Studies on Metabolism of Soybean Leaves : 1. Changes in Nitrogen Content of the Leaves as affected by girdling their Petioles

Abstract

Two experiments have been made to examine the changes in nitrogen content that proceed in soybean leaves following the application of a girdle to their petioles. Girdling was performed by killing a portion of a petiole with heat given out by a small flame. In both experiments, fully developed leaves of a definite position (i.e. sixth leaves for Experiment I and primary leaves for Experiment II) were taken as samples for analysis. Varieties used were Aomame in the former, and Shirobanasai No.1 in the latter experiment. Although the conditions involved in both experiments differed each other in many respects, the resulfs obtained showed a marked similarity and will be described as follows. In the petiole-girdled leaves (this will be reffered to as treated leaves), the amount of total nitrogen invariably increased in excess of that of the control leaves whether the control leaves, during the same experimental period, were in a tendency towards either gaining or losing nitrogen. The excess was statistically significant in Experiment II. As the protein nitrogen content of the treated leaves changes in much the same manner as that of control leaves, the difference in the total nitrogen content between the control and the treated leaves is principally due to a striking accumulation of soluble nitrogen in the latter leaves. These results have led to the idea that, at least in a fully developed leaf, the transport of nitrogenous compounds either to or from the leaf, possibly through xylem and phloem respectively, would take place simultaneously and that the nitrogen levels of the leaf could depend upon the ratios of entering rate to leaving rate of nitrogen. The changes in composition of free amino acids in the treated leaves have been briefiy examined by one dimensional paper chromatography. A marked increase in certain amino acids which were hardlv recognized in the control leaves has been clearly demonstrated, suggesting that inward and outward currents, by which nitrogenous compounds are being carried to and away from a leaf, may be different in terms of amino acid composition. The occurrence of those additional free amino acids in the treated leaves might have been, at least in part, resulted from the breakdown of the leaf proteins.