FLOWERING HORMONE MOVEMENT AND ACTION 187 



1. High Intensity Light may be Required Directly to Stabilize the 

 Flowering Hormone or Complete its Synthesis in Some Manner 



Karl Hamner and his graduate student, James Lockhart (59), 

 felt that this was the most likely explanation. They performed a 

 number of interesting experiments in which they studied the effects 

 of temperature, low light intensities, and applied auxins on the 

 processes following the dark period. 



2. Flowering Hormone may be Destroyed in Certain Leaves, and 

 High Intensity Light may Overcome this Potential for Destruction 



In Chapter 9 we interpreted high temperature and applied auxin 

 effects by postulating a destruction of flowering hormone in the leaf. 

 Does high intensity light remove this potential for destruction (e.g. 

 by lowering the auxin level)? Or does high intensity light cause a 

 movement of flowering hormone out of the leaf before it can be 

 destroyed? It is interesting to note that experiments such as that 

 shown in Fig. 10-2 do not always succeed; that is, the inhibitory 

 effect of a second dark period cannot always be demonstrated. Thus 

 it seems that environmental factors may some way condition the 

 response. Is this an effect upon the leaf's ability to destroy hormone? 



D. J. Carr (46), then at the University of Melbourne in Australia, 

 was able to overcome the requirement for high intensity light follow- 

 ing an inductive dark period by treating plants with sugar. This 

 seems to indicate that the requirement for high intensity light 

 following the dark period is a requirement for sugar produced by 

 photosynthesis. But what is the sugar doing? It might be acting as 

 an energy source for metabolic stabilization of a hormone precursor 

 (according to the idea of Lockhart and Hamner), or for metabolic 

 reactions which remove the destructive potential of the leaf (an idea 

 which I favor), or it might be providing a translocation stream for 

 movement of the stimulus out of the leaf (as Carr suggested). The 

 translocation theory also implies a destruction, however, since plants 

 are finally returned to high intensity light anyway, and if there were 

 no destruction during the second dark period, flowering would at 

 most be slightly delayed. 



One point seems clear and is agreed upon by all investigators : the 

 requirement for high intensity light following the dark period is not 

 absolute. As mentioned above, it is often difficult to demonstrate 

 it, and it is also well known that some cocklebur plants will flower 



