GENERAL SHAPE OF LIGHT CURVES 1015 



dictory. It has been suggested, as a generalization of empirical results, 

 that catalyst poisons affect only the saturation level, thus producing light 

 curve systems of the first type, while narcotics depress also the initial slope, 

 thus giving curve systems of the second type. However, not all experi- 

 mental results conform to this rule. 



In figures 28.9 A,B, the curves of Chlorella in the presence of cyanide are, 

 as expected, of the Blackman type; but the hydroxylamine curves (as ob- 

 served by Weller and Franck) are of the Bose type. With the diatom 

 Nitzschia (fig. 28.10) the effect of cyanide was different: A distinct depres- 

 sion was observed at all light intensities between 2 and 30 kerg/cm.^ sec. 

 (However, the per cent inhibition increased with increasing light intensity, 

 for example, 0.003% KCN caused an inhibition by 35% at 2.4 kerg, by 

 45% at 13 kerg and by 57% at 27 kerg.) Other observations of cyanide 

 inhibition of photosynthesis in weak light were quoted in Volume I (page 

 309). The light curves of cyanide-inhibited purple bacteria, as observed 

 by Wassink and co-workers, show a similar "semi-Bose" behavior; re- 

 versing the results obtained by Weller and Franck ^^^th Chlorella, the bac- 

 teria exhibited a Blackman type behavior toward hydroxylamine! 



The light curves of urethan-inhibited Chlorella (as given by Warbiu'g, 

 and by Wassink and co-workers, respectively) appear to be of the second 

 type, according to rule; but the third type is not entirely excluded. Bose 

 type curves were found also with urethan-inhibited Nitzschia (fig. 28.10), 

 although in this case the per cent inhibition was not constant, but rose with 

 increasing light intensity. Wassink's curves, showing the effect of urethan 

 on purple bacteria (fig. 28. HE), exhibit the reverse change — the depression 

 is more pronounced in w^eak light than in strong light. 



The effect of oxygen on the photosynthesis of Chlorella (fig. 28. 9E; 

 cj. also fig. 33, page 329, Vol. I) is of the Bose type, and the same is true of 

 the effect of copper sulfate, while that of nickel sulfate apparently is of the 

 Blackman type (fig. 28.9D). The curves of purple bacteria with pH as 

 parameter (fig. 28.12) are of the Bose type with thiosulfate, and of the 

 Blackman type with hydrogen as reductant. 



Theoretically, one can easily understand why specific catalyst poisons 

 such as cyanide, should affect only the saturation level and not the initial 

 slope of the light curves: The former is determined by the rate of a dark 

 catalytic reaction, the latter by the rate of supply of light quanta. With 

 the poisoning becoming more and more complete, the inhibition can be 

 expected to spread to lower and lower light intensities. If one assumes (as 

 suggested in Vol. I, page 307) that cyanide inhibits most strongly the 

 carboxj'lating enzyme, Ea, the observed differences in the sensitivity of dif- 

 ferent species may be attributed to variations in the amount of this en- 

 zyme. Some plants may contain a considerable reserve of Ea, and there- 



