314 



Jack Myers 



background "becomes limiting. 



The most important finding is that the initial slope of curve A 

 is identical to the initial slope of the enhanced curves F and G 

 and that the ratio between the first and second slopes of curve A 

 is identical to the maximum value for enhancement (Ej). Similar 

 results were obtained at wavelengths 680 and 690 my.. In short the 

 initial slope is not enhanceable. At very low intensities the 

 quantum yield, reckoned as increase in net oxygen evolution per 

 absorbed quantum, is maximum. 



We interpret the result in terms of the Hill and Bendall ^ 

 model : that the two photoreactions are separated by an electron 

 transport system. The simplest explanation is that F700 is, in 

 effect, a competitor of oxygen. We have proposed''"® that the com- 

 petition is direct in the sense envisioned by Lundegardh^"*": that 

 in Anacystis the same system serves both the transport of "photo- 

 synthetic" electrons from photoreaction II to F700 and also part 

 of the transport of "respiratory" electrons to oxygen. We recog- 

 nize, however, that competition could be much less direct. Hoch 

 et al . ^ have suggested a decreased "respiratory" electron flow in 

 response to a lowered ADP concentration caused, in turn, by extra 

 cycling of photoreaction I. 



The Kok effect occurs even more dramatically in Anabaena varia - 

 bilis "^^ but appears in Chlorella pyrenoidosa only as a minor non- 

 linearity.^® Where it does appear there are a number of important 

 consequences. (l) If the effect occurs in Chroococcus as it does 

 In Anacystis and Anabaena, then (as previously noted"""®) the high 

 quantum yield values obtained by Eiaerson and Lewis® on Chroococcus 

 at 660 to 700 mii probably are in error. (2) The effect can com- 

 pletely obscure the enhancement phenomenon or even give rise to 

 the "negative" enhancement noted in early work.®'"^-^ The difficul- 

 ty can be eliminated by referring enhancement to an increment 

 rather than to a single value on the light intensity curve. (3) Un- 

 less the light intensity curve is carefully determined, the effect 

 may easily be mistaken for a very early light saturation in the A-i 

 region. "'""'" (4) If there are pathways of electron transport common 

 to both respiration and photosynthesis, then it would seem that in 

 vivo absorption changes would yield more instructive information 

 if studied as changes in response to an increment of light rather 

 than (or in addition to) light-dark changes. 



