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Photosynthesis is now widely supposed to require two light reactions for the transfer of one hydrogen atom from water to carbon dioxide.1 These two reactions may take place in one and the same reaction center,2 or in two reaction centers.3 4 There is evidence, derived from difference spectroscopy,5 for the existence in the pigment system of certain algae of one "trap," known as P700. It seems to be associated with "reaction I" (reduction of ferredoxin and oxidation of cytochrome). No clear evidence exists for another trap, associated with "reaction II" (reduction of cytochrome and oxidation of water). The results presented below support the existence of this second trap. With increase in intensity of green light, I,, the differential fluorescence yield of Porphyridium (dF/dI,) increases from a lower constant value to a higher constant value, reached when photosynthesis becomes saturated. Figure 1 shows the intensity of fluorescence (measured at 693 mu by a newly constructed fluorometer6) plotted versus intensity of exciting light (544 mi) for the red alga, Porphyridium cruentum (O.D. at 680 mu = 0.03) suspended in Warburg's carbonate-bicarbonate buffer #11 (with added NaCl). In order to reach the high-intensity range, two light sources of the same spectral composition had to be used simultaneously. One light beam was from an incandescent lamp, filtered through a 544-mu Bausch and Lomb interference filter (half-band width, 10 m/A) and a green glass filter Schott VG5. The second, also from an incandescent lamp, was obtained by a Bausch and Lonlb monochromator (grating size 100 mm X 100 mm); the monochromator, too, was adjusted to provide 544-mi (half-band width, 10 miu) green light. The measuring monochromator (with a band pass of 2 mu) was provided with a sharp cut-off red glass filter (Corning 2-63) in front of the entrance slit. Fluorescence was detected by an EMI 9558-B phototube; the signal was amplified by a Keithley 150A Microvolt-Ammeter and recorded on a Brown recorder. The results, shown in Figure 1, are in agreement with earlier observations of Franck,7 Brugger,' and those of Duysens and Sweers,9 Butler,'0 Teale," and Rosenberg and Bigat.12 The ratio of the (differential) fluorescence yields at high and low intensities varies with the condition of the culture. Several experiments were made. In cultures (grown in low light intensities) which show a high quantum yield of photosynthesis (ca. 0.10), this ratio is 1.8 to 2.0 (at about 690 m1A), whereas in cultures (grown in high light intensities) with a lower quantum yield of photosynthesis (ca. 0.05), the ratio is about 1.2; in extreme cases the dF/dI curve becomes linear. The question is whether the emission spectrum changes with the change in fluorescence yield, as we pass from "low" to "saturating" light. Figure 2 shows the spectral emission curves of Porphyridium (corrected for photomultiplier sensitivity and spectral efficiency of the monochromator), measured by excitation with the two

proceedings of the national academy of sciences of the united states of america

FLUORESCENCE CHANGES IN PORPHYRIDIUM EXPOSED TO GREEN LIGHT OF DIFFERENT INTENSITY: A NEW EMISSION BAND AT 693 mμ AND ITS SIGNIFICANCE TO PHOTOSYNTHESIS