Calculating the phytochrome photoequilibrium
In the examples below we use the solar spectral data included in package ‘photobiology’ as a data frame in object sun.spct.
We can calculate the phytochrome photoequilibrium from spectral irradiance data contained in a source_spct object as follows.
## [1] 0.68341We can also calcualte the red:far-red photon ratio, in this second example, for the same spectrum as above
## Red.Smith10: FarRed.Smith10(q:q)
## 1.266704
## attr(,"radiation.unit")
## [1] "q:q ratio"which is equivalent to calculating it using package ‘photobiology’ directly
## Red.Smith10: FarRed.Smith10(q:q)
## 1.266704
## attr(,"radiation.unit")
## [1] "q:q ratio"We can, and should whenever spectral data are available, calculate the photoequilibrium as above, directly from these data. It is possible to obtain and approximation in case of the solar spectrum and other broad spectra, using the red:far-red photon ratio. The calculation, however, is only strictly valid, for di-chromatic illumination with red plus far-red light.
## Red.Smith10: FarRed.Smith10(q:q)
## 0.7051691Here we calculated the R:FR ratio from spectral data, but in practice one would use this function only when spectral data is not available as when a R plus FR sensor is used. We can see that in such a case the photoequilibrium calculated is only a rough approximation. For sunlight, in the example above when using spectral data we obtained a value of 0.683 in contrast to 0.705 when using the R:FR photon ratio. For other light sources differences can be much larger. Furthermore, here we used the true R:FR ratio calculated from the spectrum, while broad-band red:far-red sensors guive only an approximation, which is good for sunlight, but which will be innacurate for artifical light, unless a special calibration is done for each type of lamp.
In the case of monochromatic light we can still use the same functions, as the defaults are such that we can use a single value as the ‘w.length’ argument, to obtain the Pfr:P ratio. For monochromatic light, irradiance is irrelevant for the photoequilibrium (steady-state).
## [1] 0.869649## [1] 0.01749967## [1] 0.86964902 0.01749967## [1] 0.3859998We can also plot Pfr:Ptot as a function of wavelength (nm) of monochromatic light. The default is to return a vector for short input vectors, and a response_spct object otherwise, but this can be changed through argument spct.out.
autoplot(Pfr_Ptot(300:770), norm = NULL, unit.out = "photon",
w.band = Plant_bands(),
annotations = c("colour.guide", "labels", "boxes")) +
labs(y = "Phytochrome photoequilibrium, Pfr:Ptot ratio")
It is, of course, also possible to use base R plotting functions, or as shown here functions from package ‘ggplot2’
ggplot(data = Pfr_Ptot(300:770), aes(w.length, s.q.response)) +
geom_line() +
labs(x = "Wavelength (nm)",
y = "Phytochrome photoequilibrium, Pfr:Ptot ratio")
In the case of dichromatic illumination with red (660 nm) and far-red (730 nm) light, we can use a different function that takes the R:FR photon ratio as argument.
These computations are valid only for true mixes of light at these two wavelengths but not valid for broad spectra like sunlight and especially inaccurate for plant growth lamps with peaks in their output spectrum, such as most discharge lamps (sodium, mercury, multi-metal, fluorescent tubes) and many LED lamps.
## [1] 0.6919699## [1] 0.04747996## [1] 0.69196990 0.04747996It is also easy to plot Pfr:P ratio as a function of R:FR photon ratio. However we have to remember that such values are exact only for dichromatic light, and only a very rough approximation for wide-spectrum light sources. For wide-spectrum light sources, the photoequilibrium should, if possible, be calculated from spectral irradiance data.
ex6.data <- data.frame(r.fr=seq(0.01, 5.0, length.out=100), Pfr.p=numeric(100))
ex6.data$Pfr.p <- Pfr_Ptot_R_FR(ex6.data$r.fr)
ggplot(data=ex6.data, aes(r.fr, Pfr.p)) +
geom_line() +
labs(x ="R:FR photon ratio",
y = "Phytochrome photoequilibrium, Pfr:Ptot ratio")
