PHOTOCONTROL OF VEGETATIVE GROWTH 



R. J. DOWNS 



Agricultural Research Service, U. S. Department of Agriculture, 



Beltsville, Maryland 



The vegetative growth of many plants is regulated to a remarkable de- 

 gree by light. This is particularly well illustrated by the growth of a 

 great many woody plants whose preparation for winter is largely light- 

 controlled. Light affects growth in two major ways, and both are ex- 

 pressions of the same basic photoreaction (Borthwick, Hendricks, and 

 Parker, 1952; Borthwick et al, 1952; Downs, 1955; Liverman et al, 

 1955; Mohr, 1956). The first way, which is photoperiodically con- 

 trolled, determines whether woody plants shall continue to elongate, 

 and the second, which is photo-controlled, although not truly photo- 

 periodic, determines whether the elongation made under the stimulus 

 of a favorable photoperiod shall be long or short. 



The woody plant thus appears to be running two different systems 

 with the same photoregulator. In one system, the photoperiodic one, 

 the regulator seems to operate as an ordinary on-off switch in that long 

 photoperiods allow growth to continue while short photoperiods induce 

 dormancy and cessation of growth (Downs and Borthwick, 1956). In 

 the other case, the photoregulator operates as a modulating device and 

 as such permits very delicate control of the response. This modulator 

 is the ratio of the red-absorbing to the far-red-absorbing form of the 

 pigment. For example, the growth of loblolly pine (Pinus taeda L.) 

 on long photoperiods increases when the plants enter the dark period 

 with the red-absorbing form of the pigment predominating (Table I). 



The ratio of the red-absorbing to the far-red-absorbing form of the 

 pigment may be controlled by proper choice of the supplemental light 

 source. In spite of the considerable red-radiant energy emitted, the 

 high far-red emittance of the incandescent lamp is apparently adequate 

 to shift the pigment equilibrium in favor of the red-absorbing form 

 and thereby induce additional growth and elongation. The fluorescent 



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