12
One potential definition of PAR is weighting photon flux density [
µ
mol m
-2
s
-1
] at each wavelength between 300 and 800 nm
by relative quantum yield and summing the result. This is defined as yield photon flux density (YPFD) [
µ
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 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) [
µ
mol m
-2
s
-1
], was well correlated to photosynthesis, 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 approximation, 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 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.
Radiation weighting factors for PPFD
(black line, defined plant response to
radiation), YPFD (blue line, measured
plant response to radiation), and
Apogee SQ-500 Series Quantum Sensors
(green line, sensor sensitivity to
different wavelengths of radiation).