1422 INDUCTION PHENOMENA CHAP. 33 



side rapidly in air, it extends over a much longer period under anaerobic 

 conditions, where the only oxygen available is that produced by residual 

 photosynthesis (c/. section 6). 



How can one, however, explain the strong effect of the first induction 

 wave on the yield of chlorophyll fluorescence? How can such an effect be 

 caused by a "narcotic" available (assuming it is produced with a quantum 

 yield of 1) only to the extent of 1 molecule to 10 or 100 molecules of chloro- 

 phyll? One is forced to assume either that 1 molecule of the narcotic can 

 protect the fluorescence of 100 chlorophyll molecules, or else that the fluores- 

 cence of the protected molecules is so much more intense than that of the 

 unprotected ones as to raise the average yield of fluorescence by a factor of 

 two or three. Both assumptions seem artificial. 



An explanation of the "second wave" of induction was first suggested 

 in a paper by Franck, French and Puck (1941). They thought that, if 

 the first wave is an indirect effect of accumulated photoperoxides (via an 

 oxidizable metabolite), the second wave may be due to a direct effect of 

 the same intermediates, which has to wait until their quantity had become 

 sufiicient to "block" all chlorophyll. (Assuming a quantum yield of ~1 

 for the production of the photoperoxides, this should require from one half 

 to several minutes, depending on the intensity of illumination.) 



This hypothesis could explain a "second depression" in the carbon di- 

 oxide uptake curves, and also the "second wave" of fluorescence — but not 

 interruption of the steady increase in oxygen production (since the oxygen 

 "precursors" remain present in excess). Therefore, when Franck, Pring- 

 sheim and Lad (1945) found a second wave also in oxygen liberation curves, 

 a reinterpretation became necessary. This was provided by Shiau and 

 Franck (1947); as mentioned before, their hypothesis was based on com- 

 bination of normal induction with the depletion, in light, of the initially full 

 reservoir of the reduction substrate, A-COo. The suggested interplay is 

 illustrated by fig. 33.50. In A, curve a represents induction (more specifi- 

 cally of fluorescence intensity) as it would be if the reactivation of the de- 

 oxygenase were the only determining factor, and if the reservoir of reduc- 

 tion substrates remained full. This curve shows the, at first slow, and then 

 accelerated, decline of the "narcotization" of chlorophyll by the metabolic 

 poison (the concentration of which follows that of the "photoperoxides," 

 and thus, indirectly, the inactivation of the deoxygenase) . If one assumes 

 — which is the salient point of the hypothesis — that the reduction sub- 

 strates compete with the "narcotic" for adsorption on chlorophyll (as two 

 adsorbable gases would for the surface of charcoal), then, with a lower con- 

 centration of the reduction substrates, the "narcotic" would have a chance 

 to occupy more chlorophyll, and a fluorescence-time curve of type 6 would 

 result. If, at the beginning of illumination, the concentration of the reduc- 



