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GROWTH REGULATORS AND THERAPEUTANTS...THEIR ABSORPTION, 
TRANSLOCATION, AND METABOLISM IN PLANTS 
John W. Mitchell, Paul J. Linder, and Bernard C. Smale! 
INTRODUCTION 
The fate of both growth regulators and therapeutants depends not only on the kind of 
compound involved, but also on where the compound is applied to the plant. For example, 
when a growth regulator was applied to theupper part of a bean stem, five times as much 
of the chemical was found inside the plant as when the lower, more mature part of the 
stem was treated (40). Or in the case of therapeutants, when one of the commercially 
used antibiotics was applied to cherry leaves, about 2 weeks were required for the com- 
pound to be inactivated, but when applied to the fruits the therapeutant was inactivated 
within a few days (18, 54). It is thus clear that site of application affects to some extent 
the fate of regulators andtherapeutants. We willconsider, therefore, the fate of regulating 
chemicals and therapeutants in relation to the site of application as well as in relation 
to the kind of compound used. 
GROWTH REGULATORS 
Growth-regulating chemicals cannot bring about desirable effects as long as they 
remain on the surface of a plant. All growth regulators are absorbed and most of them 
are readily translocated in the phloem of plants. Translocation therefore figures promi- 
nently in the fate of these substances. Fortunately, from the standpoint of residues, 
only very minute amounts of regulating compounds are required to bring about useful 
responses in crop plants. Furthermore, regulating compounds as a class must be 
relatively nontoxic in order to be most effective for controlling the growth and behavior 
of plants. 
1Physiologistsand Pathologist, respectively, Crops Research Division, Agricultural Research Service, U. S, Department of 
Agriculture, 
181 
