1094 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



The pretreatment of the algae also seems to be much more important for the carbon 

 dioxide gush than for the steady rate of photosynthesis. Cells having a high rate of 

 respiration produced a greater carbon dioxide gush than cells with a low rate of 

 respiration; by culturing Chlorella cells in dim light throughout, cells with low respira- 

 tion, showing almost no gush, could be obtained. 



When the carbon dioxide giish was neutralized by substituting carbon- 

 ate buffers for carbonic acid solution, Emerson and Lewis obtained quan- 

 tum yields of the order of 0.1, independently of the previous illumination 

 of the cells, the age of the culture and the kind of water used in its prepara- 

 tion. (However, the presence of certain microelements — ^particularly man- 

 ganese — still appeared to be important.) 



The conclusions of Emerson and Lewis agree with the findings of Daniels and co- 

 workers (1939), who grew Chlorella with the addition of soil extracts, of a nutrient solu- 

 tion of 28 elements, or of sea water, in lake water, well water and distilled water, with- 

 out appreciable changes in the quantum jdeld. 



Emerson and Lewis did not calculate quantum jaelds from measurements of the 

 type illustrated by figure 29.3B, but merely drew from the evaluation of these experi- 

 ments the conclusion that with Chlorella the quantum yield of oxygen liberation (and of 

 carbon dioxide uptake, once the burst is completely over) does not differ significantly 

 in acid phosphate buffer and in alkaline carbonate media. If this is so, then the abso- 

 lute determination of the quantum yield is better carried out in carbonate buffer, where 

 all effects caused by carbon dioxide exchange are eliminated and therefore no need arises 

 for the use of the two-vessel method. The latter depends on comparatively small 

 differences, and has a correspondingly low precision. 



The quantum yield measurements made by Emerson and Lewis with Chlorella in 

 alkaline buffers consistently gave values between 3^ and }{i; and we will see below 

 that Warburg, Burk and co-workers have since confirmed this result (leaving 

 Eichhoff as the only investigator to have claimed that quantum yields of the order of 

 34 can be obtained with Chlorella in carbonate buffer). 



A set of manometric quantum yield determinations with Chlorella 

 pyrenoidosa was made by French and Rabideau (1945) as a check on their 

 measurements with isolated chloroplasts (cf. section 4). Using carbonate 

 buffer No. 9 as medium they found in red light (660-720 m/x) y values be- 

 tween 0.063 and 0.113, or from 9-16 quanta per reduced carbon dioxide 

 molecule, with no significant variations between 1.4 and 8 kerg/cm^ sec. 



According to Emerson and Lewis, the true maximum yield of photo- 

 synthesis does not change much from species to species. This is indicated 

 by the similarity between the above-mentioned results with Chlorella and 

 Gabrielsen's obsei-vations on the leaves of the higher plants (page 1118), 

 and confirmed by Emerson and Lewis' measurements listed in table 29. L 

 These measurements were made in carbonate buffers — after preliminary 

 tests had shown that in none of the organisms used was the yield in phos- 

 phate medium significantly higher than in carbonate buffer. 



