IL, USA
(40°N latitude) and C) Zhu et al. (2004).
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Figure 2. Schematic showing the relative induction and relaxation rates of photosynthesis-related processes during changes in light intensity. Relative rates of A) photosynthesis, B) stomatal conductance,
C) C3 cycle enzymes, and D) non-photochemical quenching are shown as a function of time during
transitions from low light (gray background) to high light (white background) and high light to low light. Curves are based on data from McAusland et al. (2016), Sassenrath-Cole and Pearcy (1994), and
Kromdijk et al. (2016).
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Figure 3. Schematic showing relevant components of C4 biochemistry and the theoretical impacts of light
fluctuations on C4 and C3 cycle coordination. A) A simplified diagram shows two types of C4 pathways,
NADP-ME and NAD-ME. The PEP-CK pathway is absent. B) A high to low light transition results in a
transient over pumping of CO2 into and subsequent CO2 leakage out of the bundle sheath. C) A low to
high light transition results in higher rates of Rubisco oxygenation. The relative substrate size represents
the concentration gradient between the mesophyll and bundle sheath cells based on measured data (see
text). Metabolites are black, enzymes are blue, and cofactors are gray. Abbreviations:
phosphoenolpyruvate (PEP), oxaloacetate (OA), aspartate (Asp), malate (M), pyruvate (Pyr), alanine
(Ala), PEP carboxylase (PEPC), NADP-malate dehydrogenase (NADP-MDH), Asp aminotransferase
(AspAT), NAD malic enzyme (NAD-ME), Ala aminotransferase (AlaAT), and Pyr phosphate dikinase
(PPDK).
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ADVANCES
• Previously used steady-state conditions for measurements and modeling are inadequate for evaluating crop photosynthesis and productivity in field conditions. The limitations to crop photosynthetic efficiency in fluctuating light present multiple opportunities for study and potentially substantial improvement in productivity.
• Manipulating regulatory enzymes, such as Rca and the enzymes involved in the xanthophyll cycle, present potential promising candidates for improving photosynthesis regulation in fluctuating light.
• Unsuccessful attempts to manipulate stomatal kinetics through single-gene mutations and recent improvements in NPQ relaxation via simultaneous manipulation of several pathway genes imply that single-gene targets are less plausible for improving photosynthetic kinetics in dynamic light conditions.
• C4 plants are more sensitive than C3 plants to dynamic light conditions, possibly due to the additional complexity of coordinating the bundle sheath C3 and mesophyll C4 cycles, but there is theoretical evidence that C4 plants may be more resilient under certain frequencies of light fluctuation due to the buffering capacity of transport metabolites.
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OUTSTANDING QUESTIONS
• Can the characteristics and/or mechanisms associated with more rapid stomatal kinetics in dumbbell-shaped guard cells be implemented into elliptical-shaped guard cells? How is mesophyll conductance regulated in fluctuating light? Can it be improved upon?
• Identification of the proteins involved in thioredox regulation of C3 cycle enzymes is underway, but what is the mechanism of enzyme activation/deactivation? Is thioredoxin involved in the oxidation mechanism, and can these be improved upon for faster upregulation and downregulation of photosynthesis in dynamic light conditions?
• Are C4 plants underutilizing the ability of intercellular metabolite pools to buffer photosynthesis during dynamic light conditions? If so, how can this potential be realized to improve crop photosynthetic efficiency and productivity?
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