Waveband definitions for photosynthetic radiation (PhR), photosynthetically active radiation (PAR = PPFD), extended photosynthetically active radiation (ePAR), phosynthesis quantum action spectrum weighted radiation (PQYR = YPD) according to different definitions in use for land plants.
Value
For PhR(), a waveband object defining a wavelength range. For
PAR(std = "McCree"), ePAR() and PQYR() a waveband
object implementing different approximations of the action spectrum of
photosynthesis in crop (land) plants as BSWF. In PAR() the BSWF is as
defined by McCree (1972b), equal action per photon. and thus including a
weighting function used in computation of energy-base PAR irradiances. The
weights are normalized to 1 at 550 nm. The waveband label is set to "PAR"
or "PhR" accordingly.
Details
Photosynthetically active radiation (400-700 nm) as proposed by McCree (1972), and currently used in plant sciences, gives equal weight to photons within its range, thus weights increase with increasing wavelength when expressed as energy. PAR is normally expressed as photon irradiance or (photosynthetic photon flux density, PPFD) using implicitly 1 as weight for all wavelengths. This is a simple photon-based Biological Spectral Weighting Function (BSWF). It is also possible, but very unusual, to express the quantity PAR as defined in McCree (1972b) as an energy irradiance, in which case a BSWF with weights different from 1 needs to be used. In this case the default normalization wavelength for the PAR BSWF is set at 550 nm (at the center of the wavelength range). In some fields, such as meteorology, PAR is simply taken as a range of wavelengths used to integrate spectral energy irradiance. This is different to McCree's definition and in this package available under the name Photosynthetic Radiation (PhR). In the case of sunlight and using 550 nm for normalization the difference between the two is very small, while for artificial light sources the differences can be larger.
Instead of using the simplified square-shapped BSWF as in PAR, some authors have used an photon-based action spectrum (or "quantum yield" spectrum) as BSWF and called the quantity Yield Flux Density (YFD). A mean action spectrum from several crop species from McCree (1972a) is one of those that has been used in the literature. Here it is available under the name PQYR (Photosynthesis Quantum Yield Radiation) using two mean action spectra, for field-grown- and controlled-environment-chamber-grown crop plants.
A recent proposal (see Zhen et al., 2021), defines extended
photosynthetically active radiation (400-750 nm) (abbreviated as ePAR) as
an alternative to PAR. The need to consider far-red photons as drivers of
photosynthesis has become apparent with the increasing use of LEDs for
plant cultivation. Far-red light contributes significantly to
photosynthesis only when added to PAR. WARNING: the proposed definition of
ePAR limits photon irradiance in the range 700-750 nm to a maximum of 30
of the total ePAR: ePAR is zero as long as PAR is zero, and never larger
than 1.4 times PAR even in the presence of far-red photons in excess,
because far-red photons have only a synergistic effect on photosynthesis
driven by VIS light. Ensuring this condition is fulfilled remains the
responsibility of the user of the wavebands returned by
PAR(std = "ePAR"), PAR(std = "Zhen"), and ePAR() as
FR's contribution can be assessed only by computing irradiances integrated
for two wavelength ranges and comparing them.
Function xPAR_irrad() from package 'photobiologyPlants' returns the
constrained ePAR under the name of xPAR as well as ePAR and its PAR and FR
components.
Some earlier definitions, described by McCree (1972a), include Gabrielsen and Gaastra's, which used the same wavelength range as PAR but assuming wavelength-invariant response to energy. Thus, in this case weights decrease with increasing wavelength when expressed as photons. McCree (1972a) also cites Nichiporovich for a similar energy based quantity but covering a wider range of wavelengths (380-710 nm). Both of these definitions, even if mostly of historical interest, are also implemented. When used to compute photon irradiances the BSWFs are normalised at 550 nm.
McCree's definition from 1972b is currently the one preferred by most researchers and used almost universally in the plant sciences. Photosynthetic radiation (400-700 nm) (PhR) is defined as a wavelength range and does not implement the spectral weighting inherent to McCree's (1972) of PAR or Gabrielsen and Gaastra definition of photosynthetic energy irradiance described by McCree (1972a).
Note
PAR() and PhR() call the same function definition with
different default arguments.
The default for the normalization wavelength at 550 nm keeps the average weights across the waveband equal to unity, except the special case of ePAR, where the photons in the extension to the range act by synergy.
