CARBON DIOXIDE CONCENTRATION AND FLUORESCENCE 941 



dine of <p in the carbon dioxide-supplied algae, rather than by an increase 

 of (f in the carbon dioxide-starved cells (as in figs. 28.25 and 28.30 A). 



Wassink, Katz and Dorrestein (1942) noticed a similar effect of carbon 

 dioxide deficienc.y on the yield of fluorescence in purple bacteria; however, 

 the change was much less pronounced than that caused by the rationing or 

 denial of the reductants (cf. below, section D2). In the absence of a reduc- 

 tant (a condition that cannot be paralleled in green plants), the removal of 

 carbon dioxide had no effect at all on the fluorescence of Chromatium (cf. 

 fig. 28.29) . In the presence of reductants (such as thiosulfate or hydrogen) , 

 the fluorescence of carbon dioxide-starved bacteria was, however, con- 

 siderably stronger than that of carbon dioxide-supplied cells (c/. fig. 28.30A). 

 This difference occurred in the range from 2 to 30 kerg./cm.^ sec. At light 

 intensities above 30 kerg, the fluorescence curves with and without carbon 

 dioxide again approached, and finally even crossed each other, so that (p 

 became higher in the absence than in the presence of carbon dioxide. In 

 other words, fluorescence was now higher when photosynthesis was pos- 

 sible than when it was suppressed ! (This is a good illustration of the fact 

 that the relation between photosynthesis and fluorescence is not a simple 

 competition; cf. i)age 820.) 



In chap. 28 we will discuss several possible explanations of the change 

 in intensity of chlorophyll fluorescence in strong light. One, is to attrib- 

 ute this change to the accumulation of the photocomplex, X- Chi -HZ 

 (a complex of primary oxidant + chlorophyll + primary reductant) 

 in a changed form due to the incapability of the catalytic dark reactions to 

 keep pace with the primary photochemical process. The changed forms 

 of the photocomplex can have a different fluorescence yield, first, because 

 they are photostable (i. e., cannot undergo the primary photochemical 

 photoprocess (e. g., X- Chi -HZ ^ XH-Chl-Z), and, secondly, because they 

 have an intrinsically different capacity for dissipating the excitation energy. 

 An alternative or additional cause is, according to Franck, the forma- 

 tion of a "narcotizing" substance, by reaction of excess oxidation inter- 

 mediates with metabolites ; this narcotic displaces the reactants from the 

 photocomplex and thus causes changes in the yield of fluorescence. 



Absence of carbon dioxide slows down the restoration of the primary 

 oxidant, HX or ACO2, after it has been reduced by light. It must thus 

 favor the accumulation of the photocomplex in a form such as HX-Chl-Z 

 or HX-Chl-HZ, and this accumulation will find its expression in a change 

 (usually an increase) in the jaeld of fluorescence. 



Franck and Herzfeld (1941) were inclined to consider the effect of car- 

 bon dioxide on fluorescence of plants as proof that carbon dioxide (in the 

 forai of a compound with an acceptor, ACO2) is a direct part of the photo- 

 sensitive complex (i. e., that X in X- Chi -HZ is identical with ACO2). 



