Light Energy Utilization Efficiency for Photosynthesis

Abstract

ABSTRACT THE radiation output of sources used in growth cham-bers and for supplemental lighting in horticulture was measured in terms of irradiance, photon flux densi-ty, and two photosynthetic responses based on the mean relative action spectrum and mean relative quantum yield of photosynthesis. The outputs of the sources were given in terms of a constant power input and showed the necessity of comparing electrical conversion efficiences on an installed rather than 'test' basis. A rating of the sources based on the calculated photosynthetic utiliza-tion efficiencies was given for the sources tested. A source with broad spectral outputs can be evaluated with either photon flux density or photosynthetic photon flux density. However, analysis of less homogenous sources with narrow spectral emissions showed that the photon flux density was not a valid index of photosynthetic utilization efficiency. Plants, lettuce and tomato, grown under daylight, blue and red spectral quality sources showed large differences in morphology between the plants. The best indicator of photosynthetic utilization efficiency of a source was found to be either the photosynthetic action photon flux density or the photosynthetic yield photon flux density as modified by an absorptance greater than that previously given in the literature. The net assimilation rates of lettuce and tomato plants grown under a blue source would predict that the mean relative action spectrum was low in the blue region.