The Metabolism of Senescent Leaves 207 



the high rates of respiration in the ripening fruits are associ- 

 ated with the breakdown of starch to sugars, although a slight 

 synthesis of protein can be detected during yellowing. 



The experimental data briefly considered above indicate 

 that the catabolic processes in senescent plants are commonly 

 associated with high rates of cellular respiration. It is appro- 

 priate now to try and distinguish some of the basic changes in 

 cellular metabolism which accompany ageing of plant tissues, 

 and to bring the data into relation with some current views on 

 the regulation of respiration and its coupling with synthetic 

 activities. 



The Relation between Cell Respiration and Protein 

 Metabolism in Ageing Tissues 



Gregory (1937), Richards (1936) and their collaborators 

 have demonstrated a close connection between respiration and 

 protein metabolism in leaves of barley. In a tentative hypo- 

 thesis Gregory and Sen (1937) suggested that a continuous 

 protein cycle operates in leaves and that under normal cir- 

 cumstances much of the respiratory C0 2 arises from the oxida- 

 tion of the carbon skeletons of amino acids. The hypothesis 

 has been further developed by Steward and his collaborators 

 (see, for example, Steward and Thompson, 1954), and, along 

 these lines, the rapid respiration of senescent leaves may be 

 related to the high levels of available substrate, resulting from 

 the breakdown of tissue proteins to amino acids. 



There are, however, other cellular mechanisms which may 

 link respiration and protein metabolism and which seem to 

 offer a more comprehensive interpretation of the catabolic 

 changes in ageing leaves. It is now widely recognised that 

 phosphorylation and phosphate carrier systems constitute 

 important mechanisms regulating glycolysis and respiration 

 and coupling them with endergonic synthesis. Experimental 

 work with both animal and plant tissues indicates that the 

 rate of respiration may be strongly influenced by the 



