McCree – photosynthetic active radiation

Spectral quantum yields

The aim of these measurements is to provide a factual basis for a standard
definition o f “photosynthetically active radiation”. The need for a standard  definition has been repeatedly emphasized (e.g., GABRIELSEN, 1940; RABINOWlTCH,  1951, pp.837-844; MCCREE, 1966), but so far, no one definition has been agreed
upon. In applied photosynthesis research, the three most c o m m o n l y used units of light and radiation measurement are: (1) the illumination in lux or footcandles  (flux of lumens, the photometric unit based on the brightness response of the eye);

Original Abstract

ABSTRACT

McC’~E, K. J., 1972. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agrie. MeteoroL, 9: 191-216. The measurements were made to provide a basis for discussion of the definition of “photo-synthetically active radiation”. The action spectrum, absorptance and spectral quantum yield of CO2 uptake were measured, for leaves of 22 species of crop plant, over the wavelength range 350 to 750 nm.

The following factors were varied:

species, variety, age of leaf, growth conditions  (field or growth chamber), test conditions such as temperature, COs concentration, flux of  monochromatic radiation, flux of supplementary white radiation, orientation of leaf (adaxial  or abaxial surface exposed), For all species and conditions the quantum yield curve had 2 broad  maxima, centered at 620 and 440 nm, with a shoulder at 670 nm. The average height of the blue  peak was 70Yo of that of the red peak. The shortwave cutoff wavelength and the height of the  blue peak varied slightly with the growth conditions and with the direction of illumination, but  for the practical purpose of defining “photosynthetically active radiation” the differences are  probably insignificant. The action spectrum for photosynthesis in wheat, obtained by HOOVER  in 1937, could be duplicated only with abnormally pale leaves.

Reference: ACTION-SPECTRUM-KJMCCREE