LIGHT CURVES OF FLUORESCENCE 1057 



species with and without external CO2 (fig. 28.28) is repeated in the fluores- 

 cence curves at low and high temperature: The yield of fluorescence 

 either remains constant, or declines at high light intensity. An additional 

 peculiarity is that the curve obtained at low temperature resembles that 

 found with carbon dioxide, and the curve found at high temperature re- 

 sembles that found without external CO2; usually, the reverse relation 

 prevails, the efi^ect of loA\ering tlie temperature being similar to that of re- 

 moving carbon dioxide. In figure 28.39 the yield at 25° C. remains con- 

 stant up to 100 kerg/cm.' sec, while the saturation of photosynthesis be- 

 gins at this temperature at about 20, and is complete at about 45 kerg/em.- 

 sec. 



A temperature effect similar to that observed in green plants was foimd 

 in purple bacteria by Wassink, Katz and Dorrestein (1942). At 16° C. the 

 transition point, I^, was at about 10 kerg and, at 29°, at about 20 kerg/cm.^ 

 sec. In more detail, observations by Wassink, Katz and Dorrestein are 

 summarized in figure 28.40. 



{d) Cyanide 



A shift of the fluorescence transition point toward lower light intensities 

 can be produced also by the addition of cyanide. A "stimulating" effect 

 of cyanide on the yield of steady fluorescence was first observed by Kautsky 

 and Hirsch (1935) in experiments with leaves. Wassink, Vermeulen, 

 Reman and Katz (1938) found no such effect in Chlorella suspensions (fig. 

 28.41); but Wassink and Katz (1939) later proved that cyanide stimula- 

 tion does occur in this organism as well, although only when the cyanide 

 concentration is high enough to cause complete inhibition of photosynthe- 

 sis. This concentration completely inhibits respiration also, which is an 

 important factor from the point of view of the "narcotization" theory of 

 Franck (since this theory assumes that the "narcotic", which protects the 

 "idhng" photosjoithetic apparatus, undergoes rapid oxidation when respira- 

 tion is strong, but can be preserved for a considerable length of time if 

 respiration is weak) . Figure 28.42 shows that the cyanide effect on steady 

 fluorescence of Chlorella is caused by the disappearance of the decline of 

 fluorescence otherwise observed after the first half-minute of illumination. 

 This decline is entirely eliminated in 2.5 X 10~^ M potassium cyanide 

 solution (p = 0.33 in fig. 28.42). The effect of cyanide on the stationary 

 intensity of fluorescence increases at first with light intensity, as shown by 

 figure 28.43; but Franck, French and Puck (1941) found that it disappears 

 again if the light intensity is increased still further. For example, 2% gase- 

 ous HCN in the atmosphere increased the yield of fluorescence of Hydrangea 

 leaves (in 1% CO2) by 12% when the incident light intensity was 2 kerg. 



