Molecular structure and penetration of growth regulators 



One of the more striking manifestations of the anti-auxin reaction is 

 defoliation. A quantity of iV-1-naphthylmaleimide as small as 0-35 gamma 

 per cm''^ leaf area will cause the complete defoliation of peach branches in 

 three days. The leaves drop entirely green, without any evidence of chemical 

 burn. Simultaneous application of the auxin naphthalene acetic acid (at O-I 

 of the concentration of the maleimide) prevents defoliation completely 

 {Figure 3). This again shows the anti-auxin nature of the maleimides. 



Since the maleimides react with SH compounds, and in addition lose their 

 anti-auxin activity when the double bond is saturated, it is reasonably 

 certain that the biochemical activitv of the maleimide molecule resides in the 



CONTROL 

 EMULSION ONLY 



NAR MAL. 



NAP. MAL 

 2XI0-3M 



eXIO'^M + 

 NA2XI0-'*M 



Figure 3. Defoliation of peach branches by naphthylmaleimide. When naphthalene acetic acid was 

 applied simultaneously with the maleimide, the defoliant effect of the latter was prevented. {From 

 van Overbeek et al., 1955, by courtesy of the American Journal oj Botany.) 



maleimide ring rather than in the substituents on the nitrogen atom.There- 

 fore, if there was no penetration factor involved, all maleimides could be 

 expected to give equal defoliation on the peach branches. This is clearly not 

 so. In fact, /^opropylmaleimide was not active at all within the range of 

 concentrations used in these tests [Figure 4). Phenylmaleimide has inter- 

 mediate activity, while 2 :4-dichlorophenylmaleimide has high activity. 

 Naphthylmaleimide has an equally high activity. 



Originally we were inclined to conclude that high anti-auxin activity is 

 obtained when the maleimide possesses an 'auxin ring' from a highly active 

 auxin. We soon learned, however, that no ring is necessary for anti-auxin 

 activity and that the substitution on the nitrogen atom may be an ordinary 

 aliphatic radical. Thus it was found that octylmaleimide has considerable 

 activity [Figure 4). 



It appears, therefore, that the degree of physiological activity of the 

 maleimides is determined by the character of their 'lipophilic tail'. The 

 latter serves to carry the molecule into and perhaps through the lipoidal 

 plasma membrane into other lipophilic bodies of the cytoplasm. The water 

 solubilities of the maleimides [Figure 4) are low in the physiologically active 

 materials and high in the inactive compounds of the group. From this and 



207 



