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  • Editor: Christian Seassal
  • Vol. 21, Iss. S6 — Nov. 4, 2013
  • pp: A909–A916
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Enhanced photosynthetic activity in Spinacia oleracea by spectral modification with a photoluminescent light converting material

Qi Xia, Miroslaw Batentschuk, Andres Osvet, Peter Richter, Donat P. Häder, Juergen Schneider, Christoph J. Brabec, Lothar Wondraczek, and Albrecht Winnacker  »View Author Affiliations


Optics Express, Vol. 21, Issue S6, pp. A909-A916 (2013)
http://dx.doi.org/10.1364/OE.21.00A909


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Abstract

The spectral conversion of incident sunlight by appropriate photoluminescent materials has been a widely studied issue for improving the efficiency of photovoltaic solar energy harvesting. By using phosphors with suitable excitation/emission properties, also the light conditions for plants can be adjusted to match the absorption spectra of chlorophyll dyes, in this way increasing the photosynthetic activity of the plant. Here, we report on the application of this principle to a high plant, Spinacia oleracea. We employ a calcium strontium sulfide phosphor doped with divalent europium (Ca0.4Sr0.6S:Eu2+, CSSE) on a backlight conversion foil in photosynthesis experiments. We show that this phosphor can be used to effectively convert green to red light, centering at a wavelength of ~650 nm which overlaps the absorption peaks of chlorophyll a/b pigments. A measurement system was developed to monitor the photosynthetic activity, expressed as the CO2 assimilation rate of spinach leaves under various controlled light conditions. Results show that under identical external light supply which is rich in green photons, the CO2 assimilation rate can be enhanced by more than 25% when the actinic light is modified by the CSSE conversion foil as compared to a purely reflecting reference foil. These results show that the phosphor could be potentially applied to modify the solar spectrum by converting the green photons into photosynthetically active red photons for improved photosynthetic activity.

© 2013 OSA

Introduction

Chlorophylls - along with other accessory pigments of high plants - capture photosynthetically active radiation (PAR) from solar energy to carry out photosynthetic reactions. The absorption spectra of chlorophyll a and b have two major peaks in the blue (400 - 500nm) and red (600 - 700nm) regions [1

1. L. Taiz and E. Zeiger, “Photosynthesis: the light reactions,” in Plant Physiology (Sinauer Associates, Inc., 2006), pp. 126–158.

]. The action spectra based on various plant species show similar profiles with major peaks in the two mentioned spectral regions [2

2. N. R. Bulley, C. D. Nelson, and E. B. Tregunna, “Photosynthesis: action spectra for leaves in normal and low oxygen,” Plant Physiol. 44(5), 678–684 (1969). [CrossRef] [PubMed]

5

5. K. Inada, “Action spectra for photosynthesis in higher plants,” Plant Cell Physiol. 17, 355–365 (1976).

]. Green photons (500 – 600 nm), however, are less active for photosynthetic actions such as CO2 assimilation and biomass production, although they account for as much as 35% of the whole solar PAR energy.

In the present report, we discuss the application of a CSSE conversion to convert incident light which is rich in green photons and its effect on the photosynthetic activity of Spinacia oleracea (S.o.), green living spinach leaves. We show that in this way, about 30% higher CO2 assimilation rates can be achieved experimentally as compared to equivalent reference conditions without spectral converter, and we directly attribute this improvement to the light conversion process.

1. Experimental

1.1 Phosphor synthesis and converter fabrication

CSSE particles with an average size of 20 µm were obtained after a post-treatment which consisted of a milling and a sedimentation step. The particles were then embedded in resin and coated on a highly reflective aluminum surface (~325 cm2) by doctor blading method. The active layer was finally encapsulated with a matt polymer foil to protect it from ambient moisture and oxidation. In the following, this multi-layer system will be referred to as the converter foil (C-foil). Correspondingly, a reference foil (R-foil) was prepared in the same way, but using neutral MgO particles (99.99%, Alfa Aesar) as filler particles instead of CSSE. Absorption, remission and photoluminescence excitation and emission spectra of both foils were recorded with a UV-VIS spectrophotometer (Perkin Elmer Lambda 900) and a high-resolution spectrofluorometer (Horiba Jobin-Yvon Fluorolog 3-22), respectively.

