204 E. W. Yemm 



The rapid formation of amides in ageing tissues may repre- 

 sent a gradual alteration in the balance between synthesis and 

 breakdown. 



There is now much evidence that the proteins and other 

 nitrogenous constituents of plant tissues are maintained in a 

 continuous dynamic equilibrium. The operation of inde- 

 pendently controlled processes of synthesis and breakdown 

 was earlier suggested by Mothes (1933) and by Gregory and 

 Sen (1937), and the use of isotopic nitrogen ( 15 N) has provided 

 direct evidence of the dynamic state of leaf proteins. The 

 experiments of Hevesy and others (1940) and of Vickery, 

 Pucher, Schoenheimer and Rittenberg (1940) demonstrated 

 the incorporation of the isotope into tissue proteins in con- 

 siderably greater quantities than could be accounted for by 

 primary synthesis. More decisive evidence has been obtained 

 from the experiments of Chibnall (Chibnall and Wiltshire, 

 1954) with leaves of runner bean and from our own experi- 

 ment with barley leaves. It has been shown that, even under 

 conditions in which a net loss of protein occurs in detached 

 and senescent leaves, there is appreciable synthetic activity 

 indicated by the active incorporation of 15 N into the tissue 

 protein. 



Some of the results of an experiment, in which a dilute 

 solution of ammonium phosphate, containing about 30 atom 

 per cent excess of 15 N, was supplied to the cut surface of 

 detached barley leaves, are summarized in Table I. The 

 protein separated from the leaves was subjected to group 

 analysis by methods similar to those described by Yemm 

 (1950) and the abundance of 15 N determined in each group. 



Under the conditions of this experiment a net loss of about 

 20 per cent of the tissue protein occurred during the period 

 of 24 hours over which the experiment extended. Neverthe- 

 less, it is evident from the data of Table I that considerable 

 synthetic activity continued in the leaves; a significant 

 incorporation of isotopic nitrogen was detected in each of the 

 main groups of amino acids into which the protein was 

 analysed. The highest abundance of 15 N was observed in the 



