MALEIC HYDRAZIDE 329 



strongly retained by the tissues and is translocated relatively well, moving 

 freely across mesophyll and along phloem. Indeed, since it is transferred 

 from phloem to xylem so easily, possibly it circulates in plants. Crafts 

 (1961 b) thinks that maleic hydrazide may move along the cell walls of 

 meristematic tissue, as do some dyes, and presents radioautographic evi- 

 dence for this. If maleic hydrazide is sprayed onto Bermuda grass shoots 

 at 45 mM in solutions of different pH, the maximal growth inhibition oc- 

 curs at pH 7, relatively little effect being seen above pH 9 or below pH 5, 

 which is difficult to explain satisfactorily on the basis of the ionization of 

 maleic hydrazide. Although the bulk of the absorbed maleic hydrazide 

 may be free to move, the bound fraction is quite stable. Callaghan and 

 Grun (1961) exposed root tips to 1 mM maleic hydrazide-C^^ for 1 hr 

 and found the initial fixation (first 24 hr) to be mainly nucleolar, but that 

 gradually a shift to the nuclei occurred, so that at 3 weeks the nuclei were 

 well labeled, and Baker (1961) observed that tobacco seedlings treated 

 with maleic hydrazide-C^^ retain radioactivity in the meristematic regions, 

 the inhibitor being very tightly bound to proteins. Although certain bac- 

 teria can utilize maleic hydrazide, its metabolism in plants has not been 

 studied. Parups et al. (1962) examined the activities of the postulated 

 metabolic products of maleic hydrazide on bud formation — maleic dia- 

 mide, fumaric diamide, maleamate, and maleate — and found that none 

 is nearly as inhibitory as the parent compound. 



Inhibition of Enzymes and Metabolism 



Relatively little work on the effects of maleic hydrazide on enzymes 

 has been done and many important SH enzymes have not been examined, 

 as may be seen in the summary presented in Table 2-7. Furthermore, the 

 results on certain important enzymes, such as succinate oxidase and in- 

 doleacetate oxidase, are inconsistent. The inhibition of phosphorylase is 

 not reversed by glutathione but dialysis seems to restore the reacted SH 

 groups in /^-amylase (Hughes and Spragg, 1958). Reactivation of enzymes 

 by thiols would not be expected, unless the maleic hydrazide readily 

 dissociates from the enzyme. Nothing is known about the equilibria of 

 the addition reactions of thiols to maleic derivatives. Baker (1961) found 

 that a diaphorase and a nitrate reductase are fairly well inhibited by maleic 

 hydrazide — in fact, to about the same extent as the respiration of tobacco 

 tissues — and postulated that maleic hydrazide might interfere in electron 

 transport systems involving vitamin K. 



The pattern of maleic hydrazide action is in some ways similar to that 

 of X-radiation, and since X-radiation causes destruction of auxin, Leopold 

 and Klein (1951) considered the possibility that maleic hydrazide acts 

 as an antiauxin. They showed in the slit pea test that maleic hydrazide 

 inhibits at low auxin concentrations but not when the auxin concentration 



