FLUORESCENCE-^IME CURVES 



1401 



tion of induction curves at different low oxygen concentrations, contained 

 in the papers of Kautsky and co-workers, is uncertain. We will neverthe- 

 less give a short summary of their results. 



Kautsky, Hirsch and Davidshofer (1932) and Kautsky and Hirsch 

 (1935) first found that, in complete absence of oxygen, the fluorescence 

 wave, ABC (fig. 33.19), disappears, and the fluorescence-time curve be- 

 comes horizontal. Kautsky and Flesch (1936) observed that the slope AB 

 first begins to flatten out in nitrogen containing less than 0.5% O2. To 

 achieve complete suppression of the "first wave" the system had to be 

 swept by pure nitrogen for much more than 1 or 2 hours. 



(a) 



(b) 



Norrnol gQO 



I 2 



TIME, sec. TIME, sec. 



Fig. 33.45. Fluorescence curves of Ulva lactuca at low O2 pressure (after 

 Kautsky and Franck 1943): (a) variation of O2 at 20° C; (b) variations of 

 temperature at 0.3% O2. / = 10 m. c. (equiv. ). 



Later Kautsky and co-workers (1936, 1937) found no substantial varia- 

 tions in the fluorescence curves of Ageratum above 0.2% O2, but noted a 

 slight change in 0.04% O2 and a strong change in pure nitrogen (estimated 

 oxygen content, 0.0005%). After 2 hours of sweeping out with this gas, 

 the fluorescence curve acquired the shape shown in figure 33.44. 



The parallelism between these results and the observations on the effects 

 of anaerobic incubation on gas production (chapter 13, part A) is ob- 

 vious; it seems natural to correlate horizontal fluorescence-time curves 

 with anaerobic inhibition of photosynthesis. 



In transition from "aerobic" to the "anaerobic" fluorescence curves, the 

 second fluorescence wave became prominent at a certain intermediary 

 stage (0.2% O2, in fig. 33.44). An enhanced second wave was found also 

 on certain intermediate temperature curves obtained at a constant, low 

 value of oxygen. 