1.2 Analyses of photosynthetic activity

The photosynthetic activity of S.o. was assessed indirectly via monitoring the CO2 assimilation rate as a function of time with and without the converter. For that, a reactor was constructed as shown in Fig. 1
Fig. 1 Schematic of the set-up for assessment of the photosynthetic activity: 1 - metal halide lamps, 2 - dichroic filters, 3 - water shielding, 4 - intact leaves of S. o. fixed on a sponge, 5 - C- or R-foil, 6 - CO2/humidity/temperature sensors, 7 - ventilation fan, 8 - gas inlet/outlet for compressed air through a water bottle, 9 - spectrometric probe, 10 –data logger.
. The primary incident light was generated by four metal halide lamps (Philips type 13117, 17 V, 150 W) combined with dichroic filters (green, Edmund Optics). The photosynthetic photon flux density (PPFD) could be adjusted from 200.5 to 4059.8 µmol/(m2s) by decreasing the distance between the lamps and the light entrance into the reaction cell. In order to prevent undesired heating of the reactor, the infrared spectral part of the incoming irradiation was filtered-out with a 7 cm-thick water shield which was placed between the reaction cell and the light source. As the reaction unit, a gas-tight glass cell with an inner volume of 5 l was used. This cell was blackened on all sides except for a top light entrance with an area of 150 cm2. Intact S. o. leaves were cut into six squares of 6 x 6 cm2 each and their surface was sterilized with an ethanol spray and rinsed three times in tap water. They were then fixed on a sterilized sponge as humidity reservoir, shaded from the top and exposed to solely either converted or reflected light from the C-/R-foils as depicted in Fig. 1. The incoming spectra for both light conditions were recorded with a UV-VIS spectrometric probe (Carry 500, Varian). The CO2 concentration, relative humidity and temperature inside the reaction cell were measured with a CO2 sensor (Voltcraft, CO-10) and a humidity/temperature sensor (Extech, 445815). Data from all sensors and the spectrometer were logged by a computer. The internal gas distribution was homogenized with a ventilation fan. For each measurement the starting CO2 concentration was fixed at 443 ± 8 ppm by flushing compressed air through a water bottle, and CO2 concentrations (ppm) were logged every minute for 20 min at starting temperatures of about 22 °C. The relative humidity was maintained above 90% with the sponge as wet medium. The absorbance the S. o. chloroplasts was also measured ex situ. For that, several S. o. leaves were milled in methanol, the obtained suspension was filtered and then filled into a cuvette which was placed into the UV-VIS spectrometer.

2. Results and discussion

2.1 Spectral modification

As shown in Fig. 3
Fig. 3 Calculated spectra of the effective incident light, derived from multiplication of the PPFD spectra of the C- and R-foil, respectively, with the absorption spectrum of the S.o. chloroplasts. The inset shows the absorption spectrum of the S. o. chloroplasts.
, the spectral distribution of the effectively absorbed PPFD can be estimated by multiplying the PPFD spectra of Fig. 2 with the relative absorbance spectrum of the chloroplasts of S. o. (inset Fig. 3). Absorbance of S. o. is substantially low at wavelengths of 500-600 nm, but exhibits strong bands in the blue and red spectral ranges. This provides a good match to the photoluminescence spectrum of the CSSE phosphor (Fig. 2), which leads - when employed - to an improvement of photon absorbance. By integrating the spectra in Fig. 3, one can quantitatively analyze the spectral distribution of the absorbed photons (Table 2

Table 2. Integrated absorbed PPFD fraction in the presence of the R-foil and the C-foil, respectively, for selected spectral regions and relative to the total PPFD integrated over the full spectrum (%).

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). We find that in the presence of the C-foil, in principle, the number of absorbed photons can be increased by a factor of 2.8.

