Photosynthetically active radiation

The plant pigments chlorophyll a and b and carotenoids are actively involved in photosynthesis . From their absorption (above) spectra has been derived wavelength dependence of photosynthesis and experimentally verified (below)

As photosynthetically active radiation ( engl. : Photosynthetically Active Radiation short PAR or Phar) is that electromagnetic radiation in the range of the light spectrum define the phototrophic organisms mainly in the photosynthesis use. It largely coincides with the range of radiation visible to humans with a wavelength between 380 nm and 780 nm. About 50 percent of global radiation is in this range .

The photosynthetic efficiency is an efficiency and depends on the wavelength of the light and the absorption behavior of the photosynthetically active substances. Its spectral distribution is of interest, such representations and effects are called action spectrum (of photosynthesis) , spectrum of action (of photosynthesis) or spectrum of action of photosynthesis .

An assimilation chamber can be used to determine its own spectrum of activity of these light rays for each plant species . This measures CO ₂ consumption or O ₂ production when the plants are irradiated with light of different wavelengths. The previously determined wavelength-dependent photosynthesis rates of individual plant species are based on different contents and different absorption maxima of the photosensitive chromatophores (e.g. chlorophylls , carotenoids , phycoerythrin or phycocyanin ), the different methods for determining and the state of the technical development of the measuring devices.

The research of PAR was in the beginning basic research on plant cultivation. Since more and more plants are grown under exclusively artificial lighting, there have been economic reasons to waste as little energy as possible on "useless" lighting.

Specification and units of measurement

The PAR can be specified depending on different sizes:

Light energy irradiated as PAR irradiance (in the spectral range 400 to 700 nm) per time and area.

as PAR photon flux (PPF from English Photosynthetic Photon Flux ): number of irradiated photons in the spectral range 400 to 700 nm per time

as PAR photon flux density (PPFD from English Photosynthetically Active Photon Flux Density ) number of irradiated photons in the spectral range 400 to 700 nm per time and area.

Research history

Theodor Wilhelm Engelmann (1843–1909) showed with his bacterial experiment how the formation of oxygen during oxygenic photosynthesis is light- dependent in different areas of the light spectrum .

Research from the last half of the 19th century onwards showed that all light that can be seen with the human eye is also capable of causing carbon dioxide uptake and oxygen excretion in chlorophyll-bearing green plants. (For literature see Gabrielsen 1940 and Rabinowitch 1951).

WB Hoover examined the PAR using the example of wheat and in 1937 created a PAR curve with the percentage rate of photosynthesis on the ordinate and wavelength on the abscissa . In 1961 Govindjee published a graph of the PAR in which the percent photosynthesis rate is plotted on the ordinate against the wavelengths on the abscissa.

McCree has also published several articles on the photosynthetic light absorption of 22 different crops raised in the field and in test chambers. Depending on the typical pigment composition in the photosensitive chromatophores , plants absorb light of different wavelengths to different degrees. This is important with artificial lighting, since different light sources have different emission spectra , which are also significantly narrower than the emission spectrum of the sun.

At the same time McCree drew the conclusion that the range between 400 and 700 nm essentially corresponds to the wavelength range of photosynthetically active radiation.

In 1999 Shinji Tazawa published absorption spectra of 61 different plant species and derived his version of a uniform curve of the PAR. Tazawa has summarized the results of Katsumi Inada, a Japanese researcher, and Keith J. McCree.

McCree curve

uniform PAR curve according to McCree (1972)

Hoover (1937) PAR curve; Representation in the form proposed by McCree in 1972 with the wavelength-dependent relative quantum yield

PAR curve according to Inada (1976); Representation in the form proposed by McCree in 1972 with the wavelength-dependent relative quantum yield

Keith J. McCree (1927–2014) measured photosynthetic activity spectra for 22 different crops in 1972. From this he developed a uniform PAR spectrum curve , the so-called McCree curve ( Fraunhofer lines are not recorded in it). The form of representation he proposes for PAR diagrams is also called the McCree curve , a representation with the wavelength on the horizontal axis and the relative quantum yield in% or values between 0 and 1 on the vertical axis. The quantum yield is relative to the highest quantum yield in the determined spectrum, which is then given as 100% or 1.

