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Articles

Effects of Chromatic Adaptation on the Photochemical Apparatus of Photosynthesis in Porphyridium cruentum^1,2

Department of Biology, University of California at San Diego, La Jolla, California 92093

Cells of Porphyridium cruentum were grown in different colors of light which would be absorbed primarily by chlorophyll (Chl) (red and blue light) or by the phycobilisomes (green or two intensities of cool-white fluorescent light), and samples of these cells were frozen to –196 C for measurements of absorption and fluorescence emission spectra. Cells grown in the high intensity white light had least of all of the photosynthetic pigments, a higher ratio of carotenoid/Chl, but essentially the same ratio of phycobilin to Chl as cells grown in the low intensity white light. The ratio of photosystem II (PSII) to photosystem I (PSI) pigments was affected by light quality; the ratios of phycobilin to Chl and of short wavelength (PSII) Chl to long wavelength (PSI) Chl were both greater in the cells grown in red or blue light.

Light quality also exerted a strong influence on the structural and functional organization of the photochemical apparatus. Data on the relative optical cross-sections of PSI and PSII as a function of excitation wavelength indicate that cells grown in light absorbed primarily by the phycobilisomes package a large fraction of their Chl into PSI (PSI Chl/PSII Chl 20), whereas cells grown in light absorbed by Chl distribute their Chl much more equitably (PSI Chl/PSII Chl 1.5). In both types of cells the phycobilisomes transfer their excitation energy predominantly to PSII Chl with little or no direct energy transfer to PSI, but the yield of energy transfer from PSII to PSI is approximately twice as large for cells grown in the phycobilin wavelengths of light. These differences in functional organization and energy distribution account for the physiological expressions of chromatic adaptation. The effects of chromatic adaptation on O₂ evolution can be predicted from our calculations of energy distribution between PSI and PSII for cells grown in the different colors of light.

¹ This work was supported by National Science Foundation Grant PCM76-0711.

² This paper is dedicated to the memory of Bessel Kok, a free spirit, a creative scientist, and a true pioneer in the field of photosynthesis.