172 Fundamentals of Auxin Action 



and Leopold, unpublished; Bonner and Bandurski, 1952); hence these 

 substances cannot be classified as anti-auxins in this specific sense. It is 

 not certain that they inhibit growth by a direct competition with 

 auxin for the same points of attachment. 



Maleic Hydrazide. This interesting compound (XXVII) was first 

 reported as a growth inhibitor with the remarkable property of retard- 

 ing overall plant growth without causing epinastic or other obvious 

 morphological abnormalities (Schoene and Hoffmann, 1949). The fact 

 that it broke apical dominance as well as inhibited growth suggested 

 that it might lower auxin effectiveness. Experiments indicate that it 

 inhibits growth in standard auxin assays, and that its inhibition effects 

 can be removed by the presence of additional auxin. Conversely, the 

 inhibition effects of excess auxin can be reversed by addition of maleic 

 hydrazide as shown in figure 77 (Leopold and Klein, 1952). This 

 property of antagonizing auxins has been utilized for preventing the 

 auxin stimulation of respiration and rij^ening of fruits (Smock et al. 

 1951), and in development of spray technitjiies for breaking apical 

 dominance in floral crops (Beach and Leopold, 195.1). The reduction 

 of auxin effectiveness by maleic hydrazitle may be explained in part by 

 the finding that it can bring about increased enzymatic destruction 

 of indoleacetic acid (Andreae and Andreae, 1953). A synergistic effect 

 with auxin has also been reported (Gautheret, 1952). The rather 

 startling effects of maleic hydrazide on dormancy are described in 

 chapter XV. It also finds some use as an herbicide, and has been shown 

 to alter flowering and growth in many plants (Zuckel, 1952; 1953). 



0' 



Ff >COOH 

 NO, 



XXVIII 



2,3,5-Triiodobenzoic acid (XXIII) was first shown to inhibit 

 growth in the presence of auxins by Galston (1947). Its synergistic 

 effects with auxins have been discussed in the preceding section, A 

 variety of halogenated benzoic acid derivatives have been found which 

 relieve the auxin inhibition of root growth (Minarik et al, 1951); the 

 most active of these is 3-nitro-4-fluorobenzoic acid (XXVIII). 2,3,5-tri- 

 iodobenzoic acid is the most active of the benzoic series tested for 

 causing abscission (Weintraub et al, 1952), a property which may be 

 a consequence of an antagonism with auxin. It has also been found 



