Role of Auxins in Control of Leaf Senescence 337 



These experiments are still in their early stages. The method of 

 applying 2,4-D to a leaf is providing us with a very valuable way of 

 studying the effects of 2,4-D on groups of cells in situ, where they are 

 surrounded by tissues in which the applied auxin is either absent or 

 in a relatively very low concentration. 



SUMMARY 



There seems little doubt that the presence of a relatively high con- 

 centration of 2,4-D in isolated groups of cells in Euonymus leaf tissue 

 can maintain within these cells an abnormally high rate of respiration. 

 The evidence from autoradiograms of Ci^-labeled leaves has shown 

 that carbon compounds either move into or are preferentially re- 

 tained in the areas of high oxygen consumption below the spot of 

 2,4-D. The values from the nitrogen determinations indicate that 

 nitrogenous compoimds are accumulated within the 2,4-D-treated 

 areas. These treated cells therefore comprise a specialized part of the 

 leaf in which there is a high rate of metabolism and no net protein 

 breakdown; they seem to act as metabolic sinks to which nitrogen and 

 possibly carbon materials are drawn from the surrounding cells, with 

 the result that there is a premature senescence in the untreated parts 

 of the leaf which contain only a relatively low concentration of the 

 applied auxin. The maintenance of differential metabolic rates 

 within a leaf by local variations in the auxin concentration could be a 

 controlling factor in determining the movement of metabolites within 

 the blade and thereby determining the differential states of senescence 

 of the cells. The roles of kinetin, benzimidazole, 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. 



ACKNOWLEDGMENTS 



We are indebted to Mr. R. G. Powell of the Agricultural Research 

 Council Unit of Experimental Agronomy, Oxford, for devising the 

 simple method for supplying the attached Euonymus leaves with C^^Oo 

 and to Dr. D. C. Smith for his advice with nitrogen determinations. 

 We also wish to thank Prof. G. E. Blackman for his constant interest 

 and encouragement. 



LITERATURE CITED 



1. Brian, P. W., Petty, J. H. P., and Richmond, P. T. Effects of gibberellic acid 

 on development of autumn colour and leaf -fall of deciduous woody plants. 

 Nature. 183: 58-59. 1959. 

 ^2. Osborne, D. J. Control of leaf senescence by auxins. Nature. 183: 1459, 1460. 

 1959. 



