940 CONCENTRATION FACTORS CHAP. 27 



quently, we proposed to review the effects of various external factors on the 

 intensity of chlorophyll fluorescence in vivo parallel with the presentation 

 of the influence of these factors on the rate of photosynthesis. 



In following this plan, we have now to describe the changes in the in- 

 tensity of fluorescence of living plant cells, associated with variations in the 

 supply of carbon dioxide. 



The general finding appears to be that reduction or complete stoppage 

 of the carbon dioxide supply usually affects the yield of fluorescence, ^, in 

 a certain range of illuminations. It has no effect on the yield of fluorescence 

 in very weak light; and probably also none in very strong light, but the 

 latter generalization is in need of experimental confirmation. In purple 

 bacteria, the commonly observed increase of ^ upon removal of carbon di- 

 oxide sometimes gives place, with increasing light intensity, to the opposite 

 effect (c/. fig. 28.30) . Since no systematic measurements of ^ for variable 

 [CO2] have been carried out, the plotting of "carbon dioxide curves" of 

 fluorescence, F = /[CO2], is not possible. Instead, "hght curves" have 

 been drawn, showing the intensity of fluorescence as a function of light 

 intensity, with [CO2] as a parameter, usually for two different concentra- 

 tions of carbon dioxide only (or simply "with carbon dioxide" and "with- 

 out carbon dioxide"). Several such curves will be reproduced in chapter 

 28 (c/. figs. 28.25, 28, 29 and 30). 



We anticipate here some of the facts to be presented there : The yield 

 of fluorescence, ^p, generally remains constant (^^i) up to or beyond the 

 intensity region in which photosynthesis becomes saturated with light; 

 but sooner or later, it increases more or less gradually, finally to reach a new 

 steady level, ^2- We designate the intensity at which the transition begins, 

 as /', and that at which it ends, as I" . The effect of removal of carbon di- 

 oxide api^ears to be a downward shift of this transitional range. Thus, 

 McAlister and Myers (1940) found that in 4% CO2, the yield of fluorescence 

 of wheat leaves was constant up to 600 kerg/(cm.^ X sec.) (kerg = 10^ erg) 

 while, in 0.03% CO2, the yield increased (by about 15-20%) between 200 

 and 600 kerg/(cm.2 X sec.) {cj. fig. 28.25). 



Similarly, Franck, French and Puck (1941) found that the steady state 

 fluorescence of a leaf of Hydrangea, at 7 = 7 kerg/cm.- sec. was about 20% 

 higher in carbon dioxide-free air than in air containing 5% CO2. At 71 

 kerg/cm.- sec, on the other hand, the yield of fluorescence in 5%C02liad 

 increased so strongly that now a change to 0.03% CO2 had no noticeable 

 effect. 



Working with a culture of diatoms (Nitzschia sp.), Wassink and Kersten 

 (1945) found the yield of fluorescence in intense light, 50 kerg/cm.- sec, 

 to be higher in the absence than in the presence of carbon dioxide. Figure 

 28.28 indicates that, in this case, the difference is brought about by a de- 



