EFFECT OF LIGHT ON RESPIRATION 

 G. Krotkov 



During this symposium it has been sugp;ested by several 

 speakers, that light has a depressing effect on respiration, 

 as measured by oxygen absorption. In this connection some of 

 the data obtained in our laboratory is of interest. 



A detached tobacco leaf was placed in a closed system, and 

 air, with the initial concentration of 100 ppm of GO2 and 21^ 

 of O2, was circulated over it. After a few minutes of illum- 

 ination the lights were turned off. Figure 1 shows observed 

 changes in the concentration of carbon dioxide around the 

 leaf during illumination and subsequent dark period. 



During illumination carbon dioxide content of air decreased 

 until compensation point was reached. In darkness carbon 

 dioxide content went up, its initial evolution being faster 

 than later. 



Figure 2 shows the rate of carbon dioxide evolution in dark- 

 ness following previous illumination at different light inten- 

 sities. Within the first minute of darkness there was a large 

 outburst of carbon dioxide evolution, the magnitude of which 

 was directly proportional to the previous light intensity. 

 This was followed by a second outburst, which was much smaller 

 and of longer duration. The steady rates of carbon dioxide 

 evolution in the dark were reached after about six minutes. 



When these experiments were repeated at concentrations of 

 oxygen in the air varying between 2-1^7% the following changes 

 in Figures 1 and 2 were observed: 



(1) In light, the higher was the oxygen content of the air 

 the higher was the compensation point. Figure 3 shows the 

 relation between the two. Since at compensation point photo- 

 synthetic and respiratory rates are equal one can conclude 

 that the rate of carbon dioxide evolution in light is increased 

 with the increasing oxygen content of the air. 



(2) In dark, the magnitude of the first outburst of photo- 

 stimulated carbon dioxide evolution was also directly propor- 

 tional to the oxygen content of the air. As is seen in 

 Figure 4, at 2% oxygen, it was completely eliminated. On the 

 other hand the final, steady rates of dark carbon dioxide 

 evolution reached after six minutes were unaffected by oxygen 

 concentration. 



Comparable results have been obtained using several other 

 kinds of leaves besides those of tobacco. 



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