94 PLANT GROWTH SUBSTANCES 



and range of responses elicited by plant growth substances appear to be 

 unique. 



With due caution to the definition of effects studied and experimental 

 conditions employed, we now return to the question of how can we 

 describe in biochemical terms the mode of action of growth substances 

 in eUciting particular responses in plants. As Dr. Thimann has pointed 

 out, the minute quantities in which growth substances bring about their 

 most interesting effects immediately suggests that they are not acting 

 as substrates but in all likelihood are serving as cofactors, constituents, 

 or antagonists of enzyme systems. If we accept this point of view we 

 can rephrase our basic query as, how can we describe in biochemical 

 terms the effect of growth substances on the enzymatically catalyzed 

 processes of the cell. This rephrasing does not simpHfy our problem 

 particularly, for it does not define which of the multiplicity of cellular 

 enzymes we must examine for growth substance effects. 



Our reasoning to this point forms a rather logical approach to our 

 problem. But now we are confronted with the necessity of choosing 

 one particular enzyme or group of enzymes and determining whether it 

 can be related to any particular effect of growth substances. Let us, for 

 example, choose herbicidal action as the growth substance effect. What 

 enzymes shall we study? Data in the hterature suggest that herbicides 

 cause the mobilization of carbohydrate in the plant, so we might logically 

 test the influence of various concentrations of herbicides in vitro on 

 amylases. Other reports show an increased percentage of nitrogen in 

 seeds from plants treated with sublethal doses of herbicides. The herbi- 

 cides may influence nitrogen anabolism or catabolism, or the observed 

 effect could merely reflect an enhanced carbohydrate depletion. Enzymes 

 concerned with transamination or reductive amination might logically 

 be examined. A considerable number of reports indicate that herbicidal 

 action is accompanied by increased respiratory activity and carbohydrate 

 depletion. In contrast we find that herbicides in high concentrations may 

 inhibit respiratory enzymes. Hence, we can adopt either of two working 

 hypotheses; first, that herbicides kill a plant by stimulating its carbo- 

 hydrate oxidation to a point where it burns itself out, and second, that a 

 herbicide acts by directly inhibiting respiratory processes. 



These two opposite possibilities reveal another problem which besets 

 the investigator — namely, the type of test material to employ. In general, 

 although there are exceptions to this statement, the concept of herbicidal 



