Chapter 13 



INHIBITION AND STIMULATION OF PHOTOSYNTHESIS 

 n. VARIOUS CHEMICAL AND PHYSICAL AGENTS 



A. Influence of Oxygen on Photosynthesis * 



The influence of oxygen in photosynthesis has two aspects: complete 

 absence of oxygen often brings photosynthesis to a standstill; while an 

 excess of oxygen invariably reduces the rate of this process. 



The necessity for traces of oxygen for photosynthesis has been much 

 discussed but not yet completely clarified. Boussingault (1865) and 

 Pringsheim (1887) were the first to notice that plants lose their capacity 

 for photosynthesis after a sojourn in hydrogen, nitrogen, or methane. 

 Willstatter and Stoll (1918) observed that some species (e. g., Pelar- 

 gonium) lost almost all their capacity for photosynthesis after only two 

 hours in an oxygen-free atmosphere, while others (e. g., Cyclamen) 

 required as much as 15 hours of "anaerobic incubation." The small 

 residual capacity of anaerobically treated leaves for photosynthesis was 

 sufficient to restore the aerobic conditions and thus to remove the 

 inhibition after several hours of exposure to light. Willstatter and 

 Stoll noted that, when this "autocatalytic" restoration of photosynthesis 

 was achieved, the partial pressure of oxygen still was exceedingly low — 

 much lower than that required for the resumption of respiration. This 

 fact, and the length of the anaerobic treatment necessary for inhibition, 

 led Willstatter and Stoll to the belief that photosynthesis requires the 

 saturation of an oxygen-acceptor complex in the cells rather than the 

 presence of free oxygen in the atmosphere. They suggested that this 

 complex dissociates slowly in an oxygen-free atmosphere, and is re- 

 generated by photosynthesis before any oxygen can escape from the 

 cells. A similar point of view was taken by Kautsky (1931, 1939), who 

 thought that the energy absorbed by all sensitizers must be transferred 

 to oxygen or (in the case of photosynthesis) to an oxygen-acceptor 

 complex before it can be utilized for chemical transformations. Since 

 this altogether implausible hypothesis was based on observations of 

 fluorescence, we shall discuss it in chapter 24 (Vol. II), which deals with 

 the fluorescence phenomena in living plants. 



Another explanation of anaerobic inhibition was suggested by Will- 

 statter (1933) and Franck (1935), who thought that the first step in 



* Bibliography, page 347. 



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