ENERGY TRANSFER IN PHOTOCHEMICAL SYSTEMS 61 



1957) is designed so that we are able to observe continuously the 

 light emitted from the chloroplasts approximately 0. 1 sec after excita- 

 tion by a flash of light. An analysis of these curves and those for inter- 

 mediate temperatures demonstrates that the room temperature emis- 

 sion consists of at least three components having different temperature 

 dependencies and having half-lives of 0.15, 2, and 15 sec, respectively. 

 Approximately 6% of the total integrated light intensity up to about 7 

 sec after the flash is due to the 0. 15-sec emission. When the chloro- 

 plasts are cooled, the slower components diminish in intensity and 

 vanish at about — 35°C. At this temperature, the decay curve is the 

 same as that obtained by subtracting the slower components from the 

 room temperature curve. When the chloroplasts are cooled still further, 

 the 0. 15-sec component diminishes in intensity, its decay constant re- 

 maining approximately the same, and it is gone at about — 100°C. At 

 about — 90°C, a fourth emission begins to grow in and gradually in- 

 creases in intensity down to liquid nitrogen temperature. This emission 

 has a half-life of about 0.3 sec. These cooling effects are completely 

 reversible. Both large and small spinach chloroplast fragments behave 

 similarly. 



The excitation and emission spectra of the luminescence were meas- 

 ured by using Corning glass filters between the flash and the sample 

 and between the sample and the detector. Such experiments demon- 

 strate that the room temperature and — 40°C emissions are excited by 

 the same bands of wavelengths that induce the electron spin resonance, 

 thus again indicating absorption by chlorophyll. These emissions con- 

 sist of wavelengths lying between 7000 A and 9000 A. The crude 

 measurements indicate that at least 90% of the emitted light is of 

 longer wavelengths than 7000 A. The in vivo fluorescence from the 

 chlorophyll singlet lies mainly between 6500 A and 7200 A. This 

 suggests that we are observing the lowest triplet state of chlorophyll 

 rather than the lowest singlet. However, better spectra- are needed to 



- Subsequent to this Conference, accurate measurements of the luminescence 

 emission spectra under various conditions were carried out (see Tollin, Fujimori, 

 and Calvin, 1958; Tollin. Sogo, and Calvin, 1958). These measurements demon- 

 strate that the luminescence of spinach chloroplasts does not originate in the 

 triplet state of chlorophyll as is tentatively suggested above but is, rather, the 

 result of the singlet state to ground state transition of chlorophyll. A fuller dis- 

 cussion of the impHcations of this finding is presented in the above-mentioned 

 references. 