The quantum yield of photosynthesis is the ratio between the photons used by a plant for photosynthesis and the photons absorbed by the plant. The photosynthetic activity is measured using the difference in the rate of degradation in the light and in the dark. ${\ displaystyle QE}$ ${\ displaystyle CO_ {2}}$

{\ displaystyle QE = k_ {2} {\ frac {\ overbrace {\ frac {k_ {1} (C_ {L} + C_ {D})} {\ lambda E}} ^ {\ text {photosynthetic activity}} } {\ underbrace {\ frac {PM_ {0}} {PM_ {0} -PM_ {s}}} _ {\ frac {1} {{\ text {degree of absorption}} \ alpha}}}}}

Here are:

{\ displaystyle k_ {1}}

: Constant with the unit

{\ displaystyle {\ frac {J} {\ mu mol}}}

${\ displaystyle k_ {2} = 1 \ mathrm {\ frac {\ mu mol} {Einstein}}}$ : Constant for converting from to absorbed Einstein ${\ displaystyle \ mathrm {\ mu mol}}$
${\ displaystyle C_ {L}}$ : -Degradation rate in light ${\ displaystyle CO_ {2}}$
${\ displaystyle C_ {D}}$ : -Degradation rate in the dark ${\ displaystyle CO_ {2}}$
${\ displaystyle \ lambda}$ : Wavelength
${\ displaystyle E}$ : Irradiance
${\ displaystyle PM_ {0}}$ : Photon rate without sample
${\ displaystyle PM_ {S}}$ : Photon rate with sample

The relative quantum yield at a wavelength is the ratio of the quantum yield at that wavelength to the maximum value of the quantum yield within the photosynthetically active region.

The spectrum of the photosynthetically active radiation is represented with a McCree curve . Because the composition of the chromatophores of a plant is species-specific, each species has its own McCree diagram, from whose similarity McCree in 1972 derived his variant of a uniform curve of the PAR .

A PAR curve (according to McCree) shows what percentage of the radiation reaching the plant of a certain wavelength triggers photosynthesis. The remaining "remainder" would be 100%

not absorbed by the chromophore dye
or for processes of photomorphogenesis
or photoperiodism can be used,
or absorbed by other cell components and converted into heat or other radiation (e.g. fluorescence ).

McCree's measurement method

McCree isolated individual leaves from the plants and placed them watered in an assimilation chamber in which the temperature and carbon dioxide content were measured (with an infrared gas analyzer ) (see also infrared spectroscopy ). The photosynthesis rates measured with this apparatus and procedure were surprisingly reproducible ("surprisingly reproducible".)

The plants were illuminated with monochrome light by means of a monochromator and the irradiance was measured, data points for the curve of the photosynthesis rate were determined for individual wavelengths between 350 and 750 nm at intervals of 25 nm size. Then, the spectral absorption of the sheet was measured at intervals of 20 nm by means of a photomultiplier photometer. For this purpose, the light reflection of a surface coated with barium sulfate with and without shading by the leaf was measured and the absorption and, taking into account the photosynthesis rates, the quantum yield Q were calculated from the difference values.

Interpretation of the curves and criticism

It was criticized that the measuring method only measured the absorption of a single leaf and not the absorption of a whole plant. Because of the low absorption of green light rays measured in this way (the so-called green gap ), green light rays would often be denied biological functions during photosynthesis, which is doubted. The low absorption of a single leaf would be offset by the greater absorption of a closed canopy. When evaluating McCree curves for light sources, it must also be taken into account that plants need light not only for photosynthesis, but also for photomorphogenesis (the control of shape, stretching and development). While light with wavelengths between 400 and 700 nm is used for photosynthesis, phytochromes use wavelengths between 320 nm (near UV light) to 800 nm (near infrared light), according to another newer source up to 720 nm for photomorphogenesis. See photoreceptors in plants .

Nowadays, modern mobile spectrophotometers also allow recording under a canopy of leaves.

After it was shown that photosynthesis works better with a mixture of different light colors than with irradiation with monochromatic light ( Emerson effect ), i.e. there is mutual influence, it was proposed in 2009 that the quantum yield of additional monochromatic light should be used to determine PAR curves to measure different wavelengths under white background lighting. The photosynthetic quantum yield of green light would be comparable to that of red light and greater than that of blue light.

Practical tests in 2004 led to more plant growth and biomass after the addition of green light (500 to 600 nm). In addition, the alignment of the leaves (towards the light source) with the help of green light can lead to more biomass due to photomorphogenesis.