Figure 4
Fig. 4 Incident PPFD in the reaction cell (left axis) and absorbed fraction of PPFD (right axis) in the presence of C- and R-foil, respectively, as a function of primary incident photon flux density PFD.
shows the total PPFD in the reaction cell and the fraction which is absorbed on the S.o. surface in the presence of the C- and R-foil, respectively, as a function of the primary incident photon flux. The values are obtained by integrating the corresponding spectra over the wavelength range of 400 to 700 nm. It is clear that in both cases, the value of the PPFD increases linearly with the number of incident primary photons, and that the presence of the C-foil results in a notable increase of overall PPFD and absorbed PPFD as compared to using the R-foil.

2.2 Photosynthesis and CO2 assimilation

In Fig. 5(a)
Fig. 5 (a) CO2 concentration inside the reaction cell recorded over time in the presence of C- and R-foil, respectively. Data in (a) was adopted from Ref [25]. (b) Specific CO2 assimilation rate in the presence of C. and R-foil, respectively, under various primary photon flux densities. The dashed lines represent fits of the data to a Levenberg-Marquardt function, Eq. (1).
, the CO2 concentration inside the cell is plotted as a function of time for exemplary experiments (primary PPF of ~1014.9 µmol/m2 s) in the presence of the C- and R-foil, respectively [25

25. Q. Xia, M. Batentschuk, A. Osvet, P. Richter, D.-P. Häder, J. Schneider, L. Wondraczek, A. Winnacker, and C. J. Brabec, “Red-emitting Ca(1-x)SrxS:Eu2+ phosphors as light converters for plant-growth applications. MRS Proc. 1342, mrss11-1342-v04-04 (2011). [CrossRef]

]. As the photosynthetic activity is sensitive to temperature and relative humidity [6

6. A. Andersen, “Comparison of fluorescent lamps as an energy source for production of tomato plants in a controlled environment,” Sci. Hortic. (Amsterdam) 28(1-2), 11–18 (1986). [CrossRef]

-27

27. G. E. Edwards and N. R. Baker, “Can CO2 assimilation in maize leaves be predicted accurately from chlorophyll fluorescence analysis?” Photosynth. Res. 37(2), 89–102 (1993). [CrossRef]

], also the environmental parameters were recorded (not shown): During each individual experiment, the temperature inside the reaction cell increased slightly from a fixed starting temperature of about 22.1 °C to 23.8 °C; the relative humidity remained constant at a value above 90%. No deviations could be observed between the cells equipped with C- or R-foil. From the CO2 concentration curves, the decline rates were evaluated to be 11.0 and 13.9 ppm/min in the presence of the R-foil and the C-foil, respectively. The small magnitudes of error bars reflect the good reproducibility of the measurements. Taking into account the experimental parameters of total leaf surface area of 216 cm2 and the cell volume of 5 l, one can calculate the specific CO2 assimilation rate for both conditions as 1.89 µmol CO2/(m2s) for the R-foil and 2.39 µmol CO2/(m2s) for the C-foil. This means that as a result of spectral conversion, ambient CO2 is assimilated by S.o. more than 25% faster.

In the presence of the C-foil, the CO2 assimilation curves were obtained for various values of primary PFD as shown in Fig. 6
Fig. 6 CO2 concentration in the presence of the C-foil under various primary photon flux densities as a function of time.
. When S.o. were held in the dark, a mitochondrial respiration rate of about 7.27 ppm/min (1.3 µmol/(m2s)) was found. With increasing primary PFD, CO2 assimilation increased significantly due to increasing photosynthetic activity. The specific CO2 assimilation rates in the presence of C- and R-foil, respectively, are compared in Fig. 5(b). Both data sets show a typical Levenberg-Marquardt profile which is characterized by an initial linear increase with increasing primary PFD followed by a light saturation region which starts at about 649 µmol/(m2s):
Px=PmI0K+I0Rx,
(1)
where Px is the specific CO2 assimilation rate, Pm is the maximum value of Px, Rx is the specific respiration rate and Io is the incident PPFD. The fitting parameters Pm and K are listed are inset in Fig. 6. In agreement with the data shown in Fig. 4, in the presence of the C-foil, the CO2 assimilation rates are about 28.1 ± 7.5% higher as compared to the R-foil. From Fig. 5(b) it can be deduced that the light compensation points - the PFD where CO2 assimilation rate equals the respiration rate - is about 77 µmol/(m2s) lower in the presence of the C-foil as compared to the R-foil. This is taken as a direct result of the higher photosynthesis activity which is achieved by spectral conversion.

