CONTROL OF LEAF GROWTH BY AN INTERACTION 

 OF CHEMICALS AND LIGHT ^' - 



JAMES L. LIVERMAN 

 Division of Biology and Medicine, U. S. Atomic Energy Commission, 



Washington, D. C.-^ 



That photoperiodically active light controls a great many different 

 processes in plants and animals has been known for about 35 years. 

 In this connection, the phenomenon of flowering in particular has re- 

 ceived wide attention (Liverman, 1955), as has also the inhibition of 

 growth of seedlings of various kinds. 



Since 1952 (Borthwick et al, 1952) it has become increasingly ap- 

 parent that the red and far-red portions of the spectrum are concerned 

 in these responses and that these two quahties of light act antagonis- 

 tically to each other, i.e., when one promotes a process, in most cases 

 the other reverses this promotion. For instance, it has been shown that 

 red hght promotes expansion of etiolated leaves (Liverman et al, 

 1955), the unbending of the hypocotyl hook (Klein et al, 1956), 

 coleoptile elongation (Schneider, 1941; Liverman and Bonner, 1953), 

 and seed germination in a great many plants (Toole et al, 1956). In 

 most cases these promotive effects are reversed by a subsequent ex- 

 posure to far-red light. Other processes involved in seedling growth 

 that are inhibited by red light include hypocotyl, internode, and meso- 

 cotyl elongation (Downs, 1955; Goodwin, 1941; Weintraub and 

 Price, 1947). It is reasonably certain, of course, that the same recep- 

 tive pigment is involved in all these responses since their action spectra 

 appear to be almost identical. The evidence presently available does 

 not indicate, however, that the reactions subsequent to the actual per- 



1 Research reported herein was supported in part by grants-in-aid from the 

 U. S. Atomic Energy Commission and the National Science Foundation. 



- Most of the experimental work reported herein constituted part of a disserta- 

 tion submitted by Dr. Ralph A. Scott to the Graduate School, Texas Agricultural 

 and Mechanical College, 1957. 



3 On leave from Department of Biochemistry and Nutrition, Texas Agricul- 

 tural Experiment Station, College Station. Texas. 



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