LIGHT CURVES OP FLUORESCENCE 1051 



2 



28.28) French and Koski (1951) reported that the chlorophyll fluorescence 

 of a red alga declined when the exciting green light was raised to 4 kerg/cm. 

 sec. The phj^coerythrin fluorescence remained proportional to I. 



2. Efifect of Various Factors on Light Curves of Fluorescence 



We have seen above that the yield of fluorescence, <p, is often afi"ected 

 by changes in the yield of photosjTithesis, 7, when the latter are produced 

 by variations of light intensity. We therefore expect ^ to be affected also 

 by other, external or internal factors that influence 7. 



(a) Carbon Dioxide 



The effect of the factor [CO2] on fluorescence was already mentioned 

 before. First, we noted the results of McAlister and Myers (fig. 28.25), 

 who found that, in wheat, (p rises at high light intensity in 0.03% CO2, 

 while no such change occurs in 4% CO2. In qualitative agreement wdth 

 McAlister's results, Franck, French and Puck (1941) found that at 17 kerg/ 

 cm. 2 sec. the yield of fluorescence of Hydrangea was about 20% higher in 

 the absence of carbon dioxide than in the presence of 5% CO2. 



As mentioned above, Wassink and Kersten (1945) found a decrease 

 rather than increase in the yield of fluorescence of diatoms at high light 

 intensities in the presence of carbon dioxide; in the carbon dioxide-free 

 suspension (p remained constant up to 100 kerg/cm. ^ sec. {cf. fig. 28.28). 

 In other words, in this type of plant, as in Hydrangea or Triticum, the jdeld, 

 <P2, in strong light was higher without than with carbon dioxide; but this 

 difference was caused by a decline of <p in the C02-supplied plant, rather 

 than, as in the other species, by an increase of (p in the starved cells. 



In evaluating figure 28.28, it must be borne in mind that according to 

 figure 28.5 the photosynthesis of Nitzschia was not completely inhibited in 

 C02-free air. This probably means that these cells produced, by dark 

 metabolism, so much carbon dioxide (or intermediates which could be used 

 directly for photos\Tithesis) that their photosynthesis could not be stopped 

 by bubbling C02-free air through the suspension. Whether this is the cor- 

 rect explanation of the continued oxygen production by Nitzschia in "CO2- 

 free" air or not, the notation "no CO2" in figure 28.28 certainly means "no 

 extemal CO2 supplied," and not "photosynthesis totally inhibited by ab- 

 sence of CO2." 



Comparison of figure 28.28 with the effects of low temperature (fig. 

 28.39) and cyanide inhibition (fig. 28.44) in the same species shows that the 

 results would be more plausible and consistent if the designations of the 

 two curv^es in figure 28.28 were reversed. 



A still different effect of carbon dioxide on the Ught curves of fluores- 



