FLUORESCENCE INDUCTION PHENOMENA 1381 



In section A2, we stated that short induction lasts for approximately the same time 

 in algae, higher aquatic plants and land plants. Some of the parallel curves of carbon 

 dioxide absorption and fluorescence in wheat (McAlister and Myers) indicate that the 

 induction period of fluorescence may be subject to stronger variation in length than that 

 of the carbon dioxide uptake. For example, in figure 33.22A, fluorescence drops to a 

 steady level after less than 1 minute, while carbon dioxide consumption continues to 

 increase for about 3 minutes. It is, however, difficult to say how much of this discrep- 

 ancy is caused by the sluggishness of gas measurements, and how much represents a 

 genuine time lag betw^een the transformation of the photosensitive complex (instantly 

 reflected by changes in fluorescence) and the change in the uptake of carbon dioxide 

 (which, after all, is only a more or less remote consequence of this transformation). 



The shape of the fluorescence-time curves is further influenced by the 

 age of the plants, or cell cultures. In the investigation of Franck, French 

 and Puck (1941), young cultures of Chlorella and Scenedesmus gave "one- 

 wave" curves of type I, whereas old cultures gave curves dominated by the 

 second wave. According to Shiau and Franck (1947), yoimg and very 

 vigorous cultures of Chlorella and Scenedesmus sometimes show no induction 

 phenomena at all, except after long anaerobic incubation; while old cul- 

 tures may show abnormally long induction periods (up to 5 minutes in air, 

 instead of the usual 1 minute). Young cultures, transferred into a medium 

 that previousl}^ contained an old culture, sometimes show, after 24 hours, a 

 marked increase in the duration and extent of induction. 



In plants of a given species and age, the induction curve may depend 

 greatly on the conditions to which the cells were exposed before and during 

 the illumination. Carbon dioxide supply is one of them. McAlister and 

 Myers (1940) obtained curves of type I with wheat in ordinary air {cf. fig. 

 33.21A); but, in air enriched with carbon dioxide (0.3 to 0.4% CO2; 

 no higher concentrations were used), the first fluorescence peak decayed 

 very rapidly, and the second wave was strongly developed (cf. 33.21B). 

 Its crest was reached 1.5 minutes after the Ijeginning of illumination, com- 

 pared with only 20 seconds in Chlorella, in figure 33.20. This may be due 

 to stronger light and lower temperature (since figs. 33.27 and 33.33 show 

 that both these factors delay the second wave). 



With Chlorella, too, McAlister and Myers found that the type of the 

 fluorescence curve depends on carbon dioxide concentration. Induction 

 curves of type I (although with a faster fluorescence decay than in wheat) 

 were obtained with cells grown in 4% CO2, and studied in 0.24% CO2 (fig. 

 33.22A). A secontl wave appeared in cells grown in ordinary air and 

 studied in carbon dioxide-enriched air (fig. 33.22B), while, in cultures 

 grown and studied in normal air, this wave was so extensive that, after the 

 first few seconds of illumination, the development of fluorescence became 

 parallel, instead of antiparallel, to that of carbon dioxide consumption 

 (fig. 33.22C). 



