Yield Photon Flux Density (YPFD) Measurements
Photosynthesis in plants does not respond equally to all photons. Relative quantum yield (plant photosynthetic
efficiency) is dependent on wavelength (green line in figure below) (McCree, 1972a; Inada, 1976). This is due to the
combination of spectral absorptivity of plant leaves (absorptivity is higher for blue and red photons than green
photons) and absorption by non-photosynthetic pigments. As a result, photons in the wavelength range of
approximately 600-630 nm are the most efficient.
One potential definition of PAR is weighting photon flux density in units of
mol m
-2
s
-1
at each wavelength
between 300 and 800 nm by measured relative quantum yield and summing the result. This is defined as yield
photon flux density (YPFD, units of
mol m
-2
s
-1
) (Sager et al., 1988). There are uncertainties and challenges
associated with this definition of PAR. Measurements used to generate the relative quantum yield data were made
on single leaves under low radiation levels and at short time scales (McCree, 1972a; Inada, 1976). Whole plants
and plant canopies typically have multiple leaf layers and are generally grown in the field or greenhouse over the
course of an entire growing season. Thus, actual conditions plants are subject to are likely different than those the
single leaves were in when measurements were made by McCree (1972a) and Inada (1976). In addition, relative
quantum yield shown in the figure above is the mean from twenty-two species grown in the field (McCree, 1972a).
Mean relative quantum yield for the same species grown in growth chambers was similar, but there were
differences, particularly at shorter wavelengths (less than 450 nm). There was also some variability between
species (McCree, 1972a; Inada, 1976).
McCree (1972b) found that equally weighting all photons between 400 and 700 nm and summing the result,
defined as photosynthetic photon flux density (PPFD, in units of
mol m
-2
s
-1
), was well correlated to
photosynthesis, and very similar to correlation between YPFD and photosynthesis. As a matter of practicality, PPFD
is a simpler definition of PAR. At the same time as McCree’s work, others had proposed PPFD as an accurate
measure of PAR and built sensors that approximated the PPFD weighting factors (Biggs et al., 1971; Federer and
Tanner, 1966). Correlation between PPFD and YPFD measurements for several radiation sources is very high (figure
below), as an approxismation, YPFD = 0.9PPFD. As a result, almost universally PAR is defined as PPFD rather than
YPFD, although YPFD has been used in some studies. The only radiation sources shown (figure below) that don’t
fall on the regression line are the high pressure sodium (HPS) lamp, reflection from a plant canopy, and
transmission below a plant canopy. A large fraction of radiation from HPS lamps is in the red range of wavelengths
where the YPFD weighting factors (measured relative quantum yield) are at or near one. The factor for converting
PPFD to YPFD for HPS lamps is 0.95, rather than 0.90. The factor for converting PPFD to YPFD for reflected and
transmitted photons is 1.00.
Defined plant response to
photons (black line, weighting
factors used to calculate PPFD),
measured plant response to
photons (green line, weighting
factors used to calculate YPFD),
and SQ-500 series quantum
sensor response to photons
(sensor spectral response).
