Respiratory Changes in Relation to Toxicity 



FREDERICK G. SMITH 



GROWTH substances cause a variety of toxic effects in plants which 

 often involve inhibition or alteration of respiratory metabolism. 

 The nature of these effects is important not only as a clue to the role of 

 growth substances in plant metaboHsm but also as a basis for their more 

 successful application as herbicides. The distinction between the normal 

 or physiological effects of growth substances and the abnormal or non- 

 physiological effects is not a sharp one, and for the present purpose it is 

 useful to consider nonphysiological or toxic action in broad terms as any 

 alteration in metabolic processes which is deleterious to plant function. 

 We must include, then, the response to intermediate levels of growth 

 substances ranging from those characteristic of physiological hormone 

 action to those used in herbicide work where toxic action may become 

 less specific and too complex for present interpretation in metabolic 

 terms. 



The object of the present discussion is to summarize the evidence 

 relating toxicity and respiratory changes and to consider its interpreta- 

 tion in terms of present views of growth substance action and plant 

 metabolism. The data are necessarily drawn from many types of experi- 

 ments often not primarily designed to study toxic action and the results 

 can be described only in broad terms. Data are confined to the effects 

 of indoleacetic acid (lAA) and 2,4-dichlorophenoxyacetic acid (2,4-D), 

 the chief natural and artificial growth substances used in this kind of 

 experimental work. 



Respiratory Changes in 

 Elongation, Streaming, and Water Uptake 



Much of the work on the nature of hormone action has shown a 

 close relation between cell elongation, protoplasmic streaming, and water 



