432 PLANT PHOTOPERIODISM 



Any of the several relations indicated in Fig. 5 can be found. This 

 has not been possible for photoperiodic control of flowering, for the 

 change from a small to a large percentage effect takes place over a 

 twofold change in irradiance. There is some question in the flowering 

 control, both of a threshold value of conversion for response and of 

 saturation of response before complete pigment conversion. 



PHYSIOLOGICAL ACTION 



How does physiological action arise from the photoreaction, or 

 simply, what is the next step after the photoreaction? Does the red- 

 absorbing form of the pigment act as an inhibitor of a transferring 

 asent, or does the far-red form act as a transferrins^ a^ent? I know of 

 no summary argument, but several favor the latter possibility. The 

 most general statement is that the rapid rise of physiological response 

 accompanying change of only a small amount of the pigment to the 

 far-red-absorbing form, as displayed in control of elongation of pinto 

 beans, indicates the appearance of a transferring agent. This is also 

 evident for light-requiring seed that lie ungerminating in darkness for 

 the order of a century, respiration being essentially absent. The releas- 

 ing red radiation starts up the whole vital reaction system, which is an 

 expression of a transferring agent being released (unblocked) or 

 created. An inhibitor in the volume of action would likely have to 

 reach concentrations of at least 10~*^ molar for half effectiveness. The 

 effective volume and concentrations of the pigment system are un- 

 known, but the extremely low incident energies required suggest con- 

 centrations of the pigment system of the order of 10~^ molar or less in 

 volumes of mitochondrial dimensions, which are in the concentration 

 ranges of many functional enzymes. 



The quantum efficiencies for photoconversion can be varied by 

 temperature manipulations as indicated in Dr. Toole's discussion. 

 These changes are evident as a change in one direction of the red 

 irradiance required for a given germination and the simultaneous 

 change in the opposite direction of the far-red irradiance for inhibition. 

 Thus, in germination of Lepidium virginicum seed, the shift in sensi- 

 tivity when attained is toward greater sensitivity to red radiation as 

 would be expected from the low concentration of A relative to AH2 as 