Conclusions

In summary, we investigated the influence of spectral photoluminescent conversion of the incident light on the photosynthetic activity of a higher green plant, S.o.. For that, the CO2 assimilation rate of S. o. leaves was monitored under controlled illumination. A calcium strontium sulfide phosphor doped with divalent europium was used on a backlight conversion foil to effectively adjust the incoming light spectrum to the region of optimal absorption of the plants chloroplasts. We show that this phosphor can be used to convert green to red light, centering at a wavelength of ~650 nm which overlaps the absorption peaks of chlorophyll a/b. Under identical external light supply which is rich in green photons, the CO2 assimilation rate can be enhanced by more than 25% when the actinic light is modified by conversion foil as compared to a purely reflecting reference foil. These results show that the phosphor could be potentially applied to modify the solar spectrum by converting the green photons into photosynthetically active red photons for improved photosynthetic activity.

Acknowledgments

The authors gratefully acknowledge the Bayerische Forschungsstiftung for kindly sponsoring the project (Project number: DOK-102-08). We also thank Dipl.-Ing. S. Krolikowski and Dr. M. Peng (WW3, FAU) for valuable assistance in the photoluminescence characterization and Dr. R. Auer, Dr. V. Kazuz, and Dipl.-Phys. T. Swonke (ZAE Bayern) for providing the light facilities.

References and links

1.

L. Taiz and E. Zeiger, “Photosynthesis: the light reactions,” in Plant Physiology (Sinauer Associates, Inc., 2006), pp. 126–158.

2.

N. R. Bulley, C. D. Nelson, and E. B. Tregunna, “Photosynthesis: action spectra for leaves in normal and low oxygen,” Plant Physiol. 44(5), 678–684 (1969). [CrossRef] [PubMed]

3.

J. B. Clark and G. R. Lister, “Photosynthetic action spectra of trees: I. Comparative photosynthetic action spectra of one deciduous and four coniferous tree species as related to photorespiration and pigment complements,” Plant Physiol. 55(2), 401–406 (1975). [CrossRef] [PubMed]

4.

K. J. McCree, “The action spectrum, absorptance and quantum yield of photosynthesis in crop plants,” Agric. Meteorol. 9, 191–216 (1972). [CrossRef]

5.

K. Inada, “Action spectra for photosynthesis in higher plants,” Plant Cell Physiol. 17, 355–365 (1976).

6.

A. Andersen, “Comparison of fluorescent lamps as an energy source for production of tomato plants in a controlled environment,” Sci. Hortic. (Amsterdam) 28(1-2), 11–18 (1986). [CrossRef]

7.

N. G. Bukhov, I. S. Drozdova, V. V. Bondar, and A. T. Mokronosov, “Blue, red and blue plus red light control of chlorophyll content and CO2 gas exchange in barley leaves: Quantitative description of the effects of light quality and fluence rate,” Physiol. Plant. 85(4), 632–638 (1992). [CrossRef]

8.

J. Ernstsen, I. E. Woodrow, and K. A. Mott, “Effects of growth-light quantity, growth-light quality and CO2 concentration on Rubisco deactivation during low PFD or darkness,” Photosynth. Res. 61(1), 65–75 (1999). [CrossRef]

9.

K. Humbeck, B. Hoffmann, and H. Senger, “Influence of energy flux and quality of light on the molecular organization of the photosynthetic apparatus in Scenedesmus,” Planta 173(2), 205–212 (1988). [CrossRef]

10.

N. G. Bukhov, I. S. Drozdova, and V. V. Bondar, “Light response curves of photosynthesis in leaves of sun-type and shade-type plants grown in blue or red light,” J. Photochem. Photobiol. B 30(1), 39–41 (1995). [CrossRef]

11.

