light sheet microscopy imaging of light absorption and photosynthesis distribution in plant tissue

1

Short title: Imaging of light absorption and photosynthesis 1

Corresponding authors: 2

Mads Lichtenberg, [email protected]; Michael Kühl, [email protected] 3

4

Light sheet microscopy imaging of light absorption and photosynthesis 5

distribution in plant tissue 6

7

Mads Lichtenberg1,a,b

, Erik C. L. Trampe1,a

, Thomas C. Vogelmann2 and Michael Kühl

1,3,b 8

9

1 Marine Biological Section, Department of Biology, University of Copenhagen, 10

Strandpromenaden 5, 3000 Helsingør Denmark 11

2 Department of Plant Biology, University of Vermont, 12

63 Carrigan Drive, Burlington, VT, USA 13

3 Climate Change Cluster (C3), University of Technology Sydney, 14

15 Broadway, Ultimo, Sydney, NSW 2007, Australia 15

a These authors contributed equally to this work 16

b Corresponding authors 17

18

19

Author contributions: ML, ET, TV and MK designed the research; ML and ET performed 20

experiments; TV and MK contributed new analytical tools; ML, ET, TV and MK analyzed data; 21

ML and ET wrote the article with contributions from TV and MK. 22

23

Funding: This study was supported by a Sapere-Aude Advanced grant from the Danish Council for 24

Independent Research ǀ Natural Sciences (MK), and grants from the Carlsberg Foundation (MK). 25

26

One sentence summary: Fine scale characterization of light absorption and photosynthesis across 27

plant tissue sections show that quantum yields of PSII are highly affected by tissue light gradients. 28

Plant Physiology Preview. Published on August 18, 2017, as DOI:10.1104/pp.17.00820

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2

Abstract 29

In vivo variable-chlorophyll-fluorescence measurements of PSII quantum yields in optically dense 30

systems are complicated by steep tissue light gradients due to scattering and absorption. 31

Consequently, externally measured effective PSII quantum yields may be composed of signals 32

derived from cells differentially exposed to actinic light, where cells located deeper inside tissues 33

receive lower irradiance than cells closer to the surface, and can display distinct photophysiological 34

status. We demonstrate how measured distributions of PSII quantum yields in plant tissue change 35

under natural tissue light gradients as compared to conventionally measured quantum yields with 36

even exposure to actinic light. This was achieved by applying actinic irradiance perpendicular to 37

one side of thallus cross-sections of the aquatic macrophyte Fucus vesiculosus L. with laser light-38

sheets of defined spectral composition, while imaging variable-chlorophyll-fluorescence from 39

cross-sections with a microscope-mounted pulse-amplitude-modulated (PAM) imaging system. We 40

show that quantum yields are highly affected by light gradients and that traditional surface-based 41

variable-chlorophyll-fluorescence measurements result in substantial under- and/or over-42

estimations, depending on incident actinic irradiance. We present a method for using chlorophyll 43

fluorescence profiles in combination with integrating sphere measurements of reflectance and 44

transmittance to calculate depth-resolved photon absorption profiles, which can be used to correct 45

apparent PSII electron transport rates to photons absorbed by PSII. Absorption profiles of the 46

investigated aquatic macrophyte were different in shape from what is typically observed in 47

terrestrial leaves, and based on this finding we discuss strategies for optimizing photon absorption 48

via modulation of the structural organization of phytoelements according to in situ light 49

environments. 50

51



3

Keywords: absorption profile, chlorophyll fluorescence, laser sheet microscopy, light attenuation, 52

photosynthesis, quantum yield. 53



4

Introduction 54

Estimating photosynthetic parameters using variable chlorophyll fluorescence techniques has 55

become increasingly popular due to its ease of use and non-invasive nature. The basic fluorescence 56

signals of ‘open’ or ‘closed’ reaction centers (F and Fm, respectively) change according to actinic 57

irradiance and are powerful monitors of the status and activity of the photosynthetic apparatus 58

(Baker, 2008). Most measurements of variable chlorophyll fluorescence in complex plant tissues, 59

and in other surface-associated cell assemblages like biofilms and sediments, rely on external 60

measurements with fiber-optic or imaging fluorimeters under the assumptions that i) different cells 61

are subjected to the same amount of measuring light and actinic irradiance, ii) that saturating pulses 62

are indeed saturating all cells, and iii) that the fluorescence detected is emitted equally from all 63

sampled cells (Serodio, 2004). These assumptions are influenced by the optical density of the 64

sample where optical dilute refers to a negligible or only moderate light attenuation through a 65

sample (e.g. a dilute algal suspension or plant tissue with only a few cell layers), while optically 66

dense samples such as algal biofilms and thicker plant tissues absorb all, or most of, the incident 67

light. As a result the assumptions are usually valid in optically dilute samples (Klughammer and 68

Schreiber, 2015), whereas steep light gradients in densely pigmented tissues or algal biofilms will 69

distort the measurements of maximal and effective PSII quantum yields. Cells located deeper inside 70

tissues will receive less actinic irradiance than cells close to the surface. Thus, externally integrated 71

measurements of variable chlorophyll fluorescence contain a complex mixture of signals originating 72

from different layers in the structure exposed to different levels of measuring and actinic light, and 73

the actual operational depth of such measurements remains unknown. This inherent limitation of 74

such measurements can e.g. lead to light-dependent overestimations of effective PSII quantum 75

yields of up to 40% e.g. in microphytobenthic assemblages (Serodio, 2004). 76



5

Previous efforts to describe the internal gradients of photosynthetic efficiencies have used 77

microfiber based pulse amplitude modulation (PAM) techniques (Schreiber et al., 1996) revealing 78

distinct differences between such internal and external variable chlorophyll fluorescence 79

measurements (Oguchi et al., 2011). Another challenge is to quantify the internal light gradients to 80

estimate the total actinic light exposure in different tissue layers, i.e. the scalar irradiance. The 81

scalar component becomes increasingly important in deeper tissue layers as light becomes 82

progressively more diffuse due to multiple scattering (Kühl and Jørgensen, 1994). This can be 83

measured with fiber optic scalar irradiance microprobes (Kühl, 2005; Rickelt et al., 2016), which 84

collect light isotropically via a small (30-150 µm wide) spherical tip cast on the end of a tapered 85

optical fiber. Such measurements enabled estimates of internal rates of PSII electron transport 86

corrected for the specific tissue light gradients in corals and plants (Lichtenberg and Kühl, 2015; 87

Lichtenberg et al., 2016). However, to obtain absolute electron transport rates (ETR) through PSII, 88

it is necessary to know the absorption factor, which describes the PSII absorption cross-section and 89

the balance between PSI and PSII photochemistry, and these parameters cannot be calculated from 90

measurements of light availability. In addition, due to the small tip size of fiber optic radiance 91

microprobes (usually <50 µm) used to detect the fluorescence, microfiber-based measurements of 92

variable chlorophyll fluorescence are also prone to reflect the natural heterogeneity of such systems 93

(Lichtenberg and Kühl, 2015; Lichtenberg et al., 2016). A method was recently proposed for 94

calculating absolute electron turnover rates of PSII, but the approach was limited to surface 95

measurements or optically thin systems (Szabó et al., 2014). It is thus of great importance to further 96

explore how steep gradients of light influence photosynthetic efficiencies in complex 97

photosynthetic tissues and surface associated phototrophic communities. 98

Internal gradients of light absorption have been quantified from fluorescence profiles in terrestrial 99

leaves (Takahashi et al., 1994; Vogelmann and Han, 2000; Slattery et al., 2016) and this technique 100



6

has been combined with fine scale measurements of CO2 fixation to investigate the relationship 101

between chlorophyll concentration, light absorption and photosynthesis at high spatial resolution 102

(Vogelmann and Evans, 2002; Evans and Vogelmann, 2003). These studies generally found a good 103

correlation between the light absorption of different spectral ranges, and the associated CO2 fixation 104

profiles. However, the CO2 fixation rates relied on freeze clamping 14

CO2 pre-incubated leaf 105

samples with concomitant paradermal sectioning, and measurements by scintillation counting, 106

which is a laborious process that is limited in the spatial resolution by the sectioning process to ~40 107

µm (Vogelmann and Evans, 2002). Here we present a novel experimental approach and show its 108

application for mapping gradients of light absorption and photosynthesis in aquatic plant tissue. 109

The lower community photosynthesis often observed in aquatic systems as compared to terrestrial 110

systems (Sand-Jensen and Krause-Jensen, 1997) can be largely explained by the inability of aquatic 111

macrophytes to obtain an optimal 3D structural organization in relation to the incident irradiance 112

(Binzer and Sand-Jensen, 2002b, a), unlike their terrestrial counterparts that e.g. can regulate leaf 113

inclination to increase canopy light utilization (McMillen and McClendon, 1979; Myers et al., 114

1997). In addition, specialized cell/tissue structures in terrestrial plants can increase photon 115

absorption, e.g. in sun-adapted leaves with well-developed palisade cells that can act as light 116

funnels directing light into the photosynthetically active mesophyll layer (Vogelmann and Martin, 117

1993), while some shade-adapted understory plants can alleviate light-limitation by focusing light 118

in the mesophyll layer via plano-convex epidermal cells and intercellular air spaces (Vogelmann et 119

al., 1996; Brodersen and Vogelmann, 2007). In contrast, most macroalgae are not recognized to 120

have specialized tissue structures to facilitate penetration of light, although there has been reports of 121

light guides in some green algae (Ramus, 1978). 122

Macroalgal members of the Fucales have morphological differentiated tissues such as the basal 123

thallus, the growing sterile frond, and fertile receptacles, while cells are differentiated into 124



7

meristoderm, cortex, and medullary layers on the tissue scale (Garbary and Kim, 2005). While all 125

cell types contain plastids (Moss, 1983), the outer meristoderm and cortex cells contain more 126

chloroplasts and thylakoids than the medullary filaments. It has been suggested that the medullary 127

filaments could play a role in longitudinal translocation of materials (Moss, 1983; Raven, 2003), 128

and further that they may play a structural role in providing elasticity in terms of a ‘cushion-like’ 129

effect protecting against wave action (Moss, 1983). The medulla layer is surrounded on both sides 130

by anatomically similar layers of cortex, meristoderm and epidermis cells (henceforth referred to as 131

cortex), in contrast to e.g. bifacial terrestrial plant leaves that display morphologically and 132

physiologically differentiated abaxial- and adaxial domains. In Fucus, steep gradients of light and 133

photosynthesis have been measured using fiber-optic microprobes and microelectrodes, although 134

this approach is rather challenging in such cohesive tissues (Spilling et al., 2010; Lichtenberg and 135

