332 2. MALE ATE 



is increased. The antagonism of maleic hydrazide by auxin was also dem- 

 onstrated in the Avena coleoptile test. Waggoner and Dimond (1953) 

 assumed that a lowering of the auxin level by maleic hydrazide should 

 lead to a suppression of the growth of crown gall due to Agrobacterium 

 tumefaciens, and found this to occur without direct effect on the pathogen. 

 Andreae and Andreae (1953) examined the oxidation of indoleacetate in 

 a complex system from germinating peas and observed that 1.5 mM 

 maleic hydrazide markedly stimulates this oxidation. It was postulated 

 that maleic hydrazide might produce some of its growth inhibition by fa- 

 cilitating the destruction of auxin in the tissues. However, subsequent study 

 of the effects of maleic hydrazide on indoleacetate oxidation has not uni- 

 formly supported this theory, and in some cases indoleacetate oxidase is 

 inhibited (Table 2-7). These recent results along with actions to be discus- 

 sed presently make it unlikely that this is an important mechanism. 



Effects on Plant Metabolism 



The possibility that the growth inhibition exerted by maleic hydrazide 

 is related to an inhibition of respiration was examined by Naylor and Davis 

 (1951) in a variety of roots. The mean results from the seven species are 

 shown in the accompanying tabulation. That the inhibition is greater at 



the lower pH is reasonable, since at pH 4 maleic hydrazide is mainly un- 

 ionized, but the reason for the stimulation usually seen at pH 6 is difficult 

 to explain. Naylor and Davis felt that the respiratory depression might 

 well have something to do with the growth inhibition. The respiration 

 of onion bulbs obtained from plants sprayed with maleic hydrazide is stim- 

 ulated by low concentrations (around 4.4 mM) and increasingly inhibited 

 up to 35 mM (Isenberg et al., 1954), and this was also taken as evidence 

 that respiratory inhibition is of some significance. The respiration of to- 

 bacco leaves is inhibited 10% by 3.3 mM and 44% by 27 mM maleic hy- 

 drazide (Callaghan and Van Norman, 1956), and the respiration of tobacco 

 seedlings is similarly reduced, the inhibition progressively increasing with 

 infiltration time up to 4 hr (Baker, 1961). On the other hand, the respi- 

 ration of wheat seedlings is increased 20-50% by application of maleic 



