1360 INDUCTION PHENOMENA CHAP. 33 



quantum yield of oxygen liberation in weaker light could then be the same 

 as it had been in the conditioning stronger hght (as suggested in fig. 

 33.13M). Often, however, the initial rate deficiency is smaller; Steemann- 

 Nielsen suggested that, in this case, light saturation was only partially de- 

 termined by the rate of chlorophyll reactivation, and partially by another, 

 chlorophyll-independent enzymatic factor, such as "catalyst B" of Franck 

 and Herzfeld. In chapters 26 and 27, pp. 871 and 923, it was shown that 

 cooperation of several reactions in the determination of the saturation 

 level is to be expected theoretically if the "ceilings" imposed by them 

 separately are not too different; on p. 1030, we discussed the possibility 

 that a dark reaction involving chlorophyll and requiring about 20 seconds 

 may impose a "ceiling'* on the rate of photosynthesis that is not too differ- 

 ent from the ceiling imposed by a reaction of "catalyst B," which requires 

 about 0.01 second. Steemann-Nielsen's explanation is in principle the 

 same, but the postulated chlorophyll recovery is much slower— requiring 

 up to 30 minutes for completion. This means that the chlorophyll "de- 

 activation," postulated by Steemann-Xielsen, can be permitted to occur 

 only once in a large number of primary photochemical reactions — perhaps 

 one in about two hundred. (Steemann-Nielsen estimated 1 deactivation 

 per 40 reduced CO2 molecules.) 



Steemann-Nielsen (1949) discussed the above-mentioned possibility 

 that the same inactivation of chlorophyll may account also for the decline 

 of photosynthesis in excessively strong light. His first experiments with 

 Cladophora insignis spoke against such a hypothesis. He observed that, 

 when the algae were exposed to a very strong light (160 klux) until the rate 

 had declined by about 30%, a sudden drop in intensity to 21 klux did not 

 reduce the rate to the very low value which was to be expected if the cause 

 of inhibition at 160 klux were the same chlorophyll inactivation to which 

 induction losses after exposures to 20-40 klux have been attributed. 



Later, however, Steemann-Nielsen (1952) found that, after one hour 

 exposure of Cladophora to 100 klux (at 18° C), during which the rate had 

 declined by about 50%, a sudden drop of light intensity to 3 klux (implying 

 transition to the light-limited state) did reveal a strong inhibition, the re- 

 covery from which required several hours. He concluded from this that 

 exposure to excessive light affects not only a rate-limiting enzyme, which 

 is not part of the photochemical apparatus proper (as suggested by Franck 

 and French, cf. section 7 below), but inhibits the latter apparatus as well. 

 The effect of a ten minute exposure to 100 klux proved to be the same as 

 that of one hour exposure to 17 klux; it could be therefore suggested that 

 inhibitions caused by exposure to excessive hght are due, at least in part 

 (and if the exposure is not too long, perhaps predominantly), to the same 

 kind of inhibition (photoxidation?) of the photochemical mechanism pro- 



