1452 



PHOTOSYNTHESIS IN INTERMITTENT LIGHT 



CHAP. 34 



Ib), further extension become useless, and the rate per flash reaches a 

 maximum vahie independent of both I and ta. 



It may seem at first as if dark intervals of insufficient length {tj < Ib) 

 should reduce the yield of weak flashes in the same proportion as that of 

 strong ones, since in both cases the same fraction of the Eb molecules, 

 supplied wath intermediates during the flash, ^^^ll fail to complete their 

 transformation during the subsequent dark interval. However, this is 

 only true of the yield of the ^rs^ (or the first few) flashes. As shown in fig- 

 ure 34.9, the proportion of working Eb molecules will be built up, by a series 

 of subsaturating flashes, until it is high enough to transform the intermedi- 

 ates at the same average rate at which they are supplied by the flashes; 

 only then will a "stationary state" be reached (assuming one may speak of a 

 stationary state in intermittent light). 



Clendenning and Ehrmantraut (1950) found that Chlorella suffers similar induction 

 losses of oxygen (after a dark interval > 2 minutes), whether it is exposed to continuous 

 light, or to 40 neon discharge flashes per second; in both cases the steady state was 

 reached only after about 10 minutes of exposure. 



2. Maximum Flash Yield 



Figure 34.10 shows the yield per flash as a function of the length of the 

 dark intervals, according to the measurements of Emerson and Arnold 



^ 5 10 15 20 25 



DARK TIME (t^), sec. 



Fig. 34.10. Flash yield as function of 

 dark time for Chlorella (after Emerson and 

 Arnold 1932). 



90 klux 



31 klux 



0.02 0.04 006 0.08 0.10 0.12 0.14 

 DARK TIME, sec 



Fig. 34.11. Yield per flash for two dif- 

 ferent light intensities given for 4.5 msec, 

 (after Weller and Franck 1941). 



(1932). The shajie of these curves is exponential, and they can be inter- 

 preted as revealing a reaction of the first order (e. g., the monomolecular 

 transformation of the complex I-Eb, where I stands for intermediate such 

 as { HCO2 1 in chapter 7, or EbHC02 in scheme 28.11). If, at the beginning 

 of the dark interval, the quantity of this complex is [I-Eb]o, the kinetic 