McCree studied the effects of monochromatic light on individual leaves or leaf parts in order to investigate the effects of different wavelengths on different plants in isolation. Nowadays, whole plants are irradiated with the light of certain plant lamps in longer practical tests in individual chambers and the results of photosynthesis are determined by comparing the biomass achieved.

PAR curves from emitting devices

A photomultiplier equipped with filters is used to determine the PAR without photosynthesis . In this way, the photons between 400 and 700 nm can be registered almost equally.

The rate of photosynthesis also depends on the illuminance , depending on whether it is a sun or shade plant . See also light compensation point

Because of the direct stoichiometric relationship between the absorbed photon (in the range of 400 to 700 nm) and the photosynthetic CO ₂ bond which was photon flux density (engl. Photosynthetically Active Photon Flux Density , PPFD or shortly PFD) in biology standard. In contrast to PAR, it is measured in µmol / (m²s). One mole corresponds to 6.022 · 10 ²³ ( Avogadro's constant ) photons, one µmol corresponds to 6.022 · 10 ¹⁷ photons.

PAR absorbed by fouling

Dividing the APAR (the absorbed PAR) by the (measurable) PAR that can be absorbed at the location results in the so-called FPAR (also specified as “fPAR”; fraction of PAR absorbed by canopy , i.e. “ PAR absorbed by fouling”) strongly correlated with the NDVI ("normalized differentiated vegetation index ") determined from satellite images .

Reflected PAR is measured by satellite and the fPAR is calculated using conversion factors. This means that the state of the vegetation can also be determined by remote sensing .

Use in plant lamps

A plant lamp is a light source that is used instead of or in addition to sunlight to improve the light-induced effect on plants (photosynthesis and photomorphogenesis).

In indoor hydroponic systems or images of grow cabinets , lamps with violet light can often be seen; these do not emit light with a wavelength of 400 nm, but an additive color mixture with strong proportions of blue and red.

From the active PAR spectra, it was determined which spectral bands a light source used as a plant lamp should have. Then DIN 5031 radiation physics in the optical field and lighting technology , part 10 "Photobiologically effective radiation, sizes, abbreviations and effect spectra" was aligned.

Photosynthetically available radiation

The abbreviation PAR is also used for Photosynthetically Available Radiation (translated photosynthetically available radiation ), those parts of the photosynthetically active radiation that are available in certain water depths or seawater water types . Derived from this are those parts that are absorbed (by algae), the Photosynthetically Usable Radiation (PUR) and from this again those parts that are bound as chemical energy, the Photosynthetically stored radiation (PSR)

literature

Sasa Fistric: The determination of photosynthetically active radiation (PAR) for heterogeneous atmospheric conditions. A method for deriving the global PAR for different daily second intervals and for the daily total based on remote sensing data and models . Dissertation, Weihenstephan Science Center 2004.
Helmut Mayer, Thomas Holst and Dirk Schindler: Microclimate in beech stands Part I: Photosynthetically active radiation . In: Forstwissenschaftliches Centralblatt , Vol. 121 (2002), Issue 6, pp. 301–321, ISSN 0015-8003 , doi: 10.1046 / j.1439-0337.2002.02038.x .

Web links

photosynthetically active radiation in the remote sensing dictionary
Comparison of Quantum (PAR) Sensors with Different Spectral Sensitivities ( Memento from February 8, 2013 in the Internet Archive ) (PDF file; 452 kB) LI-COR Biosciences, Technical Note 126

