TEMPEIIATUKE EFFECT ON FLUORESCENCE 1245 



possible for systems of competing reactions to exhiliit such complex relation 

 to temperature. 



Henrici (1921) associated the decline in the P = f{T) curves between 20° and 30° C. 

 with the formation of starch in the chloroplasts, and its inhibiting effect on photosynthe- 

 sis (c/. Vol. I, page 331 ), and attributed the renewed rise of the rate between 25° and 30° 

 to the disappearance of starch by accelerated conversion into sugars and dislocation of 

 the latter from the chloroplasts. 



5. Temperature Effect on Fluorescence 



Knowledge of the influence of temperature on the yield of chlorophyll 

 fluorescence in the living cell may contribute to the understanding of the 

 nature of the temperature effect in photosynthesis. For a given fluorescent 

 molecule and given composition of the complex in which this molecule is 

 imbedded, the yield of fluorescence should be (at least, in the first approxi- 

 mation) independent of temperature, because the probabilities of absorp- 

 tion, of reradiation, of "quenching" by photochemical reaction within the 

 complex and of energy dissipation by "internal conversion" can be ex- 

 pected not to be strongly affected by minor changes in temperature. There- 

 fore, whenever a strong dependence of fluorescence intensity on temperature 

 is observed, the most likely explanation is a change in the composition of 

 the fluorescent complex. The effect of temperature changes on the light 

 curves of fluorescence in hving plants was described in chapter 28 (page 

 1055). These effects were observed by Kautsky and Spohr (1934) and 

 Franck, French and Puck (1941) in leaves, by Wassink, Vermeulen, Reman 

 and Katz (1938) in ChloreUa, Wassink and Kersten (1945) in diatoms and 

 by Katz, Wassink and Dorrestein (1941) and Wassink, Katz and Dorrestein 

 (1942) in purple bacteria. The characteristic results are represented in 

 figures 28.36-28.40. 



In most cases, the yield of fluorescence was found to be higher at lower 

 temperatures. Franck and co-workers found, more specifically, that 

 lowering of temperature caused in Hydrangea leaves a downward shift of 

 the light intensity, Ic, at which the quantum yield of fluorescence changed 

 from the "low light value," <pi, to the "high light value," ^2 (> vi) (c/. page 

 1049 and fig. 28.26). The observations of Wassink and co-workers with 

 purple bacteria (fig. 28.40) can be interpreted in the same way. 



On the other hand, Wassink, Vermeulen, Reman and Katz (1938) found 

 no effect of temperature on <p in ChloreUa (fig. 28.36) and Wassink and Ker- 

 sten (1945) oljserved, in Nitzschia, light curves of the shape shown in figure 

 28.39, in which the transition from <pi to ^2 occurred at lower light intensity 

 at 25° than at 5° C, and ip2 was lower, instead of higher, than ^1. 



In interpreting the light curves of fluorescence in chapter 28, we sug- 

 gested that a change in the yield of fluorescence is indicative of failure of a 



