1358 



INDUCTION PHENOMENA 



CHAP. 33 



that would prevail at the same light intensity if light saturation did not 

 occur). To put it more simply, Steemann-Nielsen postulated that, upon 

 a sudden decrease of light intensity, the quantum yield at first remains un- 

 changed, and only gradually rises to the higher value characteristic of the 

 weaker light. If generally valid, this would be a significant relation; how- 

 ever, it was derived from only seven not very precise, measurements (five at 



300 



o 



0) 



</) 



^ 200 



>- 

 (/) 

 o 



o 



I 

 a. 



100 - 



41.300 lux 



40 



2.200 lux 



_j_ 



_i_ 



20 40 

 MINUTES 



20 40 60 



Fig. 33.13P. O2 induction losses of Cladophora insignis after upward 

 and downward changes in light intensity (after Steemann-Nielsen 

 1949). Winkler method. 18° C, 5 X IQ-^ mole/1. HCO3, pH 8.1. 



variable conditioning light, /', and constant illuminating light, I", and two 

 at constant I' and variable /")• In figure 33. 1 3 M, the curve represents the 

 initial inhibition as calculated, on the basis of this theory, from the hght 

 curve of photosynthesis of Fucus; the crosses represent the actually ob- 

 served inhibitions. 



Steemann-Nielsen also carried out experiments in which conditioning to the higher 

 light intensity was incomplete. The results are shown in figure 33.13N, in which the 

 ordinates represent the yields in the first 10 minutes of weaker illumination as a function 

 of the duration of preillumination with stronger hght. The curve shows a maximum 

 after about 6 minutes conditioning. Figure 33.130 shows that a similar maximum cor- 

 responds to a certain degree of "deconditioning" in the dark; it is reached about 12 

 minutes after the cessation of strong illumination. In other words, it appears that at 

 some time in the course of conditioning to strong light, and again at some time during 

 "deconditioning" in the dark, the photosynthetic apparatus passes through a state ca- 