H. Yu and B. Ong, “Effect of radiation quality on growth and photosynthesis of Acacia mangium seedlings,” Photosynthetica 41(3), 349–355 (2003). [CrossRef]

12.

G. D. Goins, N. C. Yorio, M. M. Sanwo, and C. S. Brown, “Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting,” J. Exp. Bot. 48(7), 1407–1413 (1997). [CrossRef] [PubMed]

13.

G. Tamulaitis, P. Duchovskis, Z. Bliznikas, K. Breive, R. Ulinskaite, A. Brazaeityte, A. Novickovas, and A. Zukauskas, “High-power light-emitting diode based facility for plant cultivation,” J. Phys. D Appl. Phys. 38(17), 3182–3187 (2005). [CrossRef]

14.

J. W. Heo, K. S. Shin, S. K. Kim, and K. Y. Paek, “Light quality affects in vitro growth of grape 'Teleki 5BB',” J. Plant Biol. 49(4), 276–280 (2006). [CrossRef]

15.

S. Lian, C. Li, X. Mao, and H. Zhang, “H. “On application of converting green to red of CaS:Eu in agriculture,” Chin. Rare Earths. 23, 37–40 (2002).

16.

L. Ma, D. Wang, Z. Mao, Q. Lu, and Z. Yuan, “Investigation of Eu–Mn energy transfer in A3MgSi2O8:Eu2+, Mn2+ A=Ca,Sr,Ba for light-emitting diodes for plant cultivation,” Appl. Phys. Lett. 93(14), 144101 (2008). [CrossRef]

17.

G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, 1994).

18.

G. Gao, S. Reibstein, M. Peng, and L. Wondraczek, “Tunable dual-mode photoluminescence from nanocrystalline Eu-doped Li2ZnSiO4 glass ceramic phosphors,” J. Mater. Chem. 21(9), 3156–3161 (2011). [CrossRef]

19.

G. Gao, N. Da, S. Reibstein, and L. Wondraczek, “Enhanced photoluminescence from mixed-valence Eu-doped nanocrystalline silicate glass ceramics,” Opt. Express 18(S4Suppl 4), A575–A583 (2010). [CrossRef] [PubMed]

20.

P. F. Smet, I. Moreels, Z. Hens, and D. Poelman, “Luminescence in sulfides: a rich history and a bright future,” Mater. 3(4), 2834–2883 (2010). [CrossRef]

21.

Q. Xia, M. Batentschuk, A. Osvet, A. Winnacker, and J. Schneider, “Quantum yield of Eu2+ emission in (Ca1−xSrx)S:Eu light emitting diode converter at 20–420 K,” Radiat. Meas. 45(3-6), 350–352 (2009). [CrossRef]

22.

L. Wondraczek, M. Batentschuk, M. A. Schmidt, R. Borchardt, S. Scheiner, B. Seemann, P. Schweizer, and C. J. Brabec, “Solar spectral conversion for improving the photosynthetic activity in algae reactors,” Nat Commun 4, 2047 (2013), doi:. [CrossRef] [PubMed]

23.

E. Danielson, A. Ellens, F. Jermann, W. Rossner, M. Devenney, D. Giaquinta, and M. Kobusch, “Light emitting device for generating specific colored light, including white light,” US Patent no. 6,850,002 B2 (2005).

24.

S. Lian, “Ultramicro/nano solar dual conversion material, and its preparing method and use. Chin. Patent application. no. CN 1935937 A (2007).

25.

Q. Xia, M. Batentschuk, A. Osvet, P. Richter, D.-P. Häder, J. Schneider, L. Wondraczek, A. Winnacker, and C. J. Brabec, “Red-emitting Ca(1-x)SrxS:Eu2+ phosphors as light converters for plant-growth applications. MRS Proc. 1342, mrss11-1342-v04-04 (2011). [CrossRef]

26.

