JAMES A. LOCKHARTi 



California Institute of Technology 



The Hormonal Meckanism. of Growth Inhibition 



by Visible Radiation' 



A fundamental problem of plant physiology is to gain an under- 

 standing of the mechanisms by which plants respond to their environ- 

 ment. The response may be any one of three general types: metabolic, 

 tropic, or morphogenetic. Each of these responses is the resultant of 

 an interaction between the environment and the genetic and onto- 

 genetic potential of a given organism. Hormones may participate in 

 the regulation of any of these responses, but they are probably most 

 significant in the last two. 



We are already familiar with the nature of several plant hormones 

 and can infer the existence of others. This paper is concerned specifi- 

 cally with the control of stem growth. Two hormones (or types of 

 hormones) are known to participate in the control of stem growth. 

 The first is auxin, which now appears to function primarily in the 

 tropic responses. Second is gibberellin, which apparently acts as a con- 

 trolling mechanism in many morphogenetic responses. A third hor- 

 mone, caulocaline, produced in roots, is also required for normal stem 

 growth, at least in many species (40). It has not yet been possible to 

 isolate or characterize this hormone. 



The present paper will consider evidence purporting to show that 

 gibberellin is, in fact, the controlling factor in regulation of stem 

 growth by visible radiation. 



MORPHOLOGICAL EFFECTS OF VISIBLE RADIATION 



Visible radiation has two distinct morphological effects on stem 

 growth. Irradiation reduces total length of stem and generally causes 

 it to become thicker and stiffer as well. This is the most noticeable 

 effect of high irradiances, although measurable effects are often ob- 



^ Subsequently: Hawaii Agricultural Experiment Station, University of Hawaii, 

 Honolulu 14, Hawaii. 



''This research was supported in part by a grant from the Herman Frasch Foun- 

 dation. 



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