58 



PHOTOCHEMICAL PRINCIPLES 



Strehler and co-workers (Strehler, 1951; Strehler and Arnold, 1951; 

 Strehler and Lynch, 1957; Arthur and Strehler, 1957) have demon- 

 strated the existence of temperature-dependent long-lived lumines- 

 cences in algae and in chloroplasts. 



Our own experiments in this area began in 1956 with the demon- 

 stration by Sogo of a light-induced ESR signal in dried eucalyptus 

 leaves. Inasmuch as these results were rather poorly reproducible, it 

 was decided to study isolated chloroplasts (Sogo et ai, 1957). Fur- 

 thermore, when it became apparent that the spin resonance signals de- 

 cayed fairly rapidly when the light was turned off, the possibility that 

 at least part of the energy associated with these unpaired spins might 

 appear as luminescence led us to a study of the light emission proper- 

 ties of the chloroplasts (TolHn and Calvin, 1957). 



The chloroplasts are prepared (Sogo et aL, 1957) by grinding 

 spinach leaves in a blendor and carrying out a series of differential 

 centrifugations. These enable us to obtain what we shall call intact 

 chloroplasts and large and small chloroplast fragments. 



Some typical ESR curves for wet, large chloroplast fragments are 

 shown in Fig. 4. These curves are essentially plots of microwave power 

 absorbed in the sample vs. magnetic field strength. It is seen that there 



dark, 25'C 



light, 25°C 



l5oe-i 



dark,-l40°C 



light, -I40*C 



Fig. 4. Typical spin resonance spectra from wet, large spinach chloro- 

 plast fragments. 



