568 



PHOTOSYNTHESIS AND RESPIRATION 



CHAP. 20 



urethan is incomplete and only reduces it roughly to the compensation 

 point; red (and green) light do not affect respiration, and therefore 

 leave the gas exchange close to balance, whereas blue light stimulates 

 respiration and thus causes a net consumption of oxygen. 



These results have been confirmed by the more precise observations 

 of Emerson and Lewis (1943). They found that, in Chlorella, the 

 oxygen consumption in the dark is enhanced considerably after a short 

 illumination with light belonging to the narrow band of 440-530 m/x, 

 with a maximum effect at 470 m/x. The increase in the net oxygen 

 consumption in light with time, illustrated by figure 63 (and occurring, 



100 



140 



180 220 



Time, minufes 



260 



400 



440 



Fig. 63. — Effect of light on oxygen consumption (after Emerson and Lewis 1943). 

 The rate of oxygen exchange is based on readings at one-minute intervals. Light 

 periods are indicated at the bottom by the wave length of the Ught used. Intensities: 

 8.0 X 10~* einsteins per cm.^ per minute at 480 m^; 4.1 X 10"^ at 435 m^t; and 7.3 X 

 10~^ at 560 m/x. Broken line indicates the probable development of respiration during 

 the illumination periods. The cells (260 mm.' Chlorella cells in 25 ml. carbonate buffer) 

 were in the dark for about 75 minutes preceding the observations. 



according to this figure, only at 480 m/x and not at 435 m^ or 560 m^), 

 indicates that the rate of respiration increases gradually during the 

 illumination, the maximum observed increase (shown on the extreme 

 right) being of the order of 70%. (It must be noted that the light 

 intensity used in these experiments was very weak. The net gas exchange 

 in figure 63 never exceeded the compensation point, so stronger effects 

 may perhaps occur in more intense light.) 



According to Emerson and Lewis, the light-stimulated oxygen 

 consumption is particularly strong in Chlorella cells grown in neon light. 

 These cells are yellowish green in color (indicating a low concentration 

 of chlorophyll). Some cultures showed almost no effect; the same was 

 true of the blue-green cells of Chroococcus. 



