FLUORESCENCE-TIME CURVES 



1393 



cording to chapter 28 (page 1051), carbon dioxide limitation causes a shift 

 toward lower light intensities of the transition from the "low" to the "high" 

 steady fluorescence yield {<p, -> ^2). In complete absence of carbon dioxide, 

 one would expect the fluorescence yield to be equal to <^2 — 1-7 ^i even at 

 the lowest light intensities. However, no difference between the steady 

 yields at and 0.5% CO2 appears in figure 33.34. Figure 33.35 also 

 shows no difference in shape between the curves correspondmg to 0.04 and 

 5.4% CO2; but, here, the absolute fluorescence values cannot be com- 

 pared, because of the arljitrary adjustment of the scale. 



(A) 



L 



J I I- 



Fig. 33.36. Induction phenomena in wheat in the absence of CO. (after 

 McAUster and Myers 1940). (A) O2, 15 niin. hght, 10 min. dark. (B) N,. 



Figure 33.36A (McAlister and Myers 1940) agrees with figure 33.34 

 (Franck and co-workers) in that it, too, shows the first fluorescence burst to 

 grow and decay normally even in the absence of carbon dioxide. In a 

 nitrogen atmosphere, on the other hand, the decay of fluorescence after the 

 burst is much slower in the absence than in the presence of carbon dioxide 

 (c/. fig. 33.36B). This agrees with the hypothesis that removal of the in- 

 hibitor formed (or activated) in the first few seconds of illumination occurs 

 by a reaction with oxygen (which, in nitrogen, must first be produced by 



photosynthesis) . 



The occurrence of the complete first fluorescence wave in the absence ot 

 carbon dioxide, demonstrated by figures 33.34 and 33.36A, recalls the ob- 

 servation of McAlister that preliminary illumination in carbon dioxide- 

 free atmosphere can eliminate the induction loss of carbon dioxide upon 

 subsequent admission of this gas. Despite the "autocatalytic" character 

 of the process by which the induction is liquidated, no complete photosyn- 

 thesis appears to be required for this purpose. 