Individual evidence

^ ^A ^b curve according to John Whitmarsh, Govindjee: The photossynthetic process ; from "Concepts in Photobiology: Photosynthesis and Photomorphogenesis", Edited by GS Singhal, G Renger, SK Sopory, KD Irrgang and Govindjee, Narosa Publishers / New Delhi; and Kluwer Academic / Dordrecht, pages 11-51. | GS Singhal: Concepts in Photobiology . Springer Science & Business Media, 2012, ISBN 978-9-401-14832-0 ( limited preview in Google book search).
^ Richard Pott: Special Geobotany. Springer-Verlag, 2008, ISBN 978-3-540-49357-0 , p. 17 ( limited preview in the Google book search).
↑ Gerhard Richter: Metabolic physiology of plants. Georg Thieme Verlag, 1998, ISBN 978-3-134-42006-7 , p. 106 ( limited preview in Google book search).
↑ Sight light versus growth light. Part IV: Assessment of Light, PAR
↑ Gerhart Drews: Bacteria - their discovery and importance for nature and humans. Springer-Verlag, 2015, ISBN 978-3-662-45327-8 , p. 120 ( limited preview in Google book search).
↑ EK Gabrielsen (1960) wavelength of light and photosynthesis . In: Pirson A. (editor) Die CO2-Assimilation / The Assimilation of Carbon Dioxide. Manual of Plant Physiology / Encyclopedia of Plant Physiology, vol 5. Springer, Berlin, Heidelberg
↑ see facsimile in PDF file
↑ Kharchenko, Valeriy: Handbook of Research on Renewable Energy and Electric Resources for Sustainable Rural Development. IGI Global, 2018, ISBN 978-1-522-53868-4 , p. 206 ( limited preview in Google book search).
↑ ^a ^b ^c ^d ^e K. J. McCree: The action spectrum, absorptance and quantum yield of photosynthesis in crop plants . In: Agricultural Meteorology . tape 9 , 1972, p. 191-216 , doi : 10.1016 / 0002-1571 (71) 90022-7 (English). ; PDF file
↑ Katsumi Inada: Action spectra for photosynthesis in higher plants ; Plant and Cell Physiology, Volume 17, Issue 2, April 1, 1976, pages 355-365, doi : 10.1093 / oxfordjournals.pcp.a075288
↑ Shinji Tazawa: Effects of Various Radiant Sources on Plant Growth , published in Japan Agricultural Research Quarterly, Issue 33 (1999); quoted from Sehlicht versus Wuchslicht. Part IV: Evaluation of light, PAR
↑ ^a ^b ^c ^d Bruce Bugbee: Toward an optimal spectral quality for plant growth and development: the importance of radiation capture , doi: 10.17660 / ActaHortic.2016.1134.1 ; Script (PDF file) and presentation images (PDF file)
↑ see his personal data and the list of publications on his website, stored there in the internet archive
↑ "Figure 18. The relative quantum efficiency curve, also known as the McCree Curve, as determined by the average plant response for photosynthesis rate"; Michael Henke, Gerhard H. Buck-Sorlin: Using a Full Spectral Raytracer for Calculating Light Microclimate in Functional-Structural Plant Modeling ; Computing and Informatics, Vol. 36, 2017, pages 1492-1522, doi: 10.4149 / cai_2017_6_1492 ; PDF file
^ Hans Mohr: Textbook of Plant Physiology. Springer-Verlag, 1978, ISBN 978-3-642-96453-4 , p. 311 ( limited preview in Google book search).
↑ Peter Schopfer, Axel Brennicke: Plant Physiology. Springer-Verlag, 2016, ISBN 978-3-662-49880-4 , p. 445 ( limited preview in the Google book search).
↑ ^a ^b Ichiro Terashima, Takashi Fujita, Takeshi Inoue, Wah bald Chow, Riichi Oguchi: "Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green?" ; Physiology of Plants and Cells, Volume 50, Issue 4, April 1, 2009, published: February 25, 2009, pages 684-697, doi : 10.1093 / pcp / pcp034
↑ HH Kim, GD Goins u. a .: Green-light supplementation for enhanced lettuce growth under red- and blue-light-emitting diodes. In: HortScience: a publication of the American Society for Horticultural Science. Volume 39, Number 7, December 2004, pp. 1617-1622, PMID 15770792 .
↑ SW Hogewoning, G. Trouwborst u. a .: Finding the optimal growth-light spectrum for greenhouse crops. In: Acta Horticulturae. 2012, p. 357, doi : 10.17660 / ActaHortic.2012.956.41 .
^ Redefining the McCree Curve at Utah State University - Fluence Bioengineering. In: fluence.science. May 30, 2018, accessed October 28, 2018 .
^ Fraction of Photosynthetically Active Radiation
^ German Society for Polar Research: Polar Research . German Society for Polar Research, 1999 ( limited preview in Google book search).
^ Lars Olof Björn: Photobiology. Springer Science & Business Media, 2007, ISBN 978-0-387-72655-7 , p. 131 ( limited preview in Google book search).
↑ ScholarlyEditions: Issues in Global Environment: Freshwater and Marine Environments: 2011 Edition. ScholarlyEditions, 2012, ISBN 978-1-464-96466-4 ( limited preview in Google book search).
↑ Timothy R. Parsons: Biological Oceanographic Processes. Elsevier, 2013, ISBN 978-1-483-28617-4 , p. 67 ( limited preview in Google book search).