QUANTUM YIELD 1951 



earlier that the beneficient influence of background hght is due to compen- 

 sation of respiration in the "dim" periods; now, however, it was found 

 that much weaker background illumination than needed for compensation 

 can suffice if it contains blue-green light. With the help of very weak 

 blue-green additional light, hour-long periods of uninterrupted photo- 

 synthesis with a "net" quantum requirement of I/7' = 3.2 could be ob- 

 tained (fig. 37D.28), corresponding to a "true" quantum requirement of 

 1/7 = 2.85; it was suggested that "from now on, it will be very difficult 

 to obtain bad energy yields in photosynthesis." 



As explanation of the effect of blue-green light, Warburg suggested re- 

 versible activation of a photosynthetic enzyme containing a carotenoid as 

 active group (in analogy to the known reversible photochemical stereo- 

 isomerization of carotenoids in the visual cycle). 



These new observations of Warburg and co-workers shifted the maxi- 

 mum quantum yield problem to an entirely new ground. Of their results 

 obtained in 1950-52, the [C02]-dependence of the quantum yield could not 

 be confirmed by other observers, and Warburg himself did not find it with 

 some of his cultures; while all data obtained with light of intermittent 

 intensity (with or without a constant background) appeared uncertain after 

 Emerson's demonstration of the variability and long duration of transients. 

 The results with the new Chlorella cultures are not similarly open to re- 

 interpretation ; if they prove experimentally reproducible, the controversy 

 will have to be considered as settled in favor of the plant's capacity to con- 

 vert absorbed light energy quantitatively, without any losses, into chemical 

 energy, as suggested by Warburg. Even the most convincing physical ob- 

 jections against the possibility of such a friction-free mechanism will have 

 to be withdrawn in the face of experimental evidence— if it was to be con- 

 firmed. 



The "One Quantum Process.'' A second independent discovery was re- 

 ported by Burk and Warburg (1950) . As mentioned in section 3, an extra- 

 ordinary strong interaction of photosynthesis with respiration was found 

 in these experiments. Specifically, in the first minute after return into 

 darkness (or dim light), of cells exposed for a minute to strong illumination, 

 the rate of oxygen consumption was observed to be up to twelve times the 

 steady rate of respiration ; as a corollary, a quantum yield approaching unity 

 was observed for the additional oxygen exchange during the (also one- 

 minute long) "bright" periods. 



This previously unsuspected interaction of respiration and photosyn- 

 thesis could be discovered in these experiments, according to Burk and 

 Warburg, because of the use of much higher light intensity (2 /xeinsteins 

 per minute in a narrow light pencil), and of shorter intervals of measure- 

 ment. High intensity naturally enhances the light effect, and minute-to- 

 minute measurements reveal transient changes that had remained concealed 