H. A. Mooney, C. Field, C. V. Yanes, and C. Chu, “Environmental controls on stomatal conductance in a shrub of the humid tropics,” Proc. Natl. Acad. Sci. U.S.A. 80(5), 1295–1297 (1983). [CrossRef] [PubMed]

27.

G. E. Edwards and N. R. Baker, “Can CO2 assimilation in maize leaves be predicted accurately from chlorophyll fluorescence analysis?” Photosynth. Res. 37(2), 89–102 (1993). [CrossRef]

OCIS Codes
(160.2540) Materials : Fluorescent and luminescent materials
(170.1420) Medical optics and biotechnology : Biology
(350.5130) Other areas of optics : Photochemistry
(350.6050) Other areas of optics : Solar energy

ToC Category:
Fluorescent and Luminescent Materials

History
Original Manuscript: April 11, 2013
Revised Manuscript: July 30, 2013
Manuscript Accepted: July 31, 2013
Published: September 12, 2013

Virtual Issues
Vol. 9, Iss. 1 Virtual Journal for Biomedical Optics

Citation
Qi Xia, Miroslaw Batentschuk, Andres Osvet, Peter Richter, Donat P. Häder, Juergen Schneider, Christoph J. Brabec, Lothar Wondraczek, and Albrecht Winnacker, "Enhanced photosynthetic activity in Spinacia oleracea by spectral modification with a photoluminescent light converting material," Opt. Express 21, A909-A916 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-S6-A909


