Waveband definitions for photosynthetic radiation (PhR), photosynthetically active radiation (PAR) and extended photosynthetically active radiation (ePAR) according to different definitions in use for land plants.
PAR(std = "PAR", norm = 550)PhR()
For PhR, a waveband object defining a wavelength range . For PAR, a waveband object implementing the response curve of PAR as defined by McCree (1972) 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.
a character string "Plant" (or "range"), "McCree" (or "photon", "PAR"), "Zhen" (or "ePAR"), "Gabrielsen" (or "Gaastra" or "energy") or "Nichiporovich".
normalization wavelength (nm)
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 (1972) definition, an spectral response 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 or a photon irradiation, this, of course, makes no difference.
ePAR and PAR are meant 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. They were never 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.
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 (a BSWF). It is also possible, but very unusual, to express the quantity PAR as defined in McCree (1972) 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 (my own choice).
A proposal (see Zhen et al., 2021), defines extended photosynthetically
active radiation (400-750 nm) as an alternative to PAR. The need to
consider far-red photons as drivers of photosynthesis has become apparent
with the increased use of LEDs for plant cultivation. WARNING: the proposed
definition limits photon irradiance in the range 700-750 nm to a maximum of
30
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 in PAR. Ensuring this condition is fulfilled is the
responsibility of the user of PAR(), ePAR() and PhR()
functions. Currently, ePAR can usefully complement PAR, but in my
opinion, it is far too early for it to replace PAR.
Some earlier definitions, described by McCree (1972a) citing Gabrielsen and Gaastra, used this same wavelength range but assuming wavelength-invariant response to energy within this same range, thus weights decrease with increasing wavelength when expressed as photons. McCree (1972a) cites Nichiporovich for a similar energy based quantity but covering a wider range of wavelengths (380-710 nm). McCree's definition from 1972 is currently the one preferred by most researchers and used almost universally in the plant sciences, while others are only of historical interest. Photosynthetic radiation (400-700 nm) is defined as a wavelength range and does not implement the spectral weighting inherent to McCree's (1972) definition of PAR or the earlier energy-based ones described by McCree (1972a).
McCree, K. J. (1972) The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology, 9, 191-216. tools:::Rd_expr_doi("10.1016/0002-1571(71)90022-7")
McCree, K. J. (1972a) Test of current definitions of photosynthetically active radiation against leaf photosynthesis data. Agricultural Meteorology, 10, 443-453. tools:::Rd_expr_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. tools:::Rd_expr_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
waveband
Other BSWF weighted wavebands:
CH4(),
DNA_GM(),
DNA_N(),
DNA_P(),
FLAV(),
GEN_G(),
GEN_M(),
GEN_T(),
PG(),
UV_health_hazard(),
erythema()
PAR()
PhR()
PAR("Plant")
q_irrad(sun.spct, PhR(), scale.factor = 1e6) # umol m-2 s-2
q_irrad(sun.spct, PAR(), scale.factor = 1e6) # umol m-2 s-2
e_irrad(sun.spct, PAR("Gabrielsen")) # W m-2
e_irrad(sun.spct, PhR()) # W m-2
e_irrad(sun.spct, PAR()) # BE W m-2, normalized at 700 nm
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