Kühl, 2015). 136

In this study, we aimed to resolve how photosynthetic efficiencies are affected by steep light 137

gradients in different spectral regions. This was accomplished by the use of a novel multicolor laser 138

light sheet microscopy setup to image the distribution of light absorption and photosynthetic 139

activity over transverse sections of an aquatic macrophyte to resolve how photosynthetic 140

efficiencies are affected by steep light gradients in different spectral regions. We applied laser light-141

sheets of defined spectral composition perpendicular to one side of thallus cross-sections while 142

imaging the distribution of chlorophyll fluorescence and variable-chlorophyll-fluorescence from the 143

cut surface. We compared such data with measurements obtained with equal illumination of the 144

cross-section to describe for the first time how PSII quantum yields are affected by natural light 145

gradients in optically dense tissues. This novel method can resolve such gradients routinely and 146

with higher resolution as compared to other microscale approaches such as mapping with fiber-147

optic probes (Kühl and Jørgensen, 1994; Lichtenberg et al., 2016). 148



8

149



9

Results 150

Cross-thallus chlorophyll fluorescence profiles 151

Using a novel microscopic setup (Fig. 1; see further details in the Methods section), we used both 152

even illumination of plant tissue cross-sections and illumination with a laser-sheets of defined 153

spectral composition incident perpendicularly on tissue cross sections. When illuminated 154

homogeneously across the algal thallus cross-section, both cortex layers of F. vesiculosus displayed 155

an equally high amount of chlorophyll that was 2.5-5 fold higher than in the central medulla (Fig. 2 156

and Fig. 3), assuming that relative chlorophyll content can be estimated from fluorescence using 157

epi-illumination (Vogelmann and Evans, 2002). The fluorescence profiles under light sheet 158

illumination perpendicular to one side of the cross-section showed, that blue light (425-475 nm) 159

was attenuated strongest in an exponential manner with depth and decreased to <21% of the 160

maximum fluorescence (F(max)) ~250 µm inside the thallus (Fig. 3). Fluorescence profiles over the 161

thallus cross-section using green (525-575 nm), and red (615-665 nm) light showed similar 162

attenuation but decreased to a minimum fluorescence >2 times higher than was found for blue light 163

at a similar depth in the thallus. Blue, green and red light induced fluorescence profiles all displayed 164

F(max) values close to the thallus surface. When using broadband white light illumination, a peak 165

was located at the same position as the F(max) of the blue, green and red profiles followed by an 166

intermittent decrease before reaching F(max) ~100 µm inside the thallus (Fig. 3). Common for all 167

profiles was that the fluorescence showed a peak close to the illuminated cortex followed by a 168

decrease towards the center of the medulla before increasing again towards the shaded cortex. The 169

relative largest increase towards the shaded thallus side was in the order of blue < red < < green < 170

white. The width of the peaks was of similar size and extended 150-200 µm from the surfaces 171

towards the center of the thallus (Fig. 3). 172

173



10

174

Absorption profiles 175

Integrating sphere measurements of thallus reflectance, transmittance and absorptance displayed 176

typical characteristics for densely pigmented opaque plant tissues (Fig. 4). Reflectance was 177

relatively uniform at ~3% of the incident irradiance, although slightly higher in the green/yellow 178

part of the spectrum (around 570 nm). Absorptance spectra (Fig. 4 and S1) showed in vivo 179

absorption peaks from major photopigments present in brown macroalgae, e.g., Chl a (440 and 675 180

nm (Johnsen et al., 1994)), Chl c (460; 590; 635 nm (Shibata and Haxo, 1969; Kühl et al., 1995)), 181

fucoxanthin (in vitro absorption peaks in hexane at 425, 450, 475 nm; extends to 580nm in vivo 182

(Govindjee and Braun, 1974)) and other carotenoids (400-540 nm (Govindjee and Braun, 1974)). 183

The mean absorptance averaged over PAR (400-700 nm) using broadband white light was 92% of 184

the incident irradiance. Transmittance was highest (10-13%) in the green/yellow part (around 570 185

nm) of the spectrum and was close to zero in the blue and red spectral regions, while the mean 186

transmittance was 5% of the incident irradiance (Fig. 4). 187

By normalizing the chlorophyll fluorescence profiles (Fig. 3) to the total absorption measured for 188

blue, green, red, and white light with an integrating sphere (Fig. 4) we could calculate the depth of 189

specific photon absorption inside the thallus (Fig. 4; see also Fig. S3). The different thallus regions 190

(cortex/medulla) were estimated to be on average 150 µm in thickness (Fig. 3). When illuminating 191

the thallus with the laser sheet, the apparent absorption of photons was always highest in the upper 192

and lower cortex as compared to the medulla, where the fractional absorption was lowest (Fig. 4; 193

Table 1). 194

We modelled the light availability in the F. vesiculosus thallus by using measured scalar irradiance 195

attenuation coefficients of cortex and medulla layers from a closely related brown alga F. serratus 196

(Lichtenberg and Kühl, 2015) assuming monoexponential attenuation of light in the thallus (Fig. 5; 197



11

see text in Materials and Methods). These modelled curves of light attenuation were compared with 198

curves of attenuation due to absorption found in this study (Fig. 5) to test if the found absorption 199

profiles were in the same order as the attenuation profiles, as would be expected for such densely 200

pigmented systems. We found an average light attenuation coefficient of 5.64 mm-1

(R2=0.97) over 201

the entire thallus, with higher attenuation coefficients in the cortex layers (upper cortex = 6.0 mm-1

202

(R2=0.99); lower cortex = 9.8 mm

-1 (R

2=0.96)) than in the medulla layer (4.3 mm

-1 (R

2=0.99)) (Fig. 203

5). These values were in the same order as the light attenuation coefficients of cortex and medulla 204

layers in F. serratus (6.8 mm-1

and 3.4 mm-1

for cortex and medulla, respectively (Lichtenberg and 205

Kühl, 2015)), suggesting that the distribution of photon absorption can be found by combination of 206

chlorophyll fluorescence profiles and measurements of total absorption. 207

208

PSII quantum yields and photosynthetic electron transport 209

In the dark-acclimated state, all thallus layers displayed a maximal PSII quantum yield of >0.6 210

indicating no major stress factor on photosynthetic performance due to cutting or sample handing 211

(Fig. S4). When applying actinic irradiance homogeneously over the cross-sections via the build in 212

LEDs of the microscope PAM system (Fig. 1), the effective PSII quantum yield decreased in all 213

thallus layers but more so in the medulla as compared to the cortex layers. The highest decrease was 214

found under high incident irradiance, where the effective PSII quantum yield in the medulla 215

decreased to <0.3 (Fig. 6). This pattern changed under actinic laser sheet illumination of the cross-216

section from one side. While the PSII quantum yield distribution was apparently unaffected by the 217

changed actinic light geometry in the dark-acclimated state and under very low irradiance, the PSII 218

quantum yield decreased rapidly over the illuminated cortex and reached levels of <0.1 under the 219

highest irradiance. Due to the strong light attenuation across the thallus, the PSII quantum yields in 220

the medulla and the shaded cortex layers decreased less than when illuminated homogeneously via 221



12

the Imaging PAM actinic light source, and the effective PSII quantum yield in the shaded cortex 222

remained at levels similar to dark-acclimated states (>0.6) even at the highest irradiance (Fig. 6). 223

Apparent electron transport rates (ETR) through PSII were calculated for the illuminated (upper) 224

cortex, the medulla, and the shaded (lower) cortex, and the rates were corrected for the amount of 225

absorbed photons in the respective tissue layers. In all cases, the ETR rates in the upper- and lower 226

cortex layers were very similar when corrected for absorbed light. The medulla ETR rates were 227

similar to the cortex activity on the sub-saturated part of the ETR vs. irradiance curve but saturated 228

at higher irradiance (Fig. 7). 229

The slope of the ETR vs. absorbed light curve under blue light was lower than for red and white 230

light but reached higher ETR rates (Fig. 7; Table 1). The curves appeared similar under green and 231

red light, although green light yielded a lower slope on the sub-saturated part of the ETR vs. 232

absorption curve. The ETR curves measured under broadband white light appeared qualitatively as 233

a combination of the curves measured under blue, green and red light, where saturation occurred at 234

higher irradiance, similar to the green and red curves, but reaching higher ETR values probably 235

caused by the blue light component. However, the slopes on the subsaturated part of the ETR vs. 236

irradiance curves were significantly different between white light and the average of the blue, green 237

and red curves in all thallus layers (Table 1). The green and red ETR vs. absorbed light curves were 238

similar in appearance in correspondence with their associated absorption profiles that were also 239

similar (Fig. 4 and Fig. 7). Surprisingly, the ETR curves under blue light did not reach saturation 240

and ETR rates in the cortex and medulla layers were very similar except at the highest irradiance, 241

where a decrease in the medulla layer was observed (Fig. 7). Even at the highest irradiance, PSII 242

quantum yields in the cortex layers remained high, and only a small increase in the NPQ was 243

observed (Fig. S4), which could point to better light processing properties of blue light compared 244

green and red light. 245



13

246

247

248

Discussion 249

A novel combination of multi-color light sheet microscopy with variable chlorophyll fluorescence 250

imaging enabled the mapping of light absorption and photosynthetic efficiencies in densely 251

pigmented tissues. By combining well tested methods of integrating sphere measurements and the 252

chlorophyll fluorescence profile technique (Takahashi et al., 1994; Vogelmann and Han, 2000; 253

Slattery et al., 2016) we propose a method for calculating profiles of photon absorption across plant 254

tissue sections, which can be combined with variable chlorophyll fluorescence imaging of 255

photosynthetic efficiency across tissue light gradients. 256

Conversion of PSII quantum yields measured by variable chlorophyll fluorescence to absolute rates 257

of photosynthetic electron transport activity requires precise measurements of i) mean effective 258

PAR, ii) the PSII absorption cross-section, and iii) knowledge about the partitioning between PSI 259

and PSII photochemistry (Klughammer and Schreiber, 2015). While such information can be 260

obtained in dilute suspensions of chloroplasts and microalgae (Klughammer and Schreiber, 2015), 261

to measure these parameters in dense algal solutions, plant tissue and algal biofilms is not trivial 262

(Szabó et al., 2014; Klughammer and Schreiber, 2015). In optically dense systems, light gradients 263

are affected by both multiple scattering and absorption and it is important to take diffuse light into 264

account when quantifying actinic light levels, i.e., by measuring the incident photon flux from all 265

directions with scalar irradiance microprobes (Kühl, 2005). While such sensors have tip diameters 266

down to 30 µm (Rickelt et al., 2016), it is difficult to perform scalar irradiance measurements in 267

thin, cohesive plant tissues, where measurements can be biased by tissue compression due to the 268

physical impact of the microprobe (Spilling et al., 2010; Lichtenberg and Kühl, 2015). The mean 269



