554 /. A. Lockhart 



This inhibitor concept is not, of course, irreconcilable with the 

 function of gibberellin discussed above. Incorporating the inhibitor, 

 radiation would cither decrease the gibberellin, resulting in a lower 

 gibberellin-inhibitor ratio, or increase the inhibitor with the same 

 effect on the ratio and, thus, the same effect on growth. 



The mechanism by which visible radiation may limit stem elonga- 

 tion is shown in Figure 3. 



SUMMARY 



The mechanism by which visible radiation limits stem elongation 

 as we understand it today may be outlined as follows. 



Radiant energy is intercepted by the receptor pigment(s). The 

 activated pigment transfers (and amplifies) the signal through one or 

 more thermochemical steps common to all photomorphogenetic pro- 

 cesses. From one or more "master reactions" the signal is divided and 

 passed eventually to some step in the developmental sequence of each 

 of the processes under its control. 



One of the processes affected by visible radiation is stem elonga- 

 tion. Stem elongation normally proceeds by an increase in plasticity 

 of the young cell walls. For maximum increases in plasticity, evidently, 

 many growth factors are required. Among the growth factors recog- 

 nized to be necessary for increased plasticity are auxin, gibberellin, 

 and probably caulocaline. Visible radiation, through an unknown 

 sequence of reactions, acts to reduce the amount of available gibberel- 

 lin. Thus, plasticity of the cell walls is decreased and growth is re- 

 duced. 



LITERATURE CITED 



1. Avery, G. S., Jr., Burklioldcr, P. R., and Creighton, H. B. Polarized growth and 

 cell studies in the first internode and colcoptile of Avcna in relation to light 

 and darkness. Bot. Gaz. 99: 125-113. 1937. 



2. Borthwick, H. A., Hendricks, S. B., and Parker, M. W. Action spectrum for 

 inhibition of stem growth in dark-grown seedlings of albino and nonalbino 

 barley {Hordeum vulgare). Bot. Gaz. 113: 95-105. 1951. 



3. Brian, P. W., Elson, G. W., Hemming, H. G., and Radlcy, M. Ihc plant- 

 growth-promoting properties of gibbcrellic acid, a metabolic product of the 

 fungus Gibberella fujikui-oi. Jour. Sci. Food Agr. 5: 602-612. 1951. 



1. , and Hemming, H. G. Complementary action of gibberellic acid and 



auxins in pea internode extension. Aim. Bot. II. 22: 1-17. 1958. 



5. Downs, R. J. Photoreversibility of leaf and iiypocotyl elongation of dark grown 

 red kidney bean seedlings. Plant Physiol. 30: 468-473. 1955. 



6. , Hendricks, S. B., and Borthwick, H. A. Photoreversible control of 



elongation of pinto beans and other plants under normal conditions of growth. 

 Bot. Gaz. 118: 199-208. 1957. 



7. DuBuy, H. G. Ober Wachstum and Phototropismus von Ai'cna sativa. Rec. 

 Trav. Bot. N6erl. 30: 798-925. 1933. 



8. Duggar. B. M. (ed.). Biological Effects of Radiation. Vol. H. McGraw-Hill Book 

 Co., New York. 1936. 



