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1 Title: The impacts of fluctuating light on crop performance 1 Short title: Impacts of fluctuating light on crops 2 3 Rebecca A. Slattery 1,2 , Berkley J. Walker 3 , Andreas P. M. Weber 3 , Donald R. Ort 1,2,4,* 4 1 Global Change and Photosynthesis Research Unit, Agricultural Research Service, United States 5 Department of Agriculture, Urbana, IL, USA 6 2 Carl R. Woese Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL, 7 USA 8 3 Institute of Plant Biochemistry, Cluster of Excellence on Plant Sciences (CEPLAS), Heinrich-Heine- 9 University Düsseldorf, Düsseldorf, Germany 10 4 Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL, USA 11 *Address correspondence to [email protected] 12 13 Author contributions: All authors contributed to the intellectual input and writing of the manuscript. 14 15 Summary: Recent advances in understanding photosynthetic responses to dynamic light environments 16 reveal opportunities to improve crop plant photosynthetic efficiency. 17 18 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 www.plantphysiol.org on February 17, 2020 - Published by Downloaded from Copyright © 2017 American Society of Plant Biologists. All rights reserved.

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Page 1: Summary: Recent advances in understanding …...16 Summary: Recent advances in understanding photosynthetic responses to dynamic light environments ... 84 As compared to forests, crop

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Title: The impacts of fluctuating light on crop performance 1

Short title: Impacts of fluctuating light on crops 2

3

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

13

Author contributions: All authors contributed to the intellectual input and writing of the manuscript. 14

15

Summary: Recent advances in understanding photosynthetic responses to dynamic light environments 16 reveal opportunities to improve crop plant photosynthetic efficiency. 17

18

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

40

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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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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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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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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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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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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