Red Drop and Role of Auxiliary Pigments in Photosynthesis

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Abstract
A summary of unpublished results obtained by the late Robert Emerson and co-workers, prepared and interpreted by E. Rabinowitch. The enhancement of the quantum yield of photosynthesis in "far red" light (> 680 m[mu]) by auxiliary light of shorter wavelengths is measured as function of the wavelength of auxiliary light, for green (Chlorella), brown (Navicula), red (Porphy-ridium) and blue-green algae (Anacystis). The resulting "action spectra of the Emerson effect" show sharp peaks in the regions where the contribution of auxiliary pigments (chlorophyll b in Chlorella, chlorophyll c and fucoxanthol in Navicula, phycoerythrin in Porphyridium, phycocyanin in Anacystis) to total light absorption is highest. This appears offhand as evidence that excitation of chlorophyll a alone is insufficient to produce photosynthesis (at least, not with a high yield), and simultaneous excitation of one of the auxiliary pigments is needed. However, the high yield of chlorophyll fluorescence in vivo sensitized by auxiliary pigments contradicts this interpretation, since it suggests highly efficient transfer of excitation energy from auxiliary pigments to chlorophyll a. A solution can be sought in the assumption of (at least) 2 different forms of chlorophyll a which need to be excited to produce photosynthesis. Only 1 is excited by "far red" light, the other can be excited either directly, e.g., by "near red" light, or indirectly, by energy transfer from the auxiliary pigments.