LUMINESCENCE OF PHOTOSYNTHETIC ORGANISMS 119 



nomenon (4,6). It has also been shown that photosynthetic bacteria 

 emit a hnninescence of a similar nature at longer wavelengths, al- 

 though this process has not been intensively investigated (4,7). 



4. The intensity of the luminescence is quite low — of the order 

 10~* to 10""'^ of the illuminating intensity below saturation, i.e., ca. 

 10~^ to 10~^ of the fluorescence intensity. 



EVIDENCE ON THE ENZYMATIC NATURE OF THE PROCESS 



1 . The long-lived luminescence increases with increasing illuminat- 

 ing intensity up to a certain value and thereafter remains essentially 

 constant; i.e., it displays a typical saturation as does an enzymatic 

 reaction (1). 



2. At unphysiological temperatures the luminescence is rever- 

 sibly destroyed if the heat treatment is brief. Exposure to 45° to 50° C. 

 for a few minutes irreversibly destroys the luminescence (1). 



5. Rate of luminescence as a function of temperature shows a typi- 

 cal enzymatic temperature optimum at ca. 37 °C. (the temperature 

 optimum for photosynthesis) with a heat of activation for the process 

 of CO. 10 to 15 kcal. (1). That this activation energy is characteristic 

 of the limiinescent substrate was established by measuring in a flow 

 system the temperature dependence of the emission process after 

 illumination at different temperatures (1). The intensity of light 

 emission was strongly dependent on the measuring temperature and 

 essentially independent of the temperature at which the plants 

 were illuminated (between 0° and 40°C.). That the formation of 

 luminescent substrate does involve some activation energy was 

 shown by illumination at liquid nitrogen temperature, after which no 

 luminescence is observable upon thawing. 



4. Various metabolic inhibitors, including V.\. light, produce 

 strong changes in luminescence (1,8,9). 



DECAY CHARACTERISTICS 



1. The luminescence does not decay according to any simple 

 kinetic formulation (1,2,6). About one-half of the luminescence dis- 

 appears in 0.1 second, after w^hich it decays more slowly. 



2. At least two definite components are distinguishable in the 

 decay curves (see Fig. 1) : 



a. A fast decaying component of ca. 0.01 second duration that 