Standard DIN 5031-10:2018-03 Defines two BSWFs sy1 and sy2
for photosynthesis. BSWF sy2 is not implemented but is based on the
same data as PQYR("McCree.field.mean") with a specific
interpolation. BSWF sy1 is not currently implemented in
'photobiologyWavebands'.
Warnings
PAR is sometimes described as a range of wavelengths (e.g., Both et al., 2015), which can be confusing as there is more to McCree's (1972b) definition, a spectral weighting function by which all photons within the range of PAR elicit the same strength of response. As long as PAR is expressed as a photon irradiance it is identical to PhR. Similarly, as long as Gabrielsen and Gaastra's definition is expressed as energy irradiance, it is equivalent as using PhR.
ePAR and PAR were designed to be use to quantify light sources with a broad spectrum, i.e., sources giving out white light or pale-coloured light. PAR and ePAR are technical measures of light useful for plants in the same way as illuminance is a measure of how bright light feels to an average human. None of them are meant to describe the response to be expected from an individual in particular, be it a plant or human. They are generalizations, that allow us to consistently measure light in different situations rather than directly predict the rate of photosynthesis. PQYR is similar in concept to PAR and ePAR as long as the same action spectrum is used consistently.
References
McCree, K. J. (1972a) The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology, 9, 191-216. doi:10.1016/0002-1571(71)90022-7
McCree, K. J. (1972b) Test of current definitions of photosynthetically active radiation against leaf photosynthesis data. Agricultural Meteorology, 10, 443-453. doi:10.1016/0002-1571(72)90045-3
Both, A. J.; Benjamin, L.; Franklin, J.; Holroyd, G.; Incoll, L. D.; Lefsrud, M. G. & Pitkin, G. (2015) Guidelines for measuring and reporting environmental parameters for experiments in greenhouses. Plant Methods, 11:43. doi:10.1186/s13007-015-0083-5 .
DIN (2018) Standard DIN 5031-10:2018-03 Optical radiation physics and illuminating engineering. Part 10: Photobiologically effective radiation, quantities, symbols and action spectra. Beuth Verlag, Berlin 2018.
Examples
PAR()
#> PAR
#> low (nm) 400
#> high (nm) 700
#> weighted BSWF
PhR()
#> PhR
#> low (nm) 400
#> high (nm) 700
#> weighted none
PAR("Plant")
#> PhR
#> low (nm) 400
#> high (nm) 700
#> weighted none
q_irrad(sun.spct, PhR(), scale.factor = 1e6) # umol m-2 s-2
#> Q_PhR
#> 894.1483
#> attr(,"time.unit")
#> [1] "second"
#> attr(,"radiation.unit")
#> [1] "total photon irradiance"
q_irrad(sun.spct, PAR(), scale.factor = 1e6) # umol m-2 s-2
#> Q_PAR
#> 894.1483
#> attr(,"time.unit")
#> [1] "second"
#> attr(,"radiation.unit")
#> [1] "total photon irradiance"
q_irrad(sun.spct, PAR(std = "ePAR"), scale.factor = 1e6) # umol m-2 s-2
#> Q_ePAR
#> 1031.757
#> attr(,"time.unit")
#> [1] "second"
#> attr(,"radiation.unit")
#> [1] "total photon irradiance"
q_irrad(sun.spct, PQYR(), scale.factor = 1e6) # umol m-2 s-2
#> Q_PQYR.McCree.field.mean.550
#> 752.5749
#> attr(,"time.unit")
#> [1] "second"
#> attr(,"radiation.unit")
#> [1] "total photon irradiance"
e_irrad(sun.spct, PAR("Gabrielsen")) # W m-2
#> E_PAR.Gabrielsen
#> 196.6343
#> attr(,"time.unit")
#> [1] "second"
#> attr(,"radiation.unit")
#> [1] "total energy irradiance"
e_irrad(sun.spct, PhR()) # W m-2
#> E_PhR
#> 196.6343
#> attr(,"time.unit")
#> [1] "second"
#> attr(,"radiation.unit")
#> [1] "total energy irradiance"
e_irrad(sun.spct, PAR()) # BE W m-2, normalized at 700 nm
#> E_PAR
#> 203.6806
#> attr(,"time.unit")
#> [1] "second"
#> attr(,"radiation.unit")
#> [1] "total energy irradiance"