Chemical Nature of Growth Regulators 163 



Perhaps the best known example of an auxin synergist is 2,3,5- 

 triiodobenzoic acid or TIBA (XXIII). Using an Avena test with a 

 small amount of indoleacetic acid incorporated with each application 

 of TIBA, Galston (1947) showed that this substance will inhibit 

 growth. Such inhibitory effects on growth can be seen in figure 12. A. 

 The fact that TIBA can also stimulate growth in the presence of 

 auxin was demonstrated by Thimann and Bonner (1948). They found 

 that if very small amounts of TIBA were incorporated with auxin 

 growth was stimulated (figure 72 B). By testing various molar ratios 

 of TIBA to indoleacetic acid, they found that this synergistic stimu- 

 lation of growtli was greatest at a molar ratio of approximately 1.0. 

 When TIBA was present in excess it inhibited growth, and this was 

 apparently the type of inhibition which was measured by Galston 

 previously. 



A simple means of testing for synergistic action with ai-.xin is to 

 hold the auxin level constant over a rather wide range of concentra- 

 tions of the suspected synergist. Results of such a test should show a 

 stimulation with low concentrations of the synergist, and probably 

 inhibition at higher concentrations as shown in figure 73. Of course a 

 parallel test of the suspected synergist alone should be run to establish 

 that the growth effect is not due to auxin activity. 



Compounds other than TIBA which have been found to be auxin 

 synergists include unsaturated lactones such as coumarin and proto- 

 anemonin (Thimann and Bonner, 1949), dicarboxylic acids such as 

 chelidonic acid (Leopold el al, 1952) and phthalamic acid derivatives 

 (Leopold, unpublished), indole (van Raalte, 1951), and some vitamins 

 such as ascorbic acid, niacin, pantothenic acid, p-aminobenzoic acid 

 and pyridoxine (Scheuermann, 1951; Leopold, unpublished). Also the 

 antibiotic bacitracin has been found to be a synergist with auxin 

 (Leopold and Guernsey, unpuljlished). Some examples of these are 

 included in figure 73. 



From the observations summarized here, it is clear that a large 

 variety of compounds with diverse molecular constitutions may act as 

 auxin synergists in growth. It is not clear through what phase of 

 growth they exert their effects, but some evidence is available to indi- 

 cate that three of them (protoanemonin, coumarin and chelidonic 

 acid) may act competitively against sulfhydryl compounds (Thimann 

 and Bonner, 1949; Leopold et al, 1952). 



Using a somewhat different testing method, Went (1949) has 

 described a group of compounds which increase growth in response to 

 auxins. To these compounds he has given the name, "hemi-auxin." 

 Application of these compounds to slit pea sections prior to placing 



