Test of current definitions of photosynthetically active radiation against leaf photosynthesis data

Abstract

The photosynthetic rates ( P) of an “average leaf”, in light from 2 natural and 7 artificial sources, were calculated from the action spectrum for photosynthesis (McCree, 1971) and the spectral energy distributions of the light. The flux densities of photosynthetically active radiation ( E), variously defined as: (1) the energy flux in the waveband 400–700 nm; (2) the energy flux in the waveband 380–710 nm; (3) the incident quantum flux in the waveband 400–700 nm; and (4) the absorbed quantum flux in the waveband 400–700 nm, were also calculated, from the spectral energy distributions. The ratio P/E was then calculated, for all combinations of light source and flux definition, and normalized to 1.00 for sun+sky radiation. For any one flux definition, the deviation of this number from unity is the systematic error involved in an extrapolation to natural sun+sky light, from data obtained in other types of light, due to the mismatch between the flux definition and the action spectrum for leaf photosynthesis. For incandescent light from a quartz-iodine lamp, which is relatively rich in red light, the ratios using the four definitions were: (1) 1.19; (2) 1.19; (3) 1.08; and (4) 1.08. For skylight, which is relatively rich in blue light, the corresponding ratios were: (1) 0.88; (2) 0.86; (3) 0.95; and (4) 0.94. The ratios were slightly smaller for an average growth-chamber-grown leaf than for a field-grown leaf. Non-linearity of the P versus E relationship of the leaf also decreased the deviation; this was shown by experiment. The systematic errors introduced by using the incident quantum flux definition (3) were judged to be acceptable in practice. The paper also includes a table of flux ratios, which can be used (with caution) to convert from one flux definition to another, and a table of “photosynthetic efficacies” showing that the high pressure sodium lamp gives 26% more photosynthetic flux per watt of power consumed than any other lamp tested.