BUILDING UP OF INTERMEDIATES 1411 



liberated acceptor molecules. If this uptake is not instantaneous, a sta- 

 tionary concentration of A is established. This is particularly likely if the 

 carboxylation is delayed by a limiting amount of the carboxylase, Ea, or by 

 a low concentration of carbon dioxide {i. e., if saturation ceases to be de- 

 termined exclusively or mainly by the finishing catalyst, Eb, and becomes 

 a function of [Ea] or [CO2]). 



This kind of redistribution of intermediates obviously should produce 

 an inverse induction; at least, the rate of oxygen production should start 

 high and gradually decline to a steady level. How, according to Shiau 

 and Franck, the superposition of this depletion of the reduction substrates 

 upon "ordinary" induction (due to photochemical activation of an inhibitor 

 and its subsequent oxidative removal) can explain the occurrence of a 

 "wave" of inhibition and enhanced fluorescence will be discussed in section 

 4 below. 



The absolute duration of the induction period was mentioned above as a 

 second argument against the attribution of induction to the replenishment 

 of the stock of photochemical intermediates. Since the concentration of 

 the carbon dioxide acceptor appears to be approximately equal to that of 

 chlorophyll, and the acceptor has to receive an average of considerably 

 more than 4 quanta per molecule to ensure close approach to equipartition, 

 an induction period of this origin should last long enough for each chloro- 

 phyll molecule to absorb, say, 10 or 12 quanta. Franck and Gaffron (1941) 

 pointed out that, in light just sufficient to bring about net liberation of oxy- 

 gen, this may require as long as 2 hours (whereas no induction was observed 

 at all in such weak light); in ten times stronger light, induction should 

 take 12 minutes — about ten times the actual induction period in moderate 

 light. 



This estimate was based on the conditions prevailing in the quantum 

 yield experiments of Warburg and Negelein. They used such dense Chlo- 

 rella suspensions that, in weak light, the average frequency of absorptions 

 by each individual chlorophyll molecule was only once every 12 minutes. 

 In less concentrated suspension, used in induction work, this frequency is, of 

 course, much higher, approaching, according to page 838, a limiting value of 

 1 X 10~'* X / (in lux), corresponding to once every 25 seconds at 400 lux 

 or once every 2.5 seconds at 4000 lux. In an average leaf, the mean fre- 

 quency of absorptions probably is not much less than once every 2 minutes 

 at 400 lux and once every 12 seconds at 4000 lux. An induction period 

 in a leaf, caused by replenishment of photochemical intermediates, should 

 therefore last about 20 minutes at 400 lux, and 2 minutes at 4000 lux. 

 These figures show that the argument based on the absolute duration of the 

 induction period is less conclusive than the objections derived from the 

 direction in which this duration changes with light intensity. (Shiau and 



