LIGHT CURVES OP FLUORESCENCE 



1049 



supply of carbon dioxide (1% CO2). Measurements up to 130 kerg/cm.^ 

 sec. indicated (c/. fig. 28.26) that, at very high Ught intensities, the yield 

 of fluorescence again becomes stabilized. In other words the yield in- 

 creases from an initial constant level, (pi, to approach a final, also constant 

 level, <p2 (— 1-7 <j?i). The transition occurred, in Hydrangea, approxi- 

 mately in the same intensity region in which photosynthesis became light 

 saturated (cf. lower curve in fig. 28.26). 



LIGHT INTENSITY, kerg/cm' sec 



Fig. 28.26. Rate of photosynthesis (P, lower scale) and fluorescence (<p, upper 

 scale) yield of Hydrangea leaves as function of light intensity (after Franck, et al. 

 1941). Note that upper curve represents yield, ip, while the two preceding fig- 

 ures show absolute rates, i.e., intensities, of fluorescence, F. 



New measurements with Chlorella, as well as with Scenedestnus, were 

 carried out by Shiau and Franck (1947), and an increase in the yield of 

 fluorescence at high light intensities was now found also in these unicellular 

 algae — ^in Chlorella above 20 kerg/cm.^ sec. and in Scenedesmus above 15 

 kerg/cm.^ sec. 



Extensive measurements of the fluorescence of Chronialimn (strain 

 D) were described by Katz, Wassink and Dorrestein (1942) and by Was- 

 sink, Katz and Dorrestein (1942), who found that the yield of fluoresence 

 of these bacteria increased, at the higher light intensities, even more pro- 

 nouncedly than that of green plants. This is illustrated by figures 28.30 

 to 28.33. These figures also show the influence that the supply of the re- 

 ductants has on the shape of the light curves of fluorescence. In the 

 presence of abundant reductant (either hydrogen or thiosulfate), the in- 

 crease of <p in Chromaiium first begins at 15 kerg/cm.^ sec, while witliout 



