Relationship between the Absorption and Emission Spectra and the "Red Drop" in the Action Spectra of Fluorescence In Vivo

The Stepanov equation, relating the intensity of emission, f_e($\bar{v}$), at a given frequency, and that of absorption, k($\bar{v}$), at the same frequency, is applied, in its modified form (see equation 3 in text) to suspensions of Chlorella, Porphyridium, and Anacystis and to chlorophyll solutions. This application can reveal whether the yield of fluorescence, Φ($\bar{v}$), is constant, or changes with frequency. In Chlorella (green alga) a sharp drop of Φ($\bar{v}$) is indicated towards the lower frequencies (longer waves), beginning around $\bar{v}$ = 1.48 × 10⁴cm^-1 (680 mμ); the Φ($\bar{v}$) function calculated from the Stepanov equation is in fair agreement with the directly determined action spectrum for the excitation of chlorophyll fluorescence in this organism. In Porphyridium (red alga) and Anacystis (blue-green alga) application of the Stepanov equation supports the conclusions, derived from direct measurements, of a much earlier “red drop” of the fluorescence excitation spectra. Direct measurements suggest that the drop in Porphyridium may begin at about 1.53 × 10⁴cm^-1 (654 mμ); in Anacystis, it may begin already above 1.57 × 10⁴cm^-1 (<637mμ). These results confirm the relation, postulated earlier by Duysens and others, between the action spectra of photosynthesis and of chlorophyll a fluorescence in algal cells. The relation of these findings to spectroscopic evidence, suggesting the existence of two main chlorophyll a components in vivo, in green as well as in red and blue-green algae, is discussed.