374 LIGHT AND LIFE 



These observations, like the earlier data on chloroplast prepara- 

 tions, suggest that the light-induced ESR effects involve the sequen- 

 tial formation of two components which contain unpaired electrons. 

 Tentatively, it would appear that of the two, component I is the 

 more proximal to the initial photochemical event. This component 

 cannot represent an actual light-induced excited state, because its 

 decay rate, though rapid compared to that of the second component, 

 is still small compared to that exhibited by light-excited states. There 

 is evidence that comj^onent I is not ordinarily observed (i.e., at 

 optimal photosynthetic temperatures) because it rapidly decays, per- 

 haps by means of an electron transfer process which in turn gives 

 rise to component II. However, since component II is always present 

 in the dark it is also capable of being produced as an intermediate 

 in a dark oxidation-reduction process. The data of Fig. 7 and 8 

 suggest that the steady-state concentration which can be attained by 

 component II in the light is limited, and that component I makes 

 its appearance when this limit is reached. These relationships suggest 

 that electron transfer from component I to component II normally 

 results in a zero steady-state concentration of the former, which is 

 overcome only when the transfer process is inhibited by the failure 

 of component II to decay sufficiently rapidly (as at low temperatures) . 



This description of the ESR-yield components of the photosynthetic 

 system has been presented, intentionally, in purely operational terms, 

 because the data available at the present time do not warrant any 

 other approach. It is, of course, obvious that the foregoing relation- 

 ships can be fitted into any one of a number of schemes of photo- 

 synthesis that have already been proposed. However, if this correla- 

 tion is to be meaningful, both of the ESR components which we have 

 observed must be identified with specific known constituents of the 

 photosynthetic apparatus. The data available at present do not per- 

 mit such an identification, except in the most general terms. 



All that can now be said in this matter is the following: The ESR 

 component at g = 2.005 (II) is probably an organic free radical capa- 

 ble of being produced in the dark. It is uniquely present in green 

 plant photosynthesis. This component may be a free radical form 

 of any one of a number of reversibly oxidizable components that occur 

 in this system. Conceivably, it represents impaired electrons boimd by 

 a specific electron trap in a semiconductor. If so, the presumed elec- 

 tron trap is unique to green plants, and is for example, not present 

 in R. rubrxim. Less definite conclusions can be drawn with regard 

 to the component at g = 2.002 (I) . We have observed relatively 



