1016 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



fore show poisoning effects only in strong light; in others, the concentra- 

 tion Ea may be just sufficient to maintain photosynthesis in the absence of 

 cyanide and very little inactivation is needed to cause marked retardation 

 even in moderate or weak light. However, even in this case, the per cent 

 reduction of photosynthesis should remain smaller in weak light than in 

 light of saturating intensity (as was, in fact, observed \vith cyanide-poisoned 

 Nitzschia, cf. above). This hypothesis cannot explain an apparently uni- 

 form per cent reduction of photosynthesis at all light intensities by a typical 

 catalyst poison such as hydroxylamine (fig. 28. 9B). An ad hoc interpreta- 

 tion, suggested by Franck and co-workers, was described in Volume I (page 

 312). 



What was said about the effects of catalytic poisons should apply also 

 to deficiencies of the reaction substrates (carbon dioxide in green plants, 

 carbon dioxide and reductants in bacteria). Their effects, too, should 

 gradually diminish, and finally disappear with decreasing light intensity. 



The effect of narcotics on the initial slope of light curves can be under- 

 stood if one assumes that they are adsorbed on chlorophyll (or "chloro- 

 plastin") in such a way as to prevent the access of reactants or catalysts. 

 Consequently, in the partially poisoned state, only the chlorophyll mole- 

 cules free of these adsorbents are capable of properly utilizing the ab- 

 sorbed light quanta. If it is true that light saturation is due to the limited 

 amount of a catalyst, such as Ea or Eb, which is kinetically independent 

 of chlorophyll, the fact that the light curves obtained in the presence of 

 urethan appear to be of type 2 rather than 3 (z. e., that narcotics affect also 

 the saturation rate) requires special explanation. For example, it can be 

 postulated that the narcotic becomes adsorbed on the molecules of Ea or 

 Eb as well as on those of chlorophyll. Alternatively, it can be suggested 

 that, if the light curves of narcotized plants were followed to still higher 

 light intensities, they w^ould finally approach the same saturation level as 

 the light curves of normal plants (i. e., they would actually prove to be of 

 type 3 rather than 2). 



A third explanation, based on the idea that definite catalyst molecules 

 are "assigned" to definite chlorophyll molecules, and become useless when 

 the latter are "narcotized," will be discussed in chapter 32. 



Still another phenomenon needs to be taken into consideration. It 

 will be shown later in this chapter that a saturation level of photosynthe- 

 sis probably exists which is due to the distribution of the chlorophyll com- 

 plex, during photosynthesis, between the normal photosensitive form and 

 a changed (tautomeric, or reduced) form. The latter is formed as an inter- 

 mediate in photosynthesis, and requires a certain time for reconversion to 

 the original photosensitive form. While this saturation limit may not be 

 generally apparent in non-narcotized plants, because another limit (im- 



