1322 



INDUCTION PHENOMENA 



CHAP. 33 



and 20 minutes at 4°, and thus appeared to fall into the category of "short 

 induction." 



The method used by Steemann-Nielsen (involving measuring periods 

 of 2.5 to 5 minutes) was much too slow to reveal the oxygen gush; but it 

 showed clearly the gradual increase in the rate of oxygen production, which 

 lasted for 20-30 minutes at 4-6° C, and for about 10 minutes at 18° (in 

 light of 23,000 or 2300 lux, in 2.5 X 10-» mole/1, bicarbonate solution). 

 The same temperature was maintained during the incubation and the ex- 

 posure. Figure 33.4 illustrates the results. 



5 10 15 20 



INTENSITY OF CONDITIONING LIGHT, klux 

 Fig. 33.5. Initial rate of photosynthesis in Fucus in 23 klux as function of pre- 

 vious illumination (after Steemann-Nielsen 1942). Average for first 5 min. 

 (solid curve); initial rate extrapolated to zero time (broken curve). 



Steemann-Nielsen made observations on the repetition of induction upon 

 stepwise transition from weaker to stronger light, a phenomenon previ- 

 ously noted by McAlister in experiments on CO2 consumption (section 3 

 below). After Fiicus had reached the steady rate of photosynthesis 

 in 2300 lux, an increase to 23,000 lux produced, in the first 5 minutes, only 

 65% of full rate; the latter was reached after 15 minutes additional induc- 

 tion (at 5° C). Similar results were obtained with other transitions 

 (7000 -^ 23,000 lux, 6°, 82% of maximum rate in the first 5 minutes; 

 1200 -^ 23,000 lux, 5.5°, 45% of maximum rate in the first 5 minutes). 

 Fig. 33.5 shows the first measured rate in strong light as a function of inten- 

 sity of the "conditioning" illumination. Steemann-Nielsen noted that the 

 dotted curve (which represents the rate in strong light, extrapolated to the 

 moment of light increase, as a function of conditioning intensity) is identi- 

 cal with the light curve of photosynthesis, P = f(I) , of the same plant. He 

 concluded that the "activation" of the dark-inhibited factors of the 



