412 PLANT PHOTOPERIODISM 



rather direct, nonpersistent after effects of appropriate photoperiodic 



treatment. 



Let us summarize the events which occur in the course of photo- 

 periodic treatment. We will again use the cocklebur as an example. 

 We know, first of all, that the initial photoperiodic perception occurs 

 in the leaf. The leaf to be responsive to a long night must first have 

 been subjected to a hght period of sufficiently high intensity and of 

 sufficient duration (Hamner, 1940). In this preliminary high-light- 

 intensity period we believe, on the basis of the work of Liverman and 

 Bonner (1953) and others, that photosynthesis provides substrates 

 necessary for the consummation of the subsequent dark steps. In addi- 

 tion, the light serves to maintain the pigment system in the far-red- 

 absorbing form. Subsequent to the high-light-intensity treatment, the 

 dark processes commence. We recognize today three separate processes 

 that go on in the photoperiodic induction of a short-day plant. The 

 first of these steps we may call "pigment decay" (Borthwick et al., 

 1952). During the first 21/2 to 3 hr of the dark period, the pigment 

 system of the leaf is converted from a predominantly far-red-absorbing 

 form to a predominantly red-absorbing form (Salisbury and Bonner, 

 1956). The presence of the red-absorbing form is essential, since the 

 whole series of subsequent steps may be halted in their tracks by treat- 

 ment of the plant with an appropriate energy dose of red light. The 

 time-measuring reaction follows pigment decay. In the cocklebur this 

 process takes about 6 hr to run its course. The time-measuring reac- 

 tion is temperature insensitive, and this is all that we know about it. 

 Once pigment decay and time measuring have together measured off 

 8.5 hr, the processes begin that lead to the formation of something 

 within the leaf which is ultimately transported from it and which we 

 call, for convenience, "flowering hormone." Hormone production is 

 initially proportional to the amount by which the dark period exceeds 

 8.5 hr (Salisbury, 1955). The process declines in rate and saturates in 

 cocklebur at dark lengths of about 16 hr. Hormone production we 

 know to be inhibited by applied auxin (Bonner and Thurlow, 1949; 

 Sahsbury and Bonner, 1956), inhibited by high temperature, probably 

 owing to hormone destruction, and inhibited by excessively low 

 temperatures. After the dark period, the cocklebur is returned to light. 

 In the light, however, a still further process takes place. The product 



