322 



Govindjee 



,48 



green light, in contradiction to the results of McCloud (and 

 my own earlier conclusions°°»^") . In an independent study, 

 Pickett and Myers-*-^^ found the same saturation rate at different 

 wavelengths in Chlorella . The cause of the discrepancy between 

 the newer and the earlier results is that the 700 m|j light curve 

 has a peculiar "break", which was mistaken for "saturation", 

 due to the narrow range of intensities studied. 



10 20 30 40 50 60 70 BO 90 100 lib 



LIGHT INTENSITY IN' ARBITRARY UNITS 



10 20 30 40 50 60 70 80 90 100 



LIGHT INTENSITY (7C0mjj) 



Figure 3. Rate of photosynthesis in Porphyri - 

 dium as a function of light intensitv in Lwo 

 monochromatic beams 545 raji and 700 mn measured 

 at 5° C. 



Figure A. Rates of oxygen evolution, in Por - 

 phyrldium as a function of light intensity. 



My new measurements show a clear "Knick" in the light curve 

 at 700 m|j. (figure 3). Kok had observed such broken light curves 

 (see Rabinowitch^^ for a discussion of this "Kok effect"). Hoch 

 and coworkers^O recently reported light curves of the same type 



in Anacystis , 



Jones and Myers ■'■'^*^, 



observed such curves in Anacystis , 



in an independent study, also 



B. S-Shaped Light Curves. 



Figure 4 (lowest curve) shows a light curve obtained at 

 700 mjj. in Porphyridium cruentum at 20°C under anaerobic conditions, 

 It is clearly S-shaped. The middle curve shows that addition of 

 another beam of the same wavelength, makes the curve linear. 

 With Bannister & Vrooman^^^ i believe that under nitrogen the 

 lower segment of the S-shaped "light curve" is due to immediate 

 consumption of the oxygen evolved in photosynthesis; the oxygen 

 production in photosynthesis, is therefore not registered by the 

 polarograph. At higher intensities, oxygen production increasing- 

 ly exceeds oxygen consumption (thus destroying the external 

 anaerobic condition). (Reference may be made here to a paper by 



