1102 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



significant carbon dioxide burst could have occurred during the iUumina- 

 tion period. However, this procedure would only be permissible if the gas 

 exchange in light were the result of the superposition of a photochemical 

 process (photosynthesis, with possible addition of a carbon dioxide burst) 

 upon a dark process whose rate is the same in darkness and in light. This is 

 usually assumed to be true of respiration (although some doubts exist even 

 here) ; but it is not true of the reahsorption of the carbon dioxide burst (since 

 this process occurs only in the dark). By neglecting this component of the 

 gas exchange in the dark after a period of illumination, one automatically 

 eliminates from the calculated "light effect" a part, if not practically all, 

 of the carbon dioxide burst — in the same way in which the effects of the 

 sluggishness of the manometer are eliminated in the procedure illustrated 

 by figure 29.1; no wonder that the ratios Qp for the calculated "light 

 effect" prove to be close to unity. (Whether the elimination of the burst is 

 practically complete or only partial, depends on what fraction of the burst 

 is reabsorbed during the dark period utilized in the calculation of the "light 

 effect.") 



To decide whether a significant carbon dioxide burst does occur in light 

 (and is reabsorbed in darkness), the ratios AO/ACO2 should be calculated 

 for the illumination and the dark period separately instead of calculating 

 them directly for the "light effect." The ratios Qdark and Qught might 

 each be quite different from 1 — and yet, the ratio "Qp" for the "light 

 effect" might show no significant deviation from unity. Thus, the method 

 of calculating Qp used by Warburg (1948) to prove the absence (or at least, 

 practical insignificance) of the carbon dioxide burst is inappropriate for this 

 purpose — even if the experimental data used had been adequate. 



Emerson and co-workers argued, however, that the experiment itself 

 was open to criticism. They pointed out that the value Qp = 1.07 was 

 derived from measurements lasting for about 40 minutes. The plot given 

 by Warburg shows that if only the first 10 minutes of these measurements, 

 i.e., the period of quantum yield measurements, were taken into considera- 

 tion, much smaller values of Qp would have been obtained. Furthermore, 

 the Qp measurements were made in light of 3780 erg/cm.^ sec. (ten times as 

 strong as that at which quantum yields close to 0.25 were obtained) and 

 in red light, while the 7 measurements were made in yellow^ light, Avhich is 

 considerably less strongly absorbed. 



The conditions of Qp measurement differed from those of 7 measure- 

 ment also in temperature (20° C. instead of 10° C), carbon dioxide con- 

 centration (8% CO2 in O2, as compared with 5% CO2 in air) and volume of 

 respiration (twice as high in quantum yield measurements as in Qp meas- 

 urements, indicating different culture conditions) . It was mentioned before 



