Abstract
Petiolar tissues of mature leaves of soybean plants were killed by heat and the subsequent changes in total nitrogen and free amino acids content of the petiole-treated leaves were compared with those of the petiole-untreated leaves. During the experimental period there was a consistently higher accumulation of total nitrogen in the petiole-treated leaves than in the petiole-untreated leaves. When topping treatment (a removal of a growing region of the stem) was made to the plants, however, an accumulation of total nitrogen occurred in both the petiole-treated and petiole-untreated leaves to almost the same extent. These observations suggest that the nitrogen content of a mature leaf is maintained by a counterbalance of import and export of nitrogen and that the topping treatment reduces export without seriously affecting import. Similar results were obtained with total free amino acids, indicating that amino acids may be continuously translocated from the mature (pre-senescent) leaves and the translocation of amino acids could also be reduced by the treatment. From these results it may be concluded that source and sink relationship in respect to nitrogen metabolism may generally exist between mature leaves and a growing stem apex.