14

effective PAR can also be calculated from complex measurements of the angular radiance 270

distribution with field radiance microprobes (Vogelmann and Björn, 1984; Vogelmann et al., 1989; 271

Kühl and Jørgensen, 1994). Alternatively, information on the cell-size distribution, and the inherent 272

optical properties, i.e., the scattering phase function, and the scattering and absorption coefficients 273

allow calculations of PAR gradients, but these parameters are difficult to determine in optically 274

dense media (Privoznik et al., 1978; Berberoglu et al., 2009; Klughammer and Schreiber, 2015), 275

albeit recent experimental and theoretical advances in biomedical optics have allowed detailed 276

characterization of tissue optics using combinations of optical reflection spectroscopy, optical 277

coherence tomography and Monte Carlo simulations (Wang et al., 1995; Wangpraseurt et al., 278

2016a; Wangpraseurt et al., 2017). 279

An experimental solution to the above mentioned complications relies on measuring the chlorophyll 280

fluorescence profile, which represents the net outcome of photon absorption along the actinic light 281

gradient in the tissue (Takahashi et al., 1994; Vogelmann and Han, 2000). By correlating 282

fluorescence profiles to total absorption, we could measure the direct result of absorption and the 283

values obtained are thus only affected by the quantum yield of fluorescence and energy transfer 284

between antenna pigment molecules and PSII and PSI. However, due to the invasive nature of the 285

method, some actinic light will be lost from the cut surface causing some underestimation of the 286

tissue absorption in situ (Terashima et al., 2016). Furthermore, this method allows estimates of the 287

distribution of photon absorption, but does not enable a separation of possible changes in the 288

absorption cross-section or balance between PSI and PSII absorption in different tissue layers. 289

We found total absorption values from integrating sphere measurements that were similar to 290

terrestrial leaves (Gorton et al., 2010), although the absorption of green/yellow light was higher in 291

F. vesiculosus due to the presence of accessory brown algal pigments such as fucoxanthin that 292

displays a high efficiency of energy transfer to Chl a (~95% (Yukihira et al., 2017)). Due to the low 293



15

reflection and transmission in the thallus, the absorption will be in the same order as the light 294

attenuation. Comparing, the calculated attenuation of light due to absorption with the light 295

attenuation calculated using scalar irradiance microprofile measurements from a previous study 296

(Lichtenberg and Kühl (2015); see Materials and Methods) we found a whole-thallus absorption 297

coefficient that was lower than cortex attenuation coefficients and higher than medulla attenuation 298

coefficients (Fig. 5). We predicted identical absorption coefficients in the cortex layers since these 299

layers are not anatomically different and in addition displayed similar levels of chlorophyll 300

fluorescence under uniform epi-illumination. Surprisingly, the absorption coefficient of the shaded 301

cortex was larger than the one found in the illuminated cortex (Fig. 5). We speculate that the light 302

field angularity could have caused this difference. Previously it has been shown that absorptance of 303

plant tissue can be different under collimated vs. diffuse light (Brodersen and Vogelmann, 2010; 304

Gorton et al., 2010), and here the incident light on the illuminated cortex was collimated while light 305

reaching the shaded cortex had a higher diffuse component due to internal scattering. Further, we 306

note that our calculations were based on profiles of blue light, which was almost completely 307

absorbed, making the calculations of absorption in the shaded cortex more prone to errors due to the 308

lower signal/noise ratio. Absorption profiles are further complicated due to the self-absorption of 309

red fluorescence by chlorophyll. Thus, red fluorescence profiles are likely to better represent 310

absorption profiles than profiles of far-red fluorescence (Vogelmann and Han, 2000). Here, we used 311

a long-pass filter (>670 nm) to detect fluorescence, and the resulting profiles therefore comprised 312

both red- and far-red chlorophyll fluorescence, and future studies should therefore divide the 313

detected fluorescence signals into red- and far-red fluorescence. We also note that our laser sheet 314

had a Gaussian beam profile, which makes positioning close to the edge of the cut thallus surface 315

difficult and may create a potential spill-over of photons onto the cut side. This limitation could be 316



16

resolved by shaping the beam e.g. by the generalized-phase-contrast method (Bañas et al., 2014) to 317

transform the Gaussian beam profile to a sharp rectangular shape and such work is now underway. 318

The shape of the white fluorescence profile across the thallus was slightly different in appearance 319

than the profiles for blue, green and red light. Previously, it was shown that profiles of carbon 320

assimilation and chlorophyll fluorescence profiles followed each other closely depending on the 321

spectral quality (Sun et al., 1996; Vogelmann and Han, 2000), and it has been proposed that profiles 322

of carbon fixation under white light can be described as the mean when using blue, green and red 323

light (Sun et al., 1998; Vogelmann and Han, 2000). Here we show that for plant tissue harboring a 324

range of accessory pigments such as fucoxanthin, the absorptive properties are more complex 325

resulting in a different response to white light than what can be expected from the combination of 326

measurements made under monochromatic light. Furthermore, it was shown that green light drives 327

photosynthesis more efficiently deeper in terrestrial plant tissue than blue and red light due to a 328

larger penetration depth of green light in leaf tissues (Sun et al., 1998; Terashima et al., 2009). In 329

contrast, the presence of fucoxanthin and carotenes in Fucus caused the green light (525-575 nm) to 330

be absorbed equally effective as red light. However, not all wavelengths were absorbed equally 331

effective, and e.g. the spectral region around 570-605 nm displayed reduced absorption as compared 332

to the other spectral regions of photosynthetically active radiation. Since only the white light 333

treatment covered this part of the spectrum, it is possible that illumination with broadband white 334

light caused the differently shaped absorption profile. This might be confirmed by measuring 335

additional chlorophyll fluorescence profiles using yellow light (e.g. 570-605 nm) to validate if this 336

would result in fluorescence profiles with Fmax located deeper in the thallus, similar to fluorescence 337

profiles in terrestrial leaves illuminated with green light (Vogelmann and Han, 2000). 338

339

Photosynthesis 340



17

We demonstrate that PSII quantum yields and derived apparent ETR across the thallus cross-341

sections strongly differed between homogeneous actinic light illumination of the cross-section and 342

unidirectional actinic light illumination on one side of the thallus with a laser light sheet. Under low 343

incident irradiance, the yields were very similar in all layers and between measurements. However, 344

as incident light directly on the cut side increased, the yields decreased across the tissue, with the 345

highest decreases found in the medulla (Fig. 6). This was not the case under laser light sheet 346

illumination perpendicular to one side of the thallus surface, where we found strongly reduced 347

yields in the illuminated cortex, while yields in the shaded cortex were unaffected (Fig. 6), even at 348

the highest incident photon irradiance (1108 µmol photons m-2

s-1

). Thus, when applying actinic 349

light directly on a cross-section our data show that it is possible to both underestimate and 350

overestimate PSII quantum yields as compared to yields found under natural light gradients. Using 351

a multilayer leaf model, Evans (2009) found that as irradiance increased on the adaxial side, the 352

quantum yields were progressively reduced deeper into the mesophyll. However, quantum yields at 353

the abaxial side were unaltered even under high blue irradiance, which is in good agreement with 354

the findings of this paper. Evans (2009) showed that surface based measurements of ϕPSII to 355

estimate ETR was only valid in some cases but overestimated ETR when the leaf was inverted. 356

Similarly, mesophyll conductance measurements could be influenced when estimated using surface 357

based ϕPSII values (Evans, 2009). With our new method, ϕPSII values can now be measured under 358

natural tissue light gradients and can be corrected for photon absorption thus making it possible to 359

get more detailed insights into mesophyll conductance heterogeneities e.g. by using depth resolved 360

ϕPSII data in combination with gas exchange measurements (Evans, 2009; Pons et al., 2009). 361

The differences in PSII quantum yield measured under illumination directly on the cross-section or 362

perpendicular to the side of the thallus surface indicate the difference between the photosynthetic 363

potential under equal illumination and the realized photosynthesis under tissue light gradients. We 364



18

show here that even under high incident irradiance, photosynthetic electron transport in the lower 365

cortex was not saturated, and an even illumination of tissue cross-sections will therefore 366

underestimate the PSII quantum yield as compared to shaded parts during high unidirectional 367

illumination. Apparent ETR rates in the cortex layers were very similar when corrected for 368

absorption, while the medulla layer displayed saturation and lower ETR rates at increasing 369

irradiance indicating a lower photosynthetic capacity, probably due to lower pigment content (Fig. 3 370

and Fig. 7). The slope of the initial part of the ETR curves, which is a measure of the light 371

utilization efficiency at subsaturating photon flux, were similar in all thallus layers albeit 372

consistently slightly lower in the medulla (Table 1). In a recent study, absolute ETR rates of thin-373

tissued corals were calculated (Szabó et al., 2014) and the rates found in this study were of similar 374

magnitude. Szabó et al. (2014) also found initial slopes of the sub-saturated part of the ETR vs. 375

irradiance curves that were slightly higher, probably reflecting differences in photochemical 376

acclimatization that have been shown to be tightly linked to the optical properties of coral tissues 377

(Lichtenberg et al., 2016; Wangpraseurt et al., 2016a; Wangpraseurt et al., 2016b). Fucus is often 378

found in the intertidal zone and thus, on a daily basis, experiences a variable light environment as a 379

function of water depth and concentration of organic and inorganic particles as well as dissolved 380

organic matter attenuating solar irradiance. Air exposed plant parts will therefore be subjected to the 381

full solar spectrum while the incident light field on algae situated in deeper oligotrophic waters will 382

be blue-shifted due to the absorption of red light by water. In contrast, the incident light field on 383

algae in shallow eutrophic waters will be green shifted due to absorption of blue and red 384

wavelengths by suspended phytoplankton. On a tissue scale, the medulla layer of Fucus will on 385

average experience the lowest photon irradiance compared to the cortex layers (Lichtenberg et al. 386

2015) and could therefore be thought of as a shade adapted compartment in the algal thallus. Shade-387

acclimatized phytoelements will normally display higher light use efficiencies (i.e. steeper initial 388



19

slope on the photosynthesis-irradiance curve) but lower photosynthetic capacity (Pmax) due to 389

increased pigment content and biochemical regulations in the photosynthetic machinery and/or 390

ultrastructural changes in the chloroplasts (Lichtenthaler et al., 1981; Lichtenthaler and Babani, 391

