PHOTAUTOXIDATION IN PRESENCE OF EXCESS OXYGEN 533 



They have been observed for example, in stomata-free ferns (Montfort 

 and Neydel 1928; Fockler 1938), in algae (Montfort 1930, 1933); and in 

 Chlorella cells suspended in solutions which provided an ample supply of 

 carbon dioxide (Myers and Burr 1940). 



Light inhibition can most easily be obtained in land plants and algae 

 adapted to weak hght. Weis (1903), Lubimenko (1905, 1907, 1908i'2, 

 1928i'2), and Harder (1930, 1933), among others, have found that the 

 "Hght curves" (rate of photosynthesis plotted versus hght intensity) of 

 typical "shade plants" reach a maximum in relatively moderate light 

 (e. g., 10,000 lux or less) and then decline again. Montfort (1929, 19331-2) 

 observed a similar behavior of brown and red algae collected deep under 

 the sea (cf. Vol. II, Chapter 28). 



The light curves of shade plants and sun plants will be discussed in 

 more detail in volume II, chapter 28. While the ascending parts of 

 these curves are independent of the duration of illumination, the parts 

 corresponding to saturating light intensities, often are time-dependent. 

 In "shade plants," a decline in the rate of photosynthesis at high hght 

 intensities cannot be avoided even in rapid experiments; if the illumina- 

 tion is extended, a complete inhibition may ensue and the plants may 

 suffer an irreversible light injury, or even death by "sunstroke." In 

 typical "sun plants," on the other hand, a rapid determination may 

 lead to light curves with a completely flat "saturation plateau," extending 

 far beyond the saturating light intensity; however, a suflSciently prolonged 

 and intense illumination is bound to produce a gradual inhibition in 

 these plants as well. 



Fockler (1938) observed that, after one hour of exposure to direct 

 sunlight, the photosynthesis of the fern Trichomanes radicans gave place 

 to oxygen uptake. The longer the exposure, the slower and less complete 

 was the recovery in moderate or weak light. Similarly, Stalfelt (1939) 

 found that, in lichens, ten hours of illumination with 16,000 lux caused 

 a 26% decrease in the rate of photosynthesis, and 14 hours in darkness 

 were required for recovery. 



The most extensive study of the effect of strong light on photo- 

 synthesis was carried out by Myers and Burr (1940) with the unicellular 

 green algae {Chlorella pyrenoidosa, C. vulgaris, and Protococcus) . They 

 used collimated light from a tungsten lamp, giving intensities up to 

 39,000 foot-candles {i. e., about 360,000 lux), and obtained families of 

 curves showing changes in the rate of photosynthesis as a function of 

 both intensity and duration of illumination. The use of thin suspensions 

 (giving a total light absorption of only 10%) ensured a nearly uniform 

 illumination of all cells. (This might explain why the effects observed 

 by IMyers and Burr were so much stronger than those described by 

 Emerson.) 



