12 PHOTOCHEMICAL PRINCIPLES 



SUGGESTIONS FOR FURTHER EXPERIMENTS 



With the small amount of information available at the present time, 

 it is not easy to propose a mechanism of photoperiodism. I would 

 like to suggest certain experiments to help in choosing between the 

 various hypotheses that I have set forth. 



If the phenomenon is a photauxidation, then illuminating a plant 

 that has been treated with p-phenylenediamine should be revealing. 

 Soaking a leaf in a solution of this substance (at about pH 7) and 

 illuminating with red or with far red would yield a purple substance 

 (autoxidized phenylenediamine) at a rate greater than that obtained in 

 the absence of hght. 



If the process involves photoreduction, then light-induced produc- 

 tion of formazan from its tetrazolium salt should take place. Dr. 

 Hendricks reminds me that seedling viability tests with tetrazolium 

 salts are dependent on the presence of light. This seems to me to be 

 extraordinarily interesting, and an action spectra for the formation of 

 the formazan should be detennined. Ascorbic acid and glutathione do 

 not convert tetrazolium salts at pH 7 or lower, and if this is the pH 

 of the plant cells, it would appear that the viability test is really a test 

 for photoreduction in which naturally occurring reducing agents not 

 available in dead cells participate as the electron donors for the Hght- 

 excited pigments. 



It would be interesting to see whether photoperiodism involves the 

 production of free radicals. Calcium acrylate, a divinyl monomer 

 which is water soluble is converted into a voluminous precipitate in the 

 presence of free radicals. Illumination of leaves or seeds soaked with 

 this monomer under the spectral requirements of photoperiodism 

 would reveal the presence of free radicals in the photoreaction. 



The reagents which I have proposed may well inhibit the subsequent 

 growth, germination, or flowering processes associated with photo- 

 periodism. This may not be of consequence, however, since these 

 processes are assumed to be initiated by the photoreaction, the chemical 

 nature of which one wishes to reveal in its earliest phases. 