2004; Lichtenthaler et al., 2007; Sarijeva et al., 2007). However, as the initial slopes of the ETR vs. 392

absorbed light curves in the medulla were both lower and displayed saturation at lower irradiances, 393

medulla layers cannot be described as photosynthetically shade-adapted in conventional terms. 394

Conversely, it appears that the structural organization of the thallus layers could be adapted to 395

maximize photon absorption in the outer cortex layers, while having a relatively translucent central 396

medulla with low absorptive properties. This is in contrast to terrestrial leaves, where even 397

illumination of tissue layers is achieved by increased internal scattering due to intercellular 398

airspaces with the concomitant absorption profiles following an exponential attenuation with depth 399

(Vogelmann and Han, 2000). This fundamental difference is in good agreement when considering 400

their respective position in terrestrial and aquatic habitats, as terrestrial leaves can organize their 401

position according to the angle of solar radiation, whereas aquatic macrophytes are limited in their 402

structural organization by strong drag and shearing forces imposed by waves and currents, 403

randomly exposing both sides of the thallus to direct light. By maximizing absorption in the outer 404

layers and having a translucent central layer, F. vesiculosus can thus maximize light harvesting by 405

allowing photons not absorbed in the illuminated thallus to propagate to tissue layers with unused 406

photosynthetic potential thereby ensuring a more efficient resource distribution. 407

The ETR vs. absorbed light curves measured in different tissue layers under laser light sheet 408

illumination were similar to what was previously found in Fucus (Lichtenberg and Kühl, 2015) 409

although these microfiber PAM-based measurements were associated with high standard deviations 410

due to the small measurement volume of the fiber-optic microprobe, which makes such 411

measurements prone to microscale tissue heterogeneity effects. With the experimental approach 412



20

presented in this study, it is now possible to integrate photosynthetic responses from specific tissue 413

layers much more precisely, and in addition to correct them for the amount of photons absorbed by 414

that given tissue layer. Here we used a 10x microscope objective, but in principle such 415

measurements could be performed at even higher magnification e.g. to investigate single cell 416

gradients of light and photosynthetic efficiencies e.g. in large algal cells. The combination of multi-417

color light sheet microscopy with variable chlorophyll fluorescence imaging on plant tissue cross-418

sections provides an alternative to more destructive methods such as constructing profiles of CO2 419

fixation from paradermal sectioning (Evans and Vogelmann, 2003) or nanoscale secondary-ion-420

mass-spectroscopy (Kilburn et al., 2010; Wangpraseurt et al., 2016b) and allows sequential 421

measurements on the same sample e.g. under different levels of actinic irradiance or comparisons of 422

diffuse vs. collimated light fields (Brodersen et al., 2008). Here we demonstrated the application on 423

aquatic macrophyte tissue but the technique is readily applicable to many other types of plant 424

tissues including terrestrial leaves as well as photosynthetic biofilms and symbioses. 425

426

Conclusion 427

The combination of multicolor light sheet microscopy and variable chlorophyll fluorescence 428

imaging is a powerful technique that enables fine scale characterization of light absorption and PSII 429

quantum yields across plant tissue sections. Furthermore, quantification of photon absorption from 430

light sheet induced cross-tissue fluorescence profiles can be used in concert with variable 431

chlorophyll fluorescence imaging enabling calculations of ETR rates that otherwise require 432

knowledge of the absorption cross-section, and the mean effective PAR. 433

The spectral flexibility of a white super continuum laser source allows this method to be used in 434

other photosynthetic systems with different anatomical structures and pigmentation. In this manner, 435



21

the role of specific accessory pigments in light propagation and photosynthesis can be further 436

investigated. 437

438



22

Materials and Methods 439

Sample collection and preparation 440

Stands of the brown macroalga Fucus vesiculosus were collected in the littoral zone at various 441

locations around Helsingør, Denmark during late summer and were maintained in 10 liter buckets 442

continuously flushed with 0.2 µm filtered aerated seawater (temperature=16°C; salinity=32) for up 443

to one week prior to experiments. Samples were kept under a 12:12 h light:dark cycle under a 444

photon irradiance of ~50 µmol photons m-2

s-1

(PAR, 400-700 nm) as provided by a fluorescent 445

tube (Philips Master TL-D90, 18W; Philips, Amsterdam, the Netherlands). 446

Prior to measurements, an apical thallus fragment was cut ~1 cm from the thallus tip with a razor 447

blade, and the cut side was rinsed in seawater with a transfer pipette to wash away pigments leaking 448

from cut chloroplasts. The sample holder (Fig. 1) consisted of a standard plastic cuvette, cut down 449

to a height of 11 mm to allow insertion under the microscope (internal size = 10 x 10 x 10 mm). To 450

fix the sample in the cuvette, ~300 µL of 20 g L-1

seawater agar (Sigma-Aldrich) was transferred to 451

the cuvette and allowed to cool to 20°C, after which a slit was cut parallel to the cuvette window to 452

allow insertion of the thallus fragment. The thallus was inserted flush with the edge of the cuvette, 453

filled with seawater (16°C; salinity=32) and closed with a coverslip. 454

455

Chlorophyll fluorescence profiles

456

Profiles of absorbed light as estimated from chlorophyll fluorescence profiles across tissue sections 457

were imaged with a customized microscope setup (Fig. 1). The sample cuvette (see above) was 458

mounted on an inverted microscope (IX81; Olympus, Japan) with a 10x objective (UPlanSApo 459

10x/0.40; Olympus, Japan). The sample was illuminated perpendicular to one side of the thallus 460

surface by a supercontinuum laser (SuperK Extreme, EX-B, NKT Photonics, Denmark). The laser 461

was connected to a tunable single line filter module (SuperK Varia, NKT Photonics, Denmark) via 462



23

a single mode fiber with a collimated output. The tunable single line filter could be tuned from 400-463

840 nm to produce bandwidths from 1-400nm. Light from the single line filter module was 464

delivered, via an alignment tool (SuperK Connect, NKT Photonics, Denmark), to an endlessly-465

single-mode-large-mode-area photonic crystal fiber (FD7, NKT Photonics, Denmark) with a 466

collimated output. The collimated output was connected to a cylindrical laser sheet generator (NKT 467

Photonics, Denmark) with a 14° “light-sheet half angle” yielding a 5 cm longitudinal line at 10 cm 468

distance. The generated laser sheet had a Gaussian beam profile of ~1 mm on the latitudinal axis. 469

The output laser optics was mounted on a manual micromanipulator (MM33; Märzhäuser, Wetzlar, 470

Germany) that allowed easy positioning of the laser sheet in the focal plane of the microscope. 471

The sample was positioned with the cut side facing the microscope objective and the laser sheet was 472

adjusted to hit as close to the edge of the cut as possible without illumination spill-over to the cut 473

side. After positioning, the sample was allowed to dark adapt for 15 min. 474

Images of chlorophyll fluorescence from the cross-section were taken with a sensitive charge-475

coupled device (CCD) camera (iXon, Andor, UK) using a fixed exposure time of 70 ms. 476

Chlorophyll fluorescence was detected by placing a ultra-steep longpass edge filter (BLP01-664R, 477

Semrock, USA) in the light path between objective and the camera with a transmission close to 478

100% at wavelengths >670nm and close to 0% at wavelengths <670nm. Illumination in different 479

spectral bands was achieved by control of the laser and the spectral filtering module with the 480

manufacturers software (NKTP Control, NKT Photonics, Denmark). Four different spectral 481

illumination bands were composed (Fig. 1) and adjusted to the same photon irradiance (1190.3 ± 482

1.9 µmol photons m-2

s-1

) as measured with a micro quantum sensor (MC-MQS, Walz GmbH, 483

Germany) connected to a calibrated quantum irradiance meter (ULM-500, Walz GmbH, Germany). 484

We used broadband illumination (≥50nm) to ensure excitation of both PSII and PSI, thus avoiding 485

eventual red-drop effects. 486



24

Illumination of the sample during individual image acquisitions was limited to <2 s per image, and 487

the spectral sequence was randomized between replicates. The effect of irradiance on the emitted 488

fluorescence was tested by treatment with DCMU (3-(3,4-dichlorophenyl)-1, 1-dimethyl-urea) (see 489

Supplementary Information; Fig. S2). Images were analyzed in ImageJ (v. 1.50B), where grey 490

values were extracted either by the line profile tool or by extracting all grey values. The images had 491

a spatial resolution of 0.8x0.8 µm pixel-1

. Maximum chlorophyll fluorescence was normalized to 1 492

in all images to allow comparison between images. Plots were made in OriginPro (v. 9.3, Origin 493

Lab, MA, USA). The thickness of the thallus varied, both between samples and depending on 494

location in the cross-section (Fig. 2). Therefore, profiles of fluorescence were taken at thallus 495

thicknesses of ~450 µm to allow comparison of thallus light gradients over the same tissue 496

thickness. 497

498

Integrating sphere measurements of reflectance and transmittance 499

Thallus reflectance and transmittance were measured using an integrating sphere (diameter = 10 500

cm; port diameters = 2.5 cm, Labsphere Instruments Inc., USA). The sphere had 3 port openings; 501

two located opposite each other and one orthogonal to the two other openings (Fig. S1). The 502

incident light from the supercontinuum laser was tuned to different spectral ranges each with the 503

same photon irradiance (see text above and Fig. 1). Light was measured with a calibrated spectral 504

irradiance meter (MSC15, Gigahertz Optik GmbH, Germany) connected to the orthogonally located 505

port on the integrating sphere. For transmittance measurements, a thallus fragment was mounted in 506

front of the entrance port between the light source and the integrating sphere and the port opposite 507

to the entrance was covered with a white reflecting plate. For reflectance measurements, a thallus 508

fragment was placed in the port opening opposite to the incident laser beam at an angle of 5-10° to 509



25

capture both the specular and diffuse reflection. Total absorptance (A) by the thallus was estimated 510

as: 511

512

𝐴 = ∫(𝐼−𝑅−𝑇)

𝐼

𝜆𝑛

𝜆𝑖 (1) 513

514

where I is the incident photon irradiance, R is the reflectance and T is the transmittance, all 515

integrated over the spectral region of interest [λi - λn]. 516

517

Modelling of light attenuation 518

Light attenuation profiles were modelled using attenuation coefficients, α, of blue scalar irradiance 519

(420-520 nm) in the cortex and medulla layers measured in a previous study (Lichtenberg and Kühl, 520

2015). The model assumed monoexponential attenuation of incident irradiance, I0, over tissue depth 521

intervals Δz and the light availability in different tissue depths was then calculated as: 522

523

𝐼𝑧 = 𝐼0 ∙ 𝑒−𝛼∙∆𝑧 (2) 524

525

These data were compared to the estimated attenuation, due to absorption quantified as induced 526

fluorescence and corrected for total absorption (Fig. S3) across the thallus under blue irradiance 527

(425-475 nm) in this study. The attenuation due to absorption was calculated by subtracting the 528

cumulative absorption (Fig. 4), integrated in 50 µm increments, from the incident irradiance. 529

530

Variable chlorophyll fluorescence imaging 531

Pulse-amplitude-modulated variable chlorophyll fluorescence imaging (Imaging-PAM) with the 532

saturation pulse method (Schreiber et al., 1995; Schreiber, 2004; Kühl and Polerecky, 2008) was 533



26

used to assess the photosynthetic performance over cross-sections of the algal thallus mounted in 534

cuvettes as described above. Measurements were performed with a microscope Imaging-PAM 535

system (Fig. 1) fitted with a Red-Green-Blue (RGB) LED excitation lamp (IMAG-RGB; Heinz 536

Walz GmbH, Effeltrich Germany) as described in detail elsewhere (Trampe et al., 2011). The 537

microscope was fitted with a high numerical aperture objective (10x/NA0.8, Plan-Apochromate, 538

Carl Zeiss GmbH, Germany). Fast measurements of the effective PSII quantum yield under 539

increasing photon irradiance (each irradiance step was applied for 20 s) were used to measure so-540

called rapid light curves (RLC) (White and Critchley, 1999) of relative PSII electron transport vs. 541

irradiance curves. Two different approaches were applied: i) Using increasing actinic irradiances of 542

red light (590-650nm) as provided by the system-default internal RGB-LED lamp for equal 543

excitation of the exposed cross-section of the thallus from above, in combination with the 544

customizable automated light curve function of the software provided by the system software 545

(ImagingWin, Heinz Walz GmbH, Germany); ii) Using the supercontinuum laser setup as described 546

above as an external actinic light source, illuminating the thallus with a light sheet perpendicular to 547

the thallus surface. The actinic light sheet was controlled manually in stepwise increments in sync 548

with the custom defined automated light curve function of the ImagingWin software facilitating a 549

semi-automated acquisition of RLC’s, while the system-default internal actinic RGB-LED light 550

source was disconnected. RLC’s with the laser light sheet were obtained with increasing actinic 551

irradiance of blue (425-475 nm), green (525-575 nm), red (615-665 nm) or white (400-700 nm) 552

light, respectively. Non-actinic modulated blue measuring light was provided by the build-in LEDs 553

of the ImagingPAM system during both approaches. The two setups were calibrated using a photon 554

irradiance meter connected to a cosine corrected mini quantum PAR irradiance sensor, (ULM-500, 555

MQS-B, Walz GmbH, Germany). All samples were allowed to dark adapt for 15 min before the 556

measurements started. When in the dark-acclimated state, all reactions centers of PSII were open, 557



27

enabling imaging of the minimal fluorescence yield (F0). Upon exposure to a high-intensity 558

saturation pulse, all PSII reaction centers closed permitting imaging of the maximal fluorescence 559

yield (Fm) over the thallus cross-section. From these images, the maximum PSII quantum yield 560

could be calculated as in Schreiber (2004): 561

562

𝐹𝑉/𝐹𝑚 = (𝐹𝑚 − 𝐹0)/𝐹𝑚 (3) 563

564

From imaging of the fluorescence yield, F, while the sample was illuminated with a predefined 565

level of actinic light (PAR, in μmol photons m-2

s-1

), and the maximum fluorescence yield during a 566

saturation pulse, 𝐹𝑚′, images of the effective PSII quantum yield could be calculated as: 567

568

ϕ𝑃𝑆𝐼𝐼 = (𝐹𝑚′ − 𝐹)/𝐹𝑚′ (4) 569

570

From these values, the relative photosynthetic electron transport rate (rETR), is usually derived 571

using the equation: 572

573

𝑟𝐸𝑇𝑅 = ϕ𝑃𝑆𝐼𝐼 × PAR × AF (5) 574

575

where PAR is the incident photon irradiance, and AF (the absorption factor) is a constant set to 0.42 576

assuming that 84% of the incident light is absorbed (Demmig and Björkman, 1987) and an even 577

distribution of absorbed photons between photosystem II and I (Schreiber et al., 2012). In our study, 578

we estimated internal ETR rates (in units of µmol electrons m-2

s-1

) in different regions of the 579

thallus (cortex/medulla) by multiplying average ϕ𝑃𝑆𝐼𝐼 values (in units of µmol electrons m-2

s-1

580

(µmol photons m-2

s-1

)-1

) for a given area with the total amount of absorbed photons (in units of 581



28

µmol photons m-2

s-1

) in that area. The wavelength-dependent absorption was calculated by 582

normalizing the chlorophyll fluorescence profiles to the total absorption (Fig. S3). This essentially 583

gives an estimate of the wavelength-dependent photon absorption at any given depth in the thallus, 584

assuming that one unit of fluorescence was the direct result of one unit of absorption. In reality 585

however, this is affected by factors such as the quantum yield of fluorescence and energy transfer 586

between antenna pigment molecules and PSII/PSI. The true photon absorption will therefore vary 587

slightly, and the calculated photon absorptions are thus only estimates. 588

The light saturation coefficient, i.e., the photon scalar irradiance at onset of light saturation of 589

photosynthesis, Ek was calculated as 𝐸𝑘 = 𝐸𝑇𝑅𝑚𝑎𝑥/𝛼, where 𝐸𝑇𝑅𝑚𝑎𝑥 is the maximum activity, 590

and α is the initial slope of the ETR vs. photon absorption curve; both parameters were obtained by 591

curve fitting of ETR measurements using an exponential function (Webb et al., 1974) by means of a 592

non-linear Levenberg-Marquardt fitting algorithm (OriginPro 2015, OriginLab Corporation, 593

Northampton, MA, USA). 594

595

Statistics 596

One-way ANOVAs were applied to test differences in the slopes of the ETR curves between 597

different light treatments. Data was first tested for normality (Shapiro-Wilk) and for equal variance. 598

Statistical analysis was performed in SigmaPlot (SigmaPlot v. 12.5) and significance level was set 599

to p<0.01. 600

601

Supplemental information 602

Figure S1. Schematic drawing of integrating sphere measurements of transmittance and reflectance. 603

Figure S2. Chlorophyll fluorescence profiles over cross-sections of apical thallus fragments of F. 604

vesiculosus. 605



29

Figure S3. Concept of the calculations of photon absorption profiles across plant tissue cross-606

sections. 607

Figure S4. PSII quantum yield (Y(II)) and non-photochemical quenching (NPQ) as a function of 608

absorbed photons in the upper cortex, medulla and lower cortex of Fucus vesiculosus. 609

610

Acknowledgements 611

This study was supported by a Sapere-Aude Advanced grant from the Danish Council for 612

Independent Research ǀ Natural Sciences (MK), and instrument grants from the Carlsberg 613

Foundation (MK). We thank NKT Photonics, Denmark, for generous loan of the Supercontinuum 614

Laser and for excellent technical support. 615



30

Table 1: Photosynthetic electron transport from PSII (ETR) vs. photon absorption curve parameters 616

and the fractional photon absorption (in % of total absorption) calculated for the upper cortex, 617

medulla and lower cortex under blue (425-475 nm), green (525-575 nm), red (615-665 nm) and 618

white (400-700 nm) irradiance applied perpendicular to one side of the thallus surface of F. 619

vesiculosus. Slopes on the sub-saturated part of the ETR vs. light curve, maximum ETR values and 620

the light acclimation index Ek were estimated from curve fitting of the ETR vs. photon absorption 621

curves with an exponential function (Webb et al., 1974) using a non-linear Levenberg-Marquardt 622

fitting algorithm. The RGB values were calculated as average curves of blue, green and red. 623

Pairwise superscript letters indicate statistically significant differences (One-way ANOVA; p<0.01; 624

n=5 for white; n=4 for RGB). 625

Upper Cortex Medulla Lower Cortex

Slope ETRmax Ek Abs

(%)

Slope ETRmax Ek Abs

(%)

Slope ETRmax Ek Abs

(%)

Red 0.60 197.09 327.23 44.0 0.59 80.43 135.82 26.0 0.60 192.68 319.05 30.0

Green 0.47 225.31 482.46 44.0 0.45 81.33 180.71 26.0 0.47 228.90 489.60 30.0

Blue 0.54 878.52 1620.05 57.0 0.54 254.76 469.67 21.0 0.55 599.82 1082.76 22.0

White 0.60a 378.40 629.79 45.0 0.58b 125.48 216.22 26.0 0.60c 349.31 586.39 29.0

RGB 0.54a - - - 0.53b - - - 0.54c - - -

626



31

Figure legends 627

Figure 1. Experimental setup for light sheet microscopy in combination with variable chlorophyll 628

fluorescence imaging. A) The sample holder consisted of a cuvette cut down to 11 mm height 629

(internal dimensions of 10 x 10 x 10 mm). The sample was mounted in agar in the botttom of the 630

sample holder, which was filled with seawater and then closed with a coverslip. B) Spectral 631

composition of the laser light used for measurements of chlorophyll fluorescence profiles and the 632

actinic light in measurement of variable chlorophyll fluorescence. The laser was adjusted to have 633

the same absolute photon irradiance independent of spectral composition. C) Schematic drawing of 634

the experimental setup for measuring chlorophyll fluorescence profiles. (a) The algal thallus sample 635

positioned in the cuvette, (b) microscope objective, (c) filter cube with longpass filter, (d) CCD 636

camera. Illumination of the sample was done with a PC-controlled super continuum laser connected 637

to a spectral line filter unit and a laser sheet generator. D) Schematic drawing of the experimental 638

setup for variable chlorophyll fluorescence microscopy. (a) sample fixed in the cuvette, (b) 639

microscope objective, (c) emission filter, (d) dichroic beam splitter cube, (e) dichroic filter, (f) 640

mirror (to occular), (g) CCD camera. Weak non-actinic modulated measuring light was provided by 641

a software-controlled RGB LED unit. Actinic light was provided perpendicular to one side of the 642

thallus surface by a PC-controlled super continuum laser connected to a spectral line filter unit and 643

a laser sheet generator. 644

645

Figure 2. Color-coded maps showing the distribution of Chl a fluorescence (normalized to max 646

fluorescence) of A) a Fucus vesiculosus thallus cross-section illuminated evenly over the cut side 647

with 430 nm light from a Xe-lamp (the plot is composed of multiple images taken through a 10x 648

objective, and stitched together using Adobe Photoshop), and B-E) Thallus cross-sections 649

illuminated perpendicular to one side of the thallus surface with a super continuum laser sheet of 650



32

different spectral compositions of B) blue (425-475 nm), C) green (525-575 nm), D) red (615-665 651

nm) and E) white (400-700 nm) light. 652

653

Figure 3. A) Epifluorescence microscopy image (false colors) of a F. vesiculosus thallus cross-654

section illuminated evenly with blue light (430 nm) from a Xe-lamp, and B) the associated 655

fluorescence profile (normalized to max fluorescence). C) Example of a fluorescence image (false 656

colors) of a thallus fragment irradiated perpendicularly to one side of the thallus surface (arrow) 657

with a laser sheet of red light (615-665 nm) from a super continuum laser, and D) chlorophyll 658

fluorescence profiles of cross-sections of apical thallus fragments of F. vesiculosus irradiated 659

perpendicular to the thallus surface with different spectral bands of blue (425-475nm), green (525-660

575 nm), red (615-665) and white (400-700nm) light. Data was normalized to max fluorescence and 661

actual data points are spaced 0.8 µm apart. (n=5. Error bars are not shown for clarity but mean 662

relative S.D. was ±7.7%). 663

664

Figure 4. A) Spectral measurements of reflectance, transmittance and absorptance of a F. 665

vesiculosus thallus fragment using an integrating sphere (see Fig. S1). Data were recorded using 666

either incident blue (425-475 nm; blue lines), green (525-575 nm; green lines), red (615-665 nm; 667

red lines) or white (400-700 nm; black lines) laser light. Dashed lines indicate ± 1 S.D.; n=3. B) 668

Calculated absorption profiles of cross-sections of apical thallus fragments of F. vesiculosus 669

irradiated perpendicular to one side of the thallus surface with different spectral bands of blue (425-670

475 nm), green (525-575 nm), red (615-665 nm) and white (400-700 nm) laser light. Absorption 671

was calculated from the measured chlorophyll fluorescence profiles (Fig. 3D) and was normalized 672

to the bulk absorption measured with an integrating sphere (A). Dashed lines indicate the borders of 673

the cortex and medulla tissue layers. Data points are spaced 0.8 µm apart (n=5). 674



33

675

Figure 5. Left panel shows plots of attenuation profiles of blue light (420-520 nm) calculated using 676

attenuation coefficients from the cortex (blue triangles) and medulla (red circles) layers 677

(Lichtenberg and Kühl, 2015), and a profile of the attenuation due to absorption of blue light (425-678

475 nm) estimated from the observed fluorescence profile (black squares). Right panel shows the 679

natural logarithm transformed absorption profile with linear fits in the cortex and medulla layers, 680

respectively (R2>0.95 for all fits). 681

682

Figure 6. Isopleths (A, B) and images (C-H) of effective PSII quantum yield in apical thallus 683

fragments of F. vesiculosus illuminated evenly on a cross-section or perpendicular on one side of 684

the thallus surface. Images were acquired under red light using either direct light from the build-in 685

LEDs of the variable chlorophyll fluorescence imaging system (590-650 nm), or light perpendicular 686

to the thallus surface provided by a super continuum laser (615-665 nm) connected to a tunable 687

single line filter and delivered via a laser sheet generator. The isopleths (A, B) show the influence 688

of actinic irradiance on the effective PSII quantum yield (in µmol electrons m-2

s-1

(µmol photons 689

m-2

s-1

)-1

) as function of depth in the tissue when illuminated either directly on the cross-section (A) 690

or perpendicular to the surface of the thallus (B). Illumination in B was given from left towards 691

right of the panel. Line profiles (line width = 15 pixel) were taken on thallus parts with similar 692

thickness (~250 µm) with cortex layers also displaying similar thicknesses (~50-75 µm). Panels C-693

H show images of effective PSII quantum yield in darkness, moderate (567 ± 18 µmol photons m-2

694

s-1

) and, saturating irradiance (1087 ± 30 µmol photons m-2

s-1

) under direct even illumination of the 695

cross-section (C-E) and with laser light sheet illumination perpendicular to the thallus surface 696



34

(arrows) (F-H). Illumination in F-H was given from the bottom towards the top of the panels. 697

Scale-bar = 0.2 mm. 698

Figure 7. Apparent electron transport rates through PSII corrected for absorbed photons (Fig. 4). 699

Measurements were performed with 20 s acclimation to each increasing actinic irradiance level of 700

A) blue (425-475 nm), B) green (525-575 nm), C) red (615-665 nm) and D) white (400-700 nm) 701

light as provided by a laser sheet illuminating a thallus fragment perpendicular to one side of the 702

thallus surface. Data point represents means ± S.E. (n=5, except panel A where n=4). 703

704

Figure S1. A) Schematic drawing of integrating sphere measurements of transmittance (left) and 705

reflectance (right). Spectral quantities were calculated over the wavelengths of interest as 𝐴 =706

∫(𝐼−𝑅−𝑇)

𝐼

𝜆𝑛

𝜆𝑖 where I is the incident photon irradiance, R is the reflectance and T is the 707

transmittance, all integrated over the spectral region of interest [λi - λn]. B) Attenuance spectrum 708

calculated as –log(transmittance). 709

710

Figure S2. Chlorophyll fluorescence profiles over cross-sections of apical thallus fragments of F. 711

vesiculosus treated with 10 µM DCMU for 1h and then irradiated perpendicular to one side of the 712

thallus surface with different spectral bands of blue (425-475 nm), green (525-575 nm), red (615-713

665 nm) and white (400-700 nm) light (n=4. SD not shown for clarity but mean relative SD was ± 714

4.3%). 715

716

Figure S3. Concept of the calculations of photon absorption profiles across plant tissue cross-717

sections here illustrated for blue (425-475 nm) laser light sheet measurements on the brown alga 718



35

Fucus vesiculosus. A) shows the raw fluorescence profile across the tissue as detected by the CCD 719

camera expressed in pixel grey values. B) shows the relative fluorescence normalized to the 720

maximum grey value. C) shows the wavelength-dependent, whole-thallus absorption calculated 721

from integrating sphere measurements of transmission and reflection and expressed in % of incident 722

irradiance. D) The depth distribution of photon absorption was calculated by normalizing the 723

fluorescence profile to the whole-thallus photon absorption, such that the area under the curve is 724

equal to the whole-thallus absorption. 725

726

Figure S4. PSII quantum yield (Y(II)) and non-photochemical quenching (NPQ) as a function of 727

absorbed photons in the upper cortex, medulla and lower cortex of Fucus vesiculosus. The tissue 728

was irradiated perpendicular to one side of the thallus surface with blue (425-475 nm) green (525-729

575 nm), red (615-665 nm) or white (400-700 nm) light as provided by a supercontinuum laser 730

connected to a tunable single line filter unit. See main-text for measurement details. 731



36

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879



Figure 1. Experimental setup for light sheet microscopy in combination with variable chlorophyll �uorescence imaging. A) The sample holder consisted of a cuvette cut down to 11 mm height (inter-nal dimensions of 10 x 10 x 10 mm). The sample was mounted in agar in the botttom of the sample holder, which was �lled with seawater and then closed with a coverslip. B) Spectral composition of the laser light used for measurements of chlorophyll �uorescence pro�les and the actinic light in measurement of variable chlorophyll �uorescence. The laser was adjusted to have the same absolute photon irradiance independent of spectral composition. C) Schematic drawing of the experimental setup for measuring chlorophyll �uorescence pro�les. (a) The algal thallus sample positioned in the cuvette, (b) microscope objective, (c) �lter cube with longpass �lter, (d) CCD camera. Illumination of the sample was done with a PC-controlled super continuum laser connected to a spectral line �lter unit and a laser sheet generator. D) Schematic drawing of the experimental setup for variable chloro-phyll �uorescence microscopy. (a) sample �xed in the cuvette, (b) microscope objective, (c) emission �lter, (d) dichroic beam splitter cube, (e) dichroic �lter, (f ) mirror (to occular), (g) CCD camera. Weak non-actinic modulated measuring light was provided by a software-controlled RGB LED unit. Actinic light was provided perpendicular to one side of the thallus surface by a PC-controlled super continu-um laser connected to a spectral line �lter unit and a laser sheet generator.

Figure 1.



Figure 2. Color-coded maps showing the distribution of Chl a �uorescence (normalized to max �uo-rescence) of A) a Fucus vesiculosus thallus cross-section illuminated evenly over the cut side with 430 nm light from a Xe-lamp (the plot is composed of multiple images taken through a 10x objec-tive, and stitched together using Adobe Photoshop), and B-E) Thallus cross-sections illuminated perpendicular to one side of the thallus surface with a super continuum laser sheet of di�erent spec-tral compositions of B) blue (425-475 nm), C) green (525-575 nm), D) red (615-665 nm) and E) white (400-700 nm) light.

Figure 2.



Figure 3. A) Epi�uorescence microscopy image (false colors) of a F. vesiculosus thallus cross-section illuminated evenly with blue light (430 nm) from a Xe-lamp, and B) the associated �uorescence pro�le (normalized to max �uorescence). C) Example of a �uorescence image (false colors) of a thal-lus fragment irradiated perpendicularly to one side of the thallus surface (arrow) with a laser sheet of red light (615-665 nm) from a super continuum laser, and D) chlorophyll �uorescence pro�les of cross-sections of apical thallus fragments of F. vesiculosus irradiated perpendicular to the thallus surface with di�erent spectral bands of blue (425-475nm), green (525-575 nm), red (615-665) and white (400-700nm) light. Data was normalized to max �uorescence and actual data points are spaced 0.8 µm apart. (n=5. Error bars are not shown for clarity but mean relative S.D. was ±7.7%).

Figure 3.



Figure 4. A) Spectral measurements of re�ectance, transmittance and absorptance of a F. vesiculo-sus thallus fragment using an integrating sphere (see Fig. S1). Data were recorded using either incident blue (425-475 nm; blue lines), green (525-575 nm; green lines), red (615-665 nm; red lines) or white (400-700 nm; black lines) laser light. Dashed lines indicate ± 1 S.D.; n=3. B) Calculated absorption pro�les of cross-sections of apical thallus fragments of F. vesiculosus irradiated perpen-dicular to one side of the thallus surface with di�erent spectral bands of blue (425-475 nm), green (525-575 nm), red (615-665 nm) and white (400-700 nm) laser light. Absorption was calculated from the measured chlorophyll �uorescence pro�les (Fig. 3D) and was normalized to the bulk absorption measured with an integrating sphere (A). Dashed lines indicate the borders of the cortex and medul-la tissue layers. Data points are spaced 0.8 µm apart (n=5).

Figure 4.



