212 Discussion 



diverted from its normal course and consequently plastid structures and 

 pigments in the cells tend to break down, so that yellowing is here a 

 pathological effect resulting from the diversion of protein metabolism 

 rather than extensive catabolism of protein. 



Villee: I would like to congratulate Dr. Yemm on his work; it is 

 very elegant and extremely interesting. I was interested in the com- 

 parisons which can be drawn between protein metabolism in animal 

 tissues and plant tissues. I noted from your 15 N data that the picture is 

 quite similar in the two, that the highest rate of incorporation was into 

 aspartic and glutamic acids, other than the amide N, and that the basic 

 amino acid lysine had a very low rate, suggesting that, as in animals, 

 lysine probably does not exchange its nitrogen. You stated that gluta- 

 mine accumulates rapidly, more than can be accounted for by that 

 present in the protein ; is that as glutamine or as glutamine plus glutamic 

 acid? 



Yemm: As glutamine alone. Practically the whole of the soluble 

 glutamic acid in these tissues is combined as the amide. 



Villee: I see. It would suggest a de novo synthesis of glutamic acid 

 and the formation of the amine from that. 



Yemm: Yes. The deviation of these data depends upon a knowledge 

 of the composition of the leaf proteins. We have separated tissue pro- 

 teins from the leaf and estimated their average composition in terms of 

 amino acids. We then measured how much protein disappeared over a 

 given period of time and also the amount of glutamine that accumulated. 

 When the two are compared, it is found that glutamine accumulates in 

 much greater quantities than can be accounted for simply by hydrolysis 

 of the proteins. A similar result was obtained for the other common 

 amide of plants, asparagine. 



Villee: This, of course, is quite understandable biochemically because 

 of the amination of the corresponding keto acids. Now, I was also 

 interested in the increase in respiration which occurs. Since we know 

 that substituted phenols are accelerators of respiration, I wonder 

 whether the breakdown of pigments such as anthocyanins might in some 

 way release either nitrophenols or chlorophenols which could be a 

 stimulating factor at this time. The coincidence of this with the yellow- 

 ing of the leaf suggests that perhaps in the yellowing, some of the pig- 

 ments themselves break down and release physiologically active 

 substances. 



Yemm: I have no comment on it. I think that it is difficult to account 

 for its occurrence in different types of leaf, very different in composition 

 and in their anthocyanin pigments ; yet the acceleration of respiration 

 seems to be a general phenomenon in senescent leaves and is closely 

 correlated with yellowing and with protein metabolism. 



Villee: Have you made any estimates of phosphorus and oxygen 

 ratios to see whether there is, in fact, a change in this ratio which would 

 be the crucial point as to whether phosphorylation has altered later on ? 



Yemm: I can only say that we have tried it without success. The 

 situation in plants, I think, is here perhaps a little more complicated 

 than in animals. It seems that there are, in all probability, a large 



