336 



D. J. Osborne and M. Hallaway 



Table 1 . Values for the respiration rate and for the nitrogen fractions of detached 

 Euonymus leaves 19 days after treatment with 2,4-D or ethanol. 



gen oi the treated spots. There is no evidence of a net hydrolysis of 

 protein in the 2,4-D-treated spots, and the increase in total nitrogen 

 in these areas can be accounted for by an increase in soluble nitrogen 

 which must have been accumulated from the untreated parts of the 

 blade. This confirms previous results obtained with our detached 

 cherry leaves (2). 



DISCUSSION 



It is tempting to speculate if the maintenance of differential rates 

 of metabolism within a leaf by local variations in the auxin con- 

 centration could be a controlling factor in determining the move- 

 ment of metabolites within the blade and thereby determining the 

 tUiferential states of senescence of the cells. The roles of kinetin, ben- 

 zimidazole, and gibberellin in controlling leaf senescence might also 

 be due, in part, to an effect upon the accumulation of metabolites in 

 treated parts of the blade. 



The mechanism for the stimulation of oxygen uptake in the 2,4-D- 

 treated tissues remains to be investigated more fully. Since these tis- 

 sues retain a photosynthetically active chlorophyll (2) and lose neither 

 total nor protein nitrogen, in spite of the considerable changes which 

 occur in these constituents in the surrounding cells, it might be 

 that 2,4-D is acting in a manner analogous to that suggested for the 

 thyroid hormone in animal tissues (4) by a stimulation of the basic 

 metabolic rate of the cells. 