Figure 5. Plots of attenuation profiles of blue light (420-520 nm) calculated using attenuation coefficients from the cortex (blue triangles) and medulla (red circles) layers (Lichtenberg and Kühl, 2015), and a profile of the attenuation due to absorption of blue light (425-475 nm) estimated from the observed fluorescence profile (black squares).

Figure 5.



Figure 6. Isopleths (A, B) and images (C-H) of effective PSII quantum yield in apical thallus fragments of F. vesiculosus illuminated evenly on a cross-section or perpendicular on one side of the thallus surface. Images were acquired under red light using either direct light from the build-in LEDs of the variable chlorophyll fluorescence imaging system (590-650 nm), or light perpendicular to the thallus surface provided by a super continuum laser (615-665 nm) connected to a tunable single line filter and delivered via a laser sheet generator. The isopleths (A, B) show the influence of actinic irradiance on the effective PSII quantum yield (in µmol electrons m-2 s-1 (µmol photons m-2 s-1)-1) as function of depth in the tissue when illuminated either directly on the cross-section (A) or perpendicular to the surface of the thallus (B). Illumination in B was given from left towards right of the panel. Line profiles (line width = 15 pixel) were taken on thallus parts with similar thickness (~250 µm) with cortex layers also displaying similar thicknesses (~50-75 µm). Panels C-H show images of effective PSII quantum yield in darkness, moderate (567 ± 18 µmol photons m-2 s-1) and, saturating irradiance (1087 ± 30 µmol photons m-2 s-1) under direct even illumination of the cross-section (C-E) and with laser light sheet illumination perpendicular to the thallus surface (arrows) (F-H). Illumination in F-H was given from the bottom towards the top of the panels. Scale-bar = 0.2 mm.

Figure 6.



Figure 7. Apparent electron transport rates through PSII corrected for absorbed photons (Fig. 4). Measurements were performed with 20 s acclimation to each increasing actinic irradiance level of A) blue (425-475 nm), B) green (525-575 nm), C) red (615-665 nm) and D) white (400-700 nm) light as provided by a laser sheet illuminating a thallus fragment perpendicular to one side of the thallus surface. Data point represents means ± S.E. (n=5, except panel A where n=4).

Figure 7.



Parsed CitationsBaker NR (2008) Chlorophyll fluorescence: A probe of photosynthesis in vivo. Annual Review of Plant Biology 59: 89-113

Pubmed: Author and TitleCrossRef: Author and TitleGoogle Scholar: Author Only Title Only Author and Title

Bañas A, Palima D, Villangca M, Aabo T, Glückstad J (2014) GPC light shaper for speckle-free one-and two-photon contiguous patternexcitation. Optics Express 22: 5299-5310


Berberoglu H, Gomez PS, Pilon L (2009) Radiation characteristics of Botryococcus braunii, Chlorococcum littorale, and Chlorella sp.used for CO2 fixation and biofuel production. Journal of Quantitative Spectroscopy & Radiative Transfer 110: 1879-1893


Binzer T, Sand-Jensen K (2002a) Importance of structure and density of macroalgae communities (Fucus serratus) for photosyntheticproduction and light utilisation. Marine Ecology Progress Series 235: 53-62


Binzer T, Sand-Jensen K (2002b) Production in aquatic macrophyte communities: A theoretical and empirical study of the influence ofspatial light distribution. Limnology and Oceanography 47: 1742-1750


Brodersen CR, Vogelmann TC (2007) Do epidermal lens cells facilitate the absorptance of diffuse light? American Journal of Botany 94:1061-1066


Brodersen CR, Vogelmann TC (2010) Do changes in light direction affect absorption profiles in leaves? Functional Plant Biology 37:403-412


Brodersen CR, Vogelmann TC, Williams WE, Gorton HL (2008) A new paradigm in leaf-level photosynthesis: direct and diffuse lightsare not equal. Plant, Cell and Environment 31: 159-164


Demmig B, Björkman O (1987) Photon yield of O2 evolution and chlorophyll fluorescence characteristics at 77 K among vascular plantsof diverse origins. Planta 170: 489-504


Evans JR (2009) Potential errors in electron transport rates calculated from chlorophyll fluorescence as revealed by a multilayer leafmodel. Plant and Cell Physiology 50: 698-706


Evans JR, Vogelmann TC (2003) Profiles of C-14 fixation through spinach leaves in relation to light absorption and photosyntheticcapacity. Plant Cell and Environment 26: 547-560


Garbary DJ, Kim KY (2005) Anatomical differentiation and photosynthetic adaptation in brown algae. Algae 20: 233-238Pubmed: Author and TitleCrossRef: Author and TitleGoogle Scholar: Author Only Title Only Author and Title

Gorton HL, Brodersen CR, Williams WE, Vogelmann TC (2010) Measurement of the optical properties of leaves under diffuse light.Photochemistry and Photobiology 86: 1076-1083


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http://search.crossref.org/?page=1&rows=2&q=Binzer T, Sand-Jensen K (2002a) Importance of structure and density of macroalgae communities (Fucus serratus) for photosynthetic production and light utilisation. Marine Ecology Progress Series 235: 53-62

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http://search.crossref.org/?page=1&rows=2&q=Binzer T, Sand-Jensen K (2002b) Production in aquatic macrophyte communities: A theoretical and empirical study of the influence of spatial light distribution. Limnology and Oceanography 47: 1742-1750

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Klughammer C, Schreiber U (2015) Apparent PS II absorption cross-section and estimation of mean PAR in optically thin and densesuspensions of Chlorella. Photosynthesis Research 123: 77-92


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Kühl M, Jørgensen BB (1994) The light-field of microbenthic communities - radiance distribution and microscale optics of sandycoastal sediments. Limnology and Oceanography 39: 1368-1398


Kühl M, Polerecky L (2008) Functional and structural imaging of phototrophic microbial communities and symbioses. Aquatic MicrobialEcology 53: 99-118


Lichtenberg M, Kühl M (2015) Pronounced gradients of light, photosynthesis and O2 consumption in the tissue of the brown algaFucus serratus. New Phytologist 207: 559-569


Lichtenberg M, Larkum AWD, Kühl M (2016) Photosynthetic acclimation of Symbiodinium in hospite depends on vertical position in thetissue of the scleractinian coral Montastrea curta. Frontiers in Microbiology 7: 230


Lichtenthaler HK, Ac A, Marek MV, Kalina J, Urban O (2007) Differences in pigment composition, photosynthetic rates and chlorophyllfluorescence images of sun and shade leaves of four tree species. Plant Physiology and Biochemistry 45: 577-588


Lichtenthaler HK, Babani F (2004) Light adaptation and senescence of the photosynthetic apparatus. Changes in pigment composition,chlorophyll fluorescence parameters and photosynthetic activity. In GC Papageorgiou, Govindjee, eds, Chlorophyll a Fluorescence: ASignature of Photosynthesis. Springer, Netherlands, pp 713-736

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http://www.ncbi.nlm.nih.gov/pubmed?term=Govindjee%2C Braun BZ %281974%29 Light absorption emission and photosynthesis%2E In WDP Stewart%2C ed%2C Botanical Monographs%3A Algal Physiology and Biochemistry%2C Vol 10%2E University of California Press%2C Berkeley and Los Angeles&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Johnsen G%2C Samset O%2C Granskog L%2C Sakshaug E %281994%29 In vivo absorption characteristics in 10 classes of bloom%2Dforming phytoplankton %2D Taxonomic characteristics and responses to photoadaptation by means of discriminant and HPLC analysis%2E Marine Ecology Progress Series 105%3A 149%2D157&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Johnsen G, Samset O, Granskog L, Sakshaug E (1994) In vivo absorption characteristics in 10 classes of bloom-forming phytoplankton - Taxonomic characteristics and responses to photoadaptation by means of discriminant and HPLC analysis. Marine Ecology Progress Series 105: 149-157

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http://www.ncbi.nlm.nih.gov/pubmed?term=Kilburn MR%2C Jones DL%2C Clode PL%2C Cliff JB%2C Stockdale EA%2C Herrmann AM%2C Murphy DV %282010%29 Application of nanoscale secondary ion mass spectrometry to plant cell research%2E Plant Signaling %26 Behavior 5%3A 760%2D762&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Klughammer C%2C Schreiber U %282015%29 Apparent PS II absorption cross%2Dsection and estimation of mean PAR in optically thin and dense suspensions of Chlorella%2E Photosynthesis Research 123%3A 77%2D92&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Klughammer C, Schreiber U (2015) Apparent PS II absorption cross-section and estimation of mean PAR in optically thin and dense suspensions of Chlorella. Photosynthesis Research 123: 77-92

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http://www.ncbi.nlm.nih.gov/pubmed?term=K�hl M %282005%29 Optical microsensors for analysis of microbial communities%2E Environmental Microbiology 397%3A 166%2D199&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=K�hl M (2005) Optical microsensors for analysis of microbial communities. Environmental Microbiology 397: 166-199

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http://www.ncbi.nlm.nih.gov/pubmed?term=K�hl M%2C Cohen Y%2C Dalsgaard T%2C J�rgensen BB%2C Revsbech NP %281995%29 Microenvironment and photosynthesis of zooxanthellae in scleractinian corals studied with microsensors for O2%2C pH and light%2E Marine Ecology Progress Series 117%3A 159%2D172&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=K�hl M, Cohen Y, Dalsgaard T, J�rgensen BB, Revsbech NP (1995) Microenvironment and photosynthesis of zooxanthellae in scleractinian corals studied with microsensors for O2, pH and light. Marine Ecology Progress Series 117: 159-172


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http://www.ncbi.nlm.nih.gov/pubmed?term=K�hl M%2C J�rgensen BB %281994%29 The light%2Dfield of microbenthic communities %2D radiance distribution and microscale optics of sandy coastal sediments%2E Limnology and Oceanography 39%3A 1368%2D1398&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=K�hl M%2C Polerecky L %282008%29 Functional and structural imaging of phototrophic microbial communities and symbioses%2E Aquatic Microbial Ecology 53%3A 99%2D118&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Lichtenberg M%2C K�hl M %282015%29 Pronounced gradients of light%2C photosynthesis and O2 consumption in the tissue of the brown alga Fucus serratus%2E New Phytologist 207%3A 559%2D569&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Lichtenberg M%2C Larkum AWD%2C K�hl M %282016%29 Photosynthetic acclimation of Symbiodinium in hospite depends on vertical position in the tissue of the scleractinian coral Montastrea curta%2E Frontiers in Microbiology 7%3A 230&dopt=abstract

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Oguchi R, Douwstra P, Fujita T, Chow WS, Terashima I (2011) Intra-leaf gradients of photoinhibition induced by different color lights:implications for the dual mechanisms of photoinhibition and for the application of conventional chlorophyll fluorometers. NewPhytologist 191: 146-159


