328 



VARIOUS CHEMICAL AND PHYSICAL AGENTS 



CHAP. 13 



dium (Gaffron 1940) — the same treatment produces the phenomena of hy- 

 drogen fermentation and "photoreduction," described in chapter 6, attrib- 

 uted by Gaffron to activation of hydrogenase by fermentation products. 



None of these experiments prove that the absence of oxygen during 

 the illumination — without previous anaerobic incubation — has an inhi- 

 biting effect on photosynthesis; and the results of Harvey, Franck and 

 Pringsheim, and Gaffron can be quoted as evidence against such an 

 assumption. 



Kautsky and Eberlein (1939) argued against the "fermentation the- 

 ory" of anaerobic incubation by pointing out: (1) that characteristic 

 changes in chlorophyll fluorescence can be observed after an incubation 

 of only 1-1.5 hours; (2) that the aerobic state can be restored within 

 one minute after the admission of oxygen; and (3) that the effect depends 



80 



60 



f 40 



°- ao 



— ■ (> 



20 



40 60 



Oxygen,% 



60 



100 



Fig. 32. 



-Inhibition of photosynthesis of Chlorella by excess oxygen (after Warburg). 

 Marked inhibition by 20% Oj. 



on the specific oxygen pressure used. However, an incubation period of 

 one hour suffices to develop fermentation, and an oxidation period of 

 one minute may suffice to destroy photochemically the fermentation 

 poisons in the chloroplasts. As to point (3), the extent (and even the 

 qualitative character) of fermentation may well depend on the residual 

 oxygen pressure, particularly in the region in which the rate of respiration 

 is a function of this pressure. 



The inhibition of photosynthesis by excess oxygen was discovered by 

 Warburg (1920) and studied by Wassink, Vermeulen, Reman, and Katz 

 (1938), and McAlister and Myers (1940). Warburg and McAlister and 

 Myers found a decrease of about 30% in the maximum rate of photo- 

 synthesis (in strong light and abundant carbon dioxide) of Chlorella 

 (Fig. 32) and wheat (Vol. II, Chapter 33) when the oxygen concentra- 

 tion was increased from 0.5 to 20%. Wassink and coworkers, on the other 



