214 Discussion 



viral infections and pathological conditions which leads to the gradual 

 decline in the vigour of the plants. I think that many plants can 

 reproduce themselves vegetatively for a very long time without showing 

 "senescence" in the clone as a whole. 



Rowlands: What happens in, say, the coniferous trees — does the cycle 

 of yellowing go on in the pine needle ? 



Yemm: I do not think it has ever been examined experimentally. But 

 I think there is no doubt there is an extensive breakdown of protein 

 materials before the pine needles drop because of the extremely low 

 protein and nitrogen content of pine needles after leaf fall. 



Huggett: I was interested in your finding that you have some synthesis 

 accompanying senescence breakdown, as shown by your 15 N data, be- 

 cause that fits in with some observations which I have not reported in 

 which 14 C-labelled glucose incorporates into the rabbit placental glycogen, 

 not only before the peak period but in the period after the peak period 

 when it is normally disappearing. Therefore, senescence really appears to 

 be a balance of synthesis and catabolism and, as you pointed out earlier, 

 that balance is changing. The only thing I would like to know arising 

 out of that is : are there any new synthetic mechanisms occurring during 

 senescence that are not present in full adulthood or young life — in other 

 words, replacement mechanisms that are distinctive of old age ? 



Yemm: Nothing comes to mind. Certain syntheses become more 

 obvious, but whether it is a result of a change in the whole balance of 

 metabolism one does not know. The one I was mentioning — glutamine 

 — now this is a synthesis which becomes much more conspicuous in 

 older leaves, but in our view the increase of glutamine may result from 

 an alteration in the balance between synthesis and breakdown of protein 

 as the leaf ages. 



Huggett: Are you entirely happy that oxidative metabolism rises with 

 yellowing? It looked to me from the trace of curves you showed that 

 there are several other confusing factors that did not seem to be 

 correlated ? 



Yemm: All I would say, in general, is that there is very strong evidence 

 that the rate of cellular respiration is high and probably rising in 

 senescence. The point is this, that I think we cannot explain the decline 

 in synthetic activity as being due to a loss in respiratory or oxidative 

 capacity of the tissue ; the changes are, if anything, the reverse. It seems 

 necessary to postulate some change in the efficiency of the coupling 

 between respiration and cellular metabolism. 



