Abstract
The nitrogen transported from the roots to the shoots of the young soybean plant fed with K 15NO3 for 12 hours was shown to be made up of two components, the newly absorbed nitrogen (15N) and the "old" nitrogen (14N) which had been originated from the nitrogenous constituents of the roots. The mean proportion of 15N and 14N was approximately equal. But the nitrogen reaching the apical growing region was found to contain a relatively higher proportion of 14N to 15N. The "ringing" experiment, in which a portion of the petiolar phloem tissue of each leaf was blocked by heat, revealed that at least the most recently matured leaves were supplied with nitrogen containing both 15N and 14N during the experiment. The leaf proteins were found to be in a state of turnover. By assuming that both "new" and "old" nitrogen components could be incorporated into leaf proteins at an equal rate, the protein turnover rate was calculated for the second trifoliate leaves (the youngest mature leaves used)as 0.54-0.90 per cent per hour. N-deficient treatment for 36 hours prior to 15N supply caused a protein degradation and a subsequent accumulation of soluble nitrogenous compounds in the "ringed" leaves. Whether this soluble-14N pool could be reutilized for protein synthesis following the application of 15N to the roots was tested. Although the amount of 15N incorporated into proteins was reduced to some extent in the "ringed" leaves, the ratio of protein-15N to soluble-15N was not much affected as compared with the control leaves. From this result it has been suggested that the nitrogen of the newly synthesized proteins may come largely from the nitrogen source delivered newly to the leaf, and that the nitrogen released from protein decomposition may not be reutilized efficiently within the same leaf.
