THE METABOLISM OF SENESCENT LEAVES 



E. W. Yemm 



Department of Botany, University of Bristol 



Introduction 



An important feature of the metabolism of leaves and 

 other plant organs during their senescence is the predomin- 

 ance of catabolic processes in which many of the complex 

 constituents, such as proteins and polysaccharides, are broken 

 down to simpler soluble products. Yellowing of the leaves, 

 associated with the loss of chlorophyll, is an obvious external 

 sympton of ageing which usually precedes abscission and leaf 

 fall. The products of catabolism, consisting of amino acids, 

 amides and other soluble metabolites, are to a large extent 

 translocated to other parts of the plant, where they may be 

 stored as reserve substances in seeds or other perennating 

 structures. 



In the account which follows, chief attention will be given 

 to the changes of tissue proteins and other nitrogenous 

 constituents and their relation to the respiratory activities 

 of senescent leaves. Within the cell, proteins are important 

 components of the protoplasmic structure and their break- 

 down during senescence is therefore of particular interest. 

 Moreover, there is now much evidence which suggests that 

 in plant tissues the proteins are maintained by continuous 

 synthetic activity, which is closely coupled with respiratory 

 oxidations. 



The Catabolism of Proteins in Senescent Leaves 



It is well established that yellowing in old leaves is accom- 

 panied by a marked breakdown of cytoplasmic and chloro- 

 plastic proteins of the cell. For example, Michael (1935) 



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