FLUORESCENCE-TIME CURVES 



1391 



The effect of temperature on the declining slope of D was studied also by Wassink 

 and Katz (1939) with HCN-poisoned Chlorella. In this case, in contrast to fig. 33.33, 

 maximum height of peak D was reached at the highest temperature (35°; cf. fig. 33.41). 

 This can be explained by the fact that not only the decay, DE, but also the rise, CD, is ac- 

 celerated by an increase in temperature. In nonpoisoned cells the influence of tempera- 

 ture on the purely thermal decaj' process is more important than its influence on the 

 combined (photochemical and thermal) activation process; maximum D therefore de- 



Fig. 33.33. Fluorescence time curves of Ulva lactuca at different temperatures, in 

 presence of urethan, and in absence of oxygen (after Kautsky and Franck 1943): (A) 

 first 3 sec, 10 lux; (B) first 3 min., 2.5 lux. 



dines with increasing temperature. In cyanide-poisoned cells, on the other hand, the 

 decay, DE, is inhibited, and the enhancing effect of temperature on the rise, CD, is the 

 only remaining effect. 



(d) Carbon Dioxide Concentration 



We have to distinguish between two phenomena: the effect of excess 

 carbon dioxide (in concentrations which cause a "narcotic" poisoning of 

 photosynthesis; cf. chapter 13, page 330), and the effect of low carbon 

 dioxide concentration, in the range where this concentration limits the ef- 

 ficiency of the photosynthetic apparatus. 



The influence of excess carbon dioxide was first noticed by Kautsky and 

 Hirsch (1935). The activation curve, AB, was unaffected, but the decay 

 BC, lasted for 2 minutes at 4% CO2, and 3 minutes at 10% CO2, instead of 

 1 minute at < 1% CO2. The inhibiting effect of carbon dioxide concentra- 

 tions > 10% is confirmed by figure 33.34 of Franck, French and Puck. A 

 sudden increase in carbon dioxide concentration, e. g., from 1 to 20%, dur- 

 ing the decay (or after its termination) caused an immediate burst, followed 

 by a renewed slow decay, of fluorescence. 



Figure 33.35 shows an almost complete disappearance of the first 

 fluorescence wave of Ulva in concentrated carbon dioxide. The difference 

 from the results of Franck, French and Puck (who found the height of peak 

 B to be almost unaffected, even by 80% CO2) may be due to the use of a 



