394 



Pg = aig 



T. T. Bannister and. M. J. Vrooman 



(3) 



where ip is the rate of absorption of far-red q.uanta and a is the 

 fixed fraction (<l/2) of absorbed quanta acting in the short- 

 wave reaction. Of the rate (l-a)i2 of absorption of far-red 

 quanta by the far-red system, (l-2a)i2 is in excess of the rate 

 of sensitization of the short-wave reaction, and is, therefore, 

 wasted unless balanced by short-wave illumination. 



Spill-over Model . According to this model, short-wave quanta 

 are absorbed predominantly by pigments directly associated with 

 the short-wave reaction, but the excitation energy can be trans- 

 ferred to the long-wave reaction when this reaction tends to be 

 the slower. Thus short-wave quetnta are presumably distributed 

 in a flexible manner tending to equalize the rates of both re- 

 actions. In short-wave light alone, there would be a perfectly 

 balanced distribution, half the absorbed quanta going to each 

 reaction. Then 



= i,/2 



Ct) 



where P2^ is the rate of photosynthesis and i-^ is the rate of ab- 

 sorption of short-wave quanta. 



In short-wave and far-red lights together, absorbed short- 

 wave quanta go predominantly to the short-wave reaction tending 

 to balance the predominant utilization of far-red quanta in the 

 far-red reaction. In far-red and limiting short-wave illumi- 

 nations, the rate (P12) "^^^^ remain limited to the rate of the 

 short-wave reaction: 



P12 = 3-12+^^1 



(5) 



Here, b (> I/2) is the maximum fraction of absorbed short-wave 

 quanta which can be utilized in the short-wave system -- i.e., 

 (l-b) of short-wave quanta are constrained to act in the far-red 

 system . 



As short-wave illumination increases, a point is reached where 

 the rates of introduction of quanta into the short-wave and far- 

 red systems are equal, and both rates equal one-half the total 

 rate of absorption of quanta: 



P12 = (H+i2^/2 = '^^2-'^^l = (l-a)i2+(l-^)ii (^^ 



At this point no far-red quanta are wasted and any additional 



