Lfght Saturation of Delayed 



Light Production in Green Plants* 



William Arnold 

 Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 



Three years ago it was shown in this laboratory ( Strehler and Arnold, 

 1951) that green plants emitted light for some seconds after being 

 illuminated. The action spectrum for the delayed hght was shown to 

 be the same as that for photosynthesis, and thus it was chlorophyll 

 which absorbed the light energy that was reemitted. Later experi- 

 ments have shown (Arnold and Davidson, 1954) that the emission 

 spectrum of the delayed light is the same as the fluorescent spectrum 

 of chlorophyll in the living plant, and thus that it is chlorophyll which 

 emitted the delayed light. 



Although photosynthesis and delayed light production seem to be 

 closely connected, a fact that has been emphasized in the past, in the 

 manner of light saturation they are very different. Figure 1 gives the 

 two curves for continuous hght made on aliquots of the same Chlorella 

 pyrenoidosa suspension. As can be seen, the delayed hght production 

 saturates at an intensity very much lower than photosynthesis. In the 

 hope of understanding this difference, experiments have been done 

 with short flashes of light. 



A ChloreUa suspension is pumped from a darkened container 

 through a glass tube, a length D cm of which is illuminated with 

 intensity 7. From the rate of pumping, the diameter of the tube, and 

 D, the time T that each cell spent in the light can be calculated. After 

 flowing in the dark for a short distance, the suspension passes in front 



• Work performed under Contract No. W-7405-eng-26 for the Atomic 

 Energy Commission. 



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