ON FORM AND FUNCTION OF PLANT GROWTH 



SUBSTANCES 



H. Veldstra 



Research Laboratory, Combined Quinine Works, Amsterdam 



From the literature on the subject of our present discussion it is evident that 

 the analysis of structure-activity relationships with plant growth substances 

 continues to fascinate both chemists and plant physiologists. This is not to 

 be wondered at, as there exist hardly any types of naturally occurring, 

 physiologically active substances which have so simple a structure as indole- 

 acetic acid and the features of which, essential for activity, can be incorporated 

 in such a variety of compounds. 



It may be useful, however, to see where we actually are at the moment and 

 to ask what one may expect from this work as to the solution of the primary 

 problem: where do these simple substances act and what is the essence of 

 their function ? 



As to the active agents, in our opinion, we are fairly well informed about 

 the requirements for activity and are able to discern in the chemically 

 different compounds a common active principle (cf. Veldstra, 1953). 



We must confess that in recent years we have on several occasions had the 

 impression that the synthetic work would be unlikely to provide further 

 essential information, but as frequently we were intrigued by an unexpected 

 activity or lack of activity and found ourselves again on the synthetic trail, 

 the course of which will be reported upon in this paper. 



Moreover, very interesting results obtained by other investigators clearly 

 indicate that the last word with regard to the structure (form) of the active 

 compounds has certainly not yet been said. 



Thus Jonsson (1955), in an elegant way, recently analysed in more detail 

 the 'active' position of the side chain by comparing a large number of 

 compounds. His conclusion that the formation of a pseudo-ring with the 

 COOH group near to the centre of the extended ring-system formed seems to 

 be essential, would imply that the position of the carboxyl group in all of the 

 active acids is the same as that in the active dior//?o-substituted benzoic acids 

 (cf. Veldstra, 1952). 



A real surprise in this structure-activity domain was the announcement 

 of plant growth regulating acdvity of certain dithiocarbamates by 

 van der Kerk et al. (1955) as this constitutes the first example of a 'ring-less' 

 growth substance. This most interesting work will be referred to below. 

 Apparently we have not yet exhausted all possible structures active as growth 

 substances. 



One has to be aware of the fact, however, that, fascinating as this work 

 may be, it will not afford us the answer to the primary question of the mode 

 of action. We know a lot about the active compounds, while the growth 



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