208 E. W. Yemm 



availability of free phosphate and/or phosphate acceptors, both 

 of them essential components of the enzymic systems engaged 

 in glycolysis and oxidation. The general importance of phos- 

 phorylation in regulating cell respiration in plants has been 

 discussed by Lardy (1952), by Millerd and Bonner (1953) and, 

 with particular reference to protein metabolism, by Folkes and 

 Yemm (1954) and Yemm (1954). 



Evidence that the level of phosphorylation may regulate 

 the rate of respiration in plant cells rests mainly upon the 

 action of so-called uncoupling agents, such as dinitrophenols. 

 As reviewed by Simon (1953), these reagents at low concen- 

 trations strongly inhibit phosphorylation, whereas enzymatic 

 oxidations are but little affected: under suitable conditions 

 synthetic activities, particularly protein synthesis, are pre- 

 vented, while a marked increase in cellular respiration gener- 

 ally occurs. Thus treatment of the tissues with nitrophenols 

 and other uncoupling agents produces experimentally changes 

 in metabolism similar to those associated with senescence. 

 Observations such as these have led to the view that ageing in 

 plant tissues is associated with a partial failure of the linkage 

 between phosphorylation and cell oxidations. As a result of this 

 failure, the regulating action on cell respiration is impaired 

 and there is a decline in the synthetic activities whereby 

 the proteins and other complex constituents of the cell are 

 maintained. 



Pearson and Robertson (1954) have reviewed some evidence 

 that changes, such as those outlined above, occur during the 

 ripening of apples. They have shown that the relatively low 

 rate of cell respiration during the pre-ripening stages could 

 be substantially increased by 2 : 4-dinitrophenol, but that 

 after the climacteric rise and yellowing similar treatment had 

 little effect. Some further evidence that changes of the phos- 

 phorylating mechanisms were associated with ripening was 

 obtained by Biale (1954) from a study of cytoplasmic particles, 

 probably mitochondria, prepared from the avocado fruit. 

 But, as yet, no comparable data have been obtained concern- 

 ing the metabolism of senescent leaves. 