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References

  1. L. Taiz and E. Zeiger, “Photosynthesis: the light reactions,” in Plant Physiology (Sinauer Associates, Inc., 2006), pp. 126–158.
  2. N. R. Bulley, C. D. Nelson, and E. B. Tregunna, “Photosynthesis: action spectra for leaves in normal and low oxygen,” Plant Physiol.44(5), 678–684 (1969). [CrossRef] [PubMed]
  3. J. B. Clark and G. R. Lister, “Photosynthetic action spectra of trees: I. Comparative photosynthetic action spectra of one deciduous and four coniferous tree species as related to photorespiration and pigment complements,” Plant Physiol.55(2), 401–406 (1975). [CrossRef] [PubMed]
  4. K. J. McCree, “The action spectrum, absorptance and quantum yield of photosynthesis in crop plants,” Agric. Meteorol.9, 191–216 (1972). [CrossRef]
  5. K. Inada, “Action spectra for photosynthesis in higher plants,” Plant Cell Physiol.17, 355–365 (1976).
  6. A. Andersen, “Comparison of fluorescent lamps as an energy source for production of tomato plants in a controlled environment,” Sci. Hortic. (Amsterdam)28(1-2), 11–18 (1986). [CrossRef]
  7. N. G. Bukhov, I. S. Drozdova, V. V. Bondar, and A. T. Mokronosov, “Blue, red and blue plus red light control of chlorophyll content and CO2 gas exchange in barley leaves: Quantitative description of the effects of light quality and fluence rate,” Physiol. Plant.85(4), 632–638 (1992). [CrossRef]
  8. J. Ernstsen, I. E. Woodrow, and K. A. Mott, “Effects of growth-light quantity, growth-light quality and CO2 concentration on Rubisco deactivation during low PFD or darkness,” Photosynth. Res.61(1), 65–75 (1999). [CrossRef]
  9. K. Humbeck, B. Hoffmann, and H. Senger, “Influence of energy flux and quality of light on the molecular organization of the photosynthetic apparatus in Scenedesmus,” Planta173(2), 205–212 (1988). [CrossRef]
  10. N. G. Bukhov, I. S. Drozdova, and V. V. Bondar, “Light response curves of photosynthesis in leaves of sun-type and shade-type plants grown in blue or red light,” J. Photochem. Photobiol. B30(1), 39–41 (1995). [CrossRef]
  11. H. Yu and B. Ong, “Effect of radiation quality on growth and photosynthesis of Acacia mangium seedlings,” Photosynthetica41(3), 349–355 (2003). [CrossRef]
  12. G. D. Goins, N. C. Yorio, M. M. Sanwo, and C. S. Brown, “Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting,” J. Exp. Bot.48(7), 1407–1413 (1997). [CrossRef] [PubMed]
  13. G. Tamulaitis, P. Duchovskis, Z. Bliznikas, K. Breive, R. Ulinskaite, A. Brazaeityte, A. Novickovas, and A. Zukauskas, “High-power light-emitting diode based facility for plant cultivation,” J. Phys. D Appl. Phys.38(17), 3182–3187 (2005). [CrossRef]
  14. J. W. Heo, K. S. Shin, S. K. Kim, and K. Y. Paek, “Light quality affects in vitro growth of grape 'Teleki 5BB',” J. Plant Biol.49(4), 276–280 (2006). [CrossRef]
  15. S. Lian, C. Li, X. Mao, and H. Zhang, “H. “On application of converting green to red of CaS:Eu in agriculture,” Chin. Rare Earths.23, 37–40 (2002).
  16. L. Ma, D. Wang, Z. Mao, Q. Lu, and Z. Yuan, “Investigation of Eu–Mn energy transfer in A3MgSi2O8:Eu2+, Mn2+ A=Ca,Sr,Ba for light-emitting diodes for plant cultivation,” Appl. Phys. Lett.93(14), 144101 (2008). [CrossRef]
  17. G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, 1994).
  18. G. Gao, S. Reibstein, M. Peng, and L. Wondraczek, “Tunable dual-mode photoluminescence from nanocrystalline Eu-doped Li2ZnSiO4 glass ceramic phosphors,” J. Mater. Chem.21(9), 3156–3161 (2011). [CrossRef]
  19. G. Gao, N. Da, S. Reibstein, and L. Wondraczek, “Enhanced photoluminescence from mixed-valence Eu-doped nanocrystalline silicate glass ceramics,” Opt. Express18(S4Suppl 4), A575–A583 (2010). [CrossRef] [PubMed]
  20. P. F. Smet, I. Moreels, Z. Hens, and D. Poelman, “Luminescence in sulfides: a rich history and a bright future,” Mater.3(4), 2834–2883 (2010). [CrossRef]
  21. Q. Xia, M. Batentschuk, A. Osvet, A. Winnacker, and J. Schneider, “Quantum yield of Eu2+ emission in (Ca1−xSrx)S:Eu light emitting diode converter at 20–420 K,” Radiat. Meas.45(3-6), 350–352 (2009). [CrossRef]
  22. L. Wondraczek, M. Batentschuk, M. A. Schmidt, R. Borchardt, S. Scheiner, B. Seemann, P. Schweizer, and C. J. Brabec, “Solar spectral conversion for improving the photosynthetic activity in algae reactors,” Nat Commun4, 2047 (2013), doi:. [CrossRef] [PubMed]
  23. E. Danielson, A. Ellens, F. Jermann, W. Rossner, M. Devenney, D. Giaquinta, and M. Kobusch, “Light emitting device for generating specific colored light, including white light,” US Patent no. 6,850,002 B2 (2005).
  24. S. Lian, “Ultramicro/nano solar dual conversion material, and its preparing method and use. Chin. Patent application. no. CN 1935937 A (2007).
  25. Q. Xia, M. Batentschuk, A. Osvet, P. Richter, D.-P. Häder, J. Schneider, L. Wondraczek, A. Winnacker, and C. J. Brabec, “Red-emitting Ca(1-x)SrxS:Eu2+ phosphors as light converters for plant-growth applications. MRS Proc. 1342, mrss11-1342-v04-04 (2011). [CrossRef]
  26. H. A. Mooney, C. Field, C. V. Yanes, and C. Chu, “Environmental controls on stomatal conductance in a shrub of the humid tropics,” Proc. Natl. Acad. Sci. U.S.A.80(5), 1295–1297 (1983). [CrossRef] [PubMed]
  27. G. E. Edwards and N. R. Baker, “Can CO2 assimilation in maize leaves be predicted accurately from chlorophyll fluorescence analysis?” Photosynth. Res.37(2), 89–102 (1993). [CrossRef]

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