作物の窒素代謝に関する研究 : 第19報 水稲の Source から Sink への N の転流と蓄積機構について

抄録

Protein-N in rice grains is known to be derived mainly from the leaf among the vegetative plant parts during grain development. However, in the previous report, we revealed that top-dressed N during grain filling increased the protein content of grains. The tracer experiment using ¹⁵N labelled ammonium nitrogen in the rice plant also made clear that top-dressed ¹⁵N was more effectively translocated into grain protein as compared with basal-dressed N. The present study was conducted with an attempt to investigate the translocation of ¹⁴N derived from basal-dressing or ¹⁵N from top-dressing into developing grains. For this purpose, the individual leaf blades as a nitrogen-source organ or ears as a sink were removed at the flowering stage, respectively. The results obtained are as follows: 1. As for ¹⁴N- or ¹⁵N-distribution of various plant parts, there were significant differences in the contents of ¹⁴N and ¹⁵N between the leaf positions on the culms. Particularly, ¹⁵N content in the upper leaf blades was much higher than the lower ones, and the reverse was true in the lower leaf blades. Therefore, ¹⁵N/¹⁴N ratio in the upper leaf blades was higher than the lower ones. 2. Experiments in which individual leaf blades were removed at the flowering stage showed that a decreased level of source-N in the remained leaf blades and leaf sheath + culms corresponded well to a decrease in the level of sink-N, indicating that source-N in the leaf sheath + culms was mobilized by the leaf removal treatments. Furthermore, it was found that from changes in ¹⁵N/¹⁴N ratio of the various plant parts ¹⁵N in the upper leaf blades was more rapidly transported into sink than ¹⁴N, while ¹⁴N-transport in the lower leaf blades occurred in advance of ¹⁵N-transport during the early ripening period. 3. These results suggested that ¹⁵N-compounds in the leaf blades derived from top-dressed ¹⁵N were more mobile than ¹⁴N-compounds from basal-dressed ¹⁴N, probably the former contributing to soluble protein such as Fraction-1 protein, and the latter to insoluble protein such as cell membrane protein. 4. During the ripening period, there were rapid decreases in