The action spectrum, absorptance and quantum yield of photosynthesis in crop plants

Abstract

The measurements were made to provide a basis for discussion of the definition of “photosynthetically active radiation”. The action spectrum, absorptance and spectral quantum yield of CO 2 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, CO 2 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 70% 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.