Apogee MQ-501 Panduan Pemilik - Halaman 13

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and 800 nm by relative quantum yield and summing the result. This is defined as yield photon flux density (YPFD)

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] (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,

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.

Biggs, W., A.R. Edison, J.D. Eastin, K.W. Brown, J.W. Maranville, and M.D. Clegg, 1971. Photosynthesis light sensor

and meter. Ecology 52:125-131.

Federer, C.A., and C.B. Tanner, 1966. Sensors for measuring light available for photosynthesis. Ecology 47:654-657.

Inada, K., 1976. Action spectra for photosynthesis in higher plants. Plant and Cell Physiology 17:355-365.

McCree, K.J., 1972a. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants.

Agricultural Meteorology 9:191-216.

McCree, K.J., 1972b. Test of current definitions of photosynthetically active radiation against leaf photosynthesis

data. Agricultural Meteorology 10:443-453.

Sager, J.C., W.O. Smith, J.L. Edwards, and K.L. Cyr, 1988. Photosynthetic efficiency and phytochrome photoequilibria

determination using spectral data. Transactions of the ASAE 31:1882-1889.

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] at each wavelength between 300

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], was well correlated to photosynthesis, very

Correlation between photosynthetic photon flux

density (PPFD) and yield photon flux density (YPFD)

for multiple different radiation sources. YPFD is

approximately 90 % of PPFD. Measurements were

made with a spectroradiometer (Apogee

Instruments model PS-200) and weighting factors

shown in previous figure were used to calculate

PPFD and YPFD.