The Metabolism of Senescent Leaves 203 



studied the changes of chlorophyll and protein content in 

 leaves of nasturtium (Tropaeolum majus) and demonstrated 

 a close correlation between the amounts of chlorophyll and 

 protein over a wide range of conditions. Most of the detailed 

 information concerning the biochemical mechanism underlying 

 protein catabolism in senescent leaves has been obtained with 

 leaves detached from the plant. Under these conditions many 

 of the changes associated with normal ageing are greatly 

 accelerated, so that after relatively short periods of time 

 yellowing and the breakdown of protein can readily be 

 detected. Experiments with detached leaves of tobacco 

 (Vickery, Pucher, Wakman and Leavenworth, 1937), various 

 species of grass (Wood and Cruickshank, 1944) and barley 

 (Yemm, 1937, 1950) have shown consistently that appreciable 

 protein catabolism occurs within a few hours of detaching 

 the leaf. Moreover, in old leaves the breakdown is relatively 

 independent of the conditions under which the leaves are 

 kept ; it occurs in the dark and in the light and is not prevented 

 when sugars are artificially supplied to the tissues. Both cyto- 

 plasmic and chloroplastic proteins are involved, although in 

 some species there is evidence that the plastid proteins may 

 be at first more rapidly attacked. Investigations of the pro- 

 ducts of protein breakdown, which are formed in the leaves, 

 strongly suggest that the catabolic changes extend beyond 

 simple hydrolysis of the proteins. In barley leaves, for ex- 

 ample, it has been found that the amount of the amide, 

 glutamine, which accumulates in freshly detached leaves 

 commonly exceeds the amount which could arise directly 

 by proteolysis, thus indicating that amino acids are rapidly 

 drawn into secondary changes. It has been further shown that 

 the extent of these secondary changes increases with ageing 

 of the leaf; much greater quantities of glutamine accumulate 

 when old as distinct from young leaves are detached from 

 the growing plant and treated under comparable conditions 

 (Yemm, 1949). Many features of the metabolism of amides, and 

 in particular glutamine, indicate that they play an important 

 part in both the synthesis and breakdown of protein in plants. 



