6 G. L. McNew 



THE DESIGN OF A PLANT REGULATING MOLECULE 



Unfortunately, the golden era of the 1940's and 1950's in practical 

 achievements of horticulture has not been matched by comparable 

 progress in understanding the underlying principles. Too many 

 people have developed pet theories without having first turned to 

 the living cell to seek proper orientation of their ideas. The secret 

 lies inside the living cell, and it will be unlocked only by careful 

 analysis of the cell constituents on a quantitative basis or by tracing 

 the metabolic fate of the plant giowth incitant. Fortunately, with 

 the wonderful new tools that are coming into use for tracing minute 

 quantities of chemicals and unravelling local changes in metabolism 

 of a few cells, perhaps this neglected area will be investigated more 

 fully, as it should be. 



There has been much written about the design requirements of 

 a plant regulating molecule. These specifications have been modified 

 from time to time — almost every time a new group of compounds 

 has been introduced. Unfortunately, the theories have done very 

 little to promote progress because they failed to analyze completely 

 the various factors involved. Any change in chemical structure must 

 be analyzed completely from the viewpoint of five effects on the chem- 

 ical and physical attributes of the molecule; namely, selective solubility 

 involved in cell permeation, translocatability, reactivity with specific 

 cell metabolites, detoxication by extraneous reactions or physical or 

 chemical binding to nonvital cell constituents, and type of degradation 

 products formed during its metabolism by the cell. 



Very rarely have plant physiologists stopped, for example, to con- 

 sider the effect of lengthening a carbon chain or adding a parachloro- 

 phenyl group on lipide solubility. There are ample data on the 

 effects of such changes in fungicides and bactericides on the partition 

 coefficients between lipides and aqueous components of a mixed sys- 

 tem. Undoubtedly any such change is going to affect the rate and 

 completeness with which a molecule can penetrate cuticularized bar- 

 riers from an aqueous spray dispersion and pass from the interstitial 

 spaces in the cell wall through the lipoprotein barrier of the cell 

 membrane. However, if its lipophylic-hydrophilic balance is not very 

 carefully adjusted, it cannot pass from the lipide phase of the cell 

 membrane into the aqueous substratum of the cytoplasm and thence 

 to the site of enzyme activity, possibly by contact with the lipide phase 

 of mitochondria. It is obvious that the first consideration in evalu- 

 ation of any change in chemical structure on growth-regulant activ- 

 ity must be upon the simple physical attribute of selective solubilities 

 in a complex medium of various lipides and aqueous solutions. 



The potential capacity of the molecule to disperse through tissues 



