908 ADVENTURES IN RADIOISOTOPE RESEARCH 



The dry weight of the young leaves investigated amounted to 17.4 gm, that 

 of the old leaves to 11.4 gm. The nitrogen content of the different fractions is 

 seen in Table 1. 



The dry substance of the three different types of leaves was found to contain 

 4.28, 1.80, and 0.73 per cent nitrogen, respectively. 



The result of the mass-spectrographic investigation is seen in Table 2, 



CONCLUSIONS 



As already mentioned, the "old" leaves did not grow further while 

 kept in the culture solution containing heavy nitrogen. The protein 

 i^N content of these leaves is, thus, due solely to an exchange process, 

 involving the enzymatic splitting off of the nitrogen content of a part 

 of the protein molecules present and their replacement by an enzymatic 

 restitution with incorporation of ^^N atoms. As seen in Table 2, the 

 extent of replacement amounted, in the course of 8 days, to 12.3 per 

 cent of the total protein nitrogen present in the old leaves. We arrived 

 at this figure by comparing the ^'"^N excess of the old leaves with that 

 of the new leaves under the assumption that the new leaves were exclu- 

 sively formed while the plant was kept in the culture solution containing 

 heavy nitrogen. As to the intermediary leaves, the assumption that 

 these leaves were exclusively grown in the last 12 days of the experiment 

 is certainly not justified and the corresponding figure, quoted in the 

 last column of Table 2, represents therefore an upper limit to the re- 

 placement. As already mentioned on p. 906 the growth of these leaves 

 amounted certainly to less than i/o of their original size; the amount 

 of i^N incorporated into the proteins of these leaves must, therefore, 

 have been appreciably larger than that incorporated into the old leaves. 

 This result suggests a faster rate of protein replacement in growing than 

 in fully grown leaves. The strong enzymatic action going on in growing 

 leaves leads presumably to an enhanced disintegration and consecutive 

 rebuilding of protein molecules under incorporation of i^N. 



The metabolism of nitrogen in the leaves of the buckwheat plant was 

 investigated by a similar method as desribed above by Vickery, Pucher, 

 ScHOENHEiMER and RiTTENBERG (1939). In these experiments, the 

 plants were treated for 47 hours only with a culture solution in which 

 the ordinary ammonium chloride was replaced by heavy nitrogen. 

 Disregarding the growth of the leaves during that interval of 47 hours, 

 they conclude from the fact that the "heavy" nitrogen was diluted 

 only 17 times when it was introduced into the leaf protein, that no less 

 than 6 per cent of the protein nitrogen must have undergone replace- 

 ment by interaction with the simpler nitrogenous substances of the cells 

 which were ammonia and amide nitrogen of augmented isotopic ratio. 

 The protein N replacement of 6 per cent in the course of 2 days is com- 



