1394 



INDUCTION PHENOMENA 



CHAP. 33 



The absence of induction losses upon transition from a carbon dioxide- 

 free atmosphere to an atmosphere containing carbon cHoxide may be, 

 however, not a general rule. At least, fluorescence has been observed to 

 undergo a very characteristic change following an alteration of this concen- 

 tration, by McAlister and Myers in wheat, and by Franck, French, and 

 Puck in Hydrangea. As shown by figure 33.37A, the change consists of a 

 dip of fluorescence, followed by a burst, a second dip and finally an ap- 

 proacli to a new steady value (which may be lower than the value at the 

 original, low concentration of carbon dioxide, if the latter was rate-limiting). 



CO2 from 

 003-40% 



COi from 

 04-40% 



o 



o 



LlI 



cr 

 o 



3 



(B) 



TIME, min. 



Fig. 33.37. Effect of changes of CO2 concentration on fiuorescence. (A) 

 Wheat; high Hght intensity (after McAHster and Myers 1940). (B) Hydrangea 

 (after Franck, French and Puck 1941). 



According to figure 33.37A the effect disappears when the initial carbon 

 dioxide concentration itself is saturating (e. g., 0.4%; cf. third curve). 

 Franck and co-workers observed, however, a wave of fluorescence upon a 

 transition from 1 to 20% CO2, and found that only the dip was absent if the 

 initial concentration was saturating. A sudden decrease of carbon dioxide 

 concentration (e. g., from 25 to 0.03%) also caused a dip, followed by a wave 

 of fluorescence (c/. fig. 33.37B). 



Changes of carbon dioxide concentration often have a very pronounced 

 effect on the second fluorescence wave. As mentioned in section A3, 

 McAlister and Myers (1940) found in wheat one- wave curves in ordinary 

 air, and two-wave curves, with a second maximum around 1.5 minutes in 

 0.1 to 0.3% CO2 {cf. fig. 33.21B). 



