DAPHNE J. OSBORNE 



Department of Agriculture, Oxford 



MARY HALLAWAY 



Department of Botany, Oxford 



The Rote of Auxins In the Control of Leaf 



Senescence. Some Effects of Local Applications 



of 2 A-Oichlorophenoxy acetic Acid on Carbon 



and Nitrogen Metabolism. 



It has long been recognized that senescence in detached leaves is re- 

 tarded when root initials are formed along the petiole. The green 

 color is then retained, and net protein syntheses and the accumula- 

 tion of metabolites occur as long as the blade remains attached to 

 living roots. If the roots are removed, the processes of senescence 

 again take place. It has now been shown that the aging processes in 

 detached leaves can be arrested by treatment with certain chemicals 

 and that the presence of roots is therefore not essential for preventing 

 senescence of the blade. For example, Richmond and Lang (5) found 

 that chlorophyll and protein breakdown in detached Xanthium leaves 

 was retarded by treatment with kinetin. Person, Samborski, and For- 

 syth (3) showed similar effects in detached wheat leaves following 

 treatments with benzimidazole, and Brian, Petty, and Richmond (1) 

 reported that both autumnal yellowing and subsequent leaf fall could 

 be retarded in a number of deciduous species by spraying gibberellic 

 acid on the leafy branches. More recent work (2) has shown that suit- 

 able applications of both 2,4-dichloro- and 2,4,5-trichlorophenoxy- 

 acetic acids (2,4-D and 2,4,5-T) are also effective in delaying certain 

 degradative processes of leaf aging, and it is clear that the role of 

 auxins in the control of leaf senescence must also be considered. 



The surface treatment of both attached and detached autumn 

 leaves of Prunus serrulata (2) revealed that droplet applications of 

 the butyl ester of 2,4-D (10 to 100 ^ag.) in ethanol, would result in a 

 retention of green and photosynthetically active chlorophyll below the 

 spot, while the rest of the blade became yellow and senescent. Under 

 these conditions, no roots were formed and there was no apparent 

 cell enlargement or cell division. Autoradiograms of leaves treated 



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