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Privoznik KG, Daniel KJ, Incropera FP (1978) Absorption, extinction and phase function measurements for algal suspensions ofChlorella pyrenoidosa. Journal of Quantitative Spectroscopy & Radiative Transfer 20: 345-352


Ramus J (1978) Seaweed anatomy and photosynthetic performance - ecological significance of light guides, heterogeneous absorptionand multiple scatter. Journal of Phycology 14: 352-362


Raven JA (2003) Long-distance transport in non-vascular plants. Plant, Cell and Environment 26: 73-85Pubmed: Author and TitleCrossRef: Author and TitleGoogle Scholar: Author Only Title Only Author and Title

Rickelt L, Lichtenberg M, Trampe E, Kühl M (2016) Fiber-optic probes for small scale measurements of scalar irradiance.Photochemistry and Photobiology 92: 331-342


Sand-Jensen K, Krause-Jensen D (1997) Broad-scale comparison of photosynthesis in terrestrial and aquatic plant communities. Oikos80: 203-208


Sarijeva G, Knapp M, Lichtenthaler HK (2007) Differences in photosynthetic activity, chlorophyll and carotenoid levels, and inchlorophyll fluorescence parameters in green sun and shade leaves of Ginkgo and Fagus. Plant Physiology 164: 950-955


Schreiber U (2004) Pulse-Amplitude-Modulation (PAM) fluorometry and saturation pulse method: An overview. In GC Papageorgiou,Govindjee, eds, Chlorophyll a Fluorescence, Vol 19. Springer Netherlands, pp 279-319

Pubmed: Author and Title www.plantphysiol.orgon April 9, 2019 - Published by Downloaded from

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http://search.crossref.org/?page=1&rows=2&q=McMillen GG, McClendon JH (1979) Leaf angle: An adaptive feature of sun and shade leaves. Botanical Gazette 140: 437-442

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http://www.ncbi.nlm.nih.gov/pubmed?term=Moss BL %281983%29 Sieve elements in the Fucales%2E New Phytologist 93%3A 433%2D437&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Moss BL (1983) Sieve elements in the Fucales. New Phytologist 93: 433-437

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http://www.ncbi.nlm.nih.gov/pubmed?term=Myers DA%2C Jordan DN%2C Vogelmann TC %281997%29 Inclination of sun and shade leaves influences chloroplast light harvesting and utilization%2E Physiologia Plantarum 99%3A 395%2D404&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Myers DA, Jordan DN, Vogelmann TC (1997) Inclination of sun and shade leaves influences chloroplast light harvesting and utilization. Physiologia Plantarum 99: 395-404

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Myers&hl=en&c2coff=1

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http://www.ncbi.nlm.nih.gov/pubmed?term=Oguchi R%2C Douwstra P%2C Fujita T%2C Chow WS%2C Terashima I %282011%29 Intra%2Dleaf gradients of photoinhibition induced by different color lights%3A implications for the dual mechanisms of photoinhibition and for the application of conventional chlorophyll fluorometers%2E New Phytologist 191%3A 146%2D159&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Oguchi R, Douwstra P, Fujita T, Chow WS, Terashima I (2011) Intra-leaf gradients of photoinhibition induced by different color lights: implications for the dual mechanisms of photoinhibition and for the application of conventional chlorophyll fluorometers. New Phytologist 191: 146-159

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http://www.ncbi.nlm.nih.gov/pubmed?term=Privoznik KG%2C Daniel KJ%2C Incropera FP %281978%29 Absorption%2C extinction and phase function measurements for algal suspensions of Chlorella pyrenoidosa%2E Journal of Quantitative Spectroscopy %26 Radiative Transfer 20%3A 345%2D352&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Privoznik KG, Daniel KJ, Incropera FP (1978) Absorption, extinction and phase function measurements for algal suspensions of Chlorella pyrenoidosa. Journal of Quantitative Spectroscopy & Radiative Transfer 20: 345-352

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http://www.ncbi.nlm.nih.gov/pubmed?term=Ramus J %281978%29 Seaweed anatomy and photosynthetic performance %2D ecological significance of light guides%2C heterogeneous absorption and multiple scatter%2E Journal of Phycology 14%3A 352%2D362&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Ramus J (1978) Seaweed anatomy and photosynthetic performance - ecological significance of light guides, heterogeneous absorption and multiple scatter. Journal of Phycology 14: 352-362

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http://www.ncbi.nlm.nih.gov/pubmed?term=Raven JA %282003%29 Long%2Ddistance transport in non%2Dvascular plants%2E Plant%2C Cell and Environment 26%3A 73%2D85&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Rickelt L%2C Lichtenberg M%2C Trampe E%2C K�hl M %282016%29 Fiber%2Doptic probes for small scale measurements of scalar irradiance%2E Photochemistry and Photobiology 92%3A 331%2D342&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Sand%2DJensen K%2C Krause%2DJensen D %281997%29 Broad%2Dscale comparison of photosynthesis in terrestrial and aquatic plant communities%2E Oikos 80%3A 203%2D208&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Sarijeva G%2C Knapp M%2C Lichtenthaler HK %282007%29 Differences in photosynthetic activity%2C chlorophyll and carotenoid levels%2C and in chlorophyll fluorescence parameters in green sun and shade leaves of Ginkgo and Fagus%2E Plant Physiology 164%3A 950%2D955&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Sarijeva G, Knapp M, Lichtenthaler HK (2007) Differences in photosynthetic activity, chlorophyll and carotenoid levels, and in chlorophyll fluorescence parameters in green sun and shade leaves of Ginkgo and Fagus. Plant Physiology 164: 950-955

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http://www.ncbi.nlm.nih.gov/pubmed?term=Schreiber U %282004%29 Pulse%2DAmplitude%2DModulation %28PAM%29 fluorometry and saturation pulse method%3A An overview%2E In GC Papageorgiou%2C Govindjee%2C eds%2C Chlorophyll a Fluorescence%2C Vol 19%2E Springer Netherlands%2C pp 279%2D319&dopt=abstract


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Schreiber U, Klughammer C, Kolbowski J (2012) Assessment of wavelength-dependent parameters of photosynthetic electrontransport with a new type of multi-color PAM chlorophyll fluorometer. Photosynthesis Research 113: 127-144


Schreiber U, Kühl M, Klimant I, Reising H (1996) Measurement of chlorophyll fluorescence within leaves using a modified PAMFluorometer with a fiber-optic microprobe. Photosynthesis Research 47: 103-109


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Terashima I, Ooeda H, Fujita T, Oguchi R (2016) Light environment within a leaf. II. Progress in the past one-third century. Journal of www.plantphysiol.orgon April 9, 2019 - Published by Downloaded from Copyright © 2017 American Society of Plant Biologists. All rights reserved.

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http://scholar.google.com/scholar?as_q=&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Schreiber&as_ylo=2004&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Schreiber U%2C Bilger W%2C Neubauer C %281995%29 Chlorophyll fluorescence as a nonintrusive indicator for rapid assessment of In vivo photosynthesis%2E In E%2DD Schulze%2C MM Caldwell%2C eds%2C Ecophysiology of Photosynthesis%2E Springer Berlin Heidelberg%2C Berlin%2C Heidelberg%2C pp 49%2D70&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Schreiber U, Bilger W, Neubauer C (1995) Chlorophyll fluorescence as a nonintrusive indicator for rapid assessment of In vivo photosynthesis. In E-D Schulze, MM Caldwell, eds, Ecophysiology of Photosynthesis. Springer Berlin Heidelberg, Berlin, Heidelberg, pp 49-70


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http://www.ncbi.nlm.nih.gov/pubmed?term=Schreiber U%2C Klughammer C%2C Kolbowski J %282012%29 Assessment of wavelength%2Ddependent parameters of photosynthetic electron transport with a new type of multi%2Dcolor PAM chlorophyll fluorometer%2E Photosynthesis Research 113%3A 127%2D144&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Schreiber U, Klughammer C, Kolbowski J (2012) Assessment of wavelength-dependent parameters of photosynthetic electron transport with a new type of multi-color PAM chlorophyll fluorometer. Photosynthesis Research 113: 127-144


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http://www.ncbi.nlm.nih.gov/pubmed?term=Schreiber U%2C K�hl M%2C Klimant I%2C Reising H %281996%29 Measurement of chlorophyll fluorescence within leaves using a modified PAM Fluorometer with a fiber%2Doptic microprobe%2E Photosynthesis Research 47%3A 103%2D109&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Vogelmann TC%2C Bj�rn LO %281984%29 Measurement of light gradients and spectral regime in plant%2Dtissue with a fiber optic probe%2E Physiologia Plantarum 60%3A 361%2D368&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Vogelmann TC%2C Bornman JF%2C Josserand S %281989%29 Photosynthetic light gradients and spectral regime within leaves of Medicago sativa%2E Philosophical Transactions of the Royal Society of London B%3A Biological Sciences 323%3A 411%2D421&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Vogelmann TC%2C Bornman JF%2C Yates DJ %281996%29 Focusing of light by leaf epidermal cells%2E Physiologia Plantarum 98%3A 43%2D56&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Vogelmann TC%2C Evans JR %282002%29 Profiles of light absorption and chlorophyll within spinach leaves from chlorophyll fluorescence%2E Plant Cell and Environment 25%3A 1313%2D1323&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Vogelmann TC%2C Han T %282000%29 Measurement of gradients of absorbed light in spinach leaves from chlorophyll fluorescence profiles%2E Plant Cell and Environment 23%3A 1303%2D1311&dopt=abstract

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http://scholar.google.com/scholar?as_q=The functional-significance of palisade tissue - penetration of directional versus diffuse light&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Vogelmann&as_ylo=1993&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Wang LH%2C Jacques SL%2C Zheng LQ %281995%29 Monte%2DCarlo modeling of light transport in multilayered tissues%2E Computer Methods and Programs in Biomedicine 47%3A 131%2D146&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Wangpraseurt D%2C Jacques SL%2C Petrie T%2C K�hl M %282016a%29 Monte Carlo modeling of photon propagation reveals highly scattering coral tissue%2E Frontiers in Plant Science 7%3A1404&dopt=abstract

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Yukihira N, Sugai Y, Fujiwara M, Kosumi D, Iha M, Sakaguchi K, Katsumura S, Gardiner AT, Cogdell RJ, Hashimoto H (2017) Strategiesto enhance the excitation energy-transfer efficiency in a light-harvesting system using the intramolecular charge transfer character ofcarotenoids. Faraday Discussions 198: 59-71



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light sheet microscopy imaging of light absorption and photosynthesis distribution in plant tissue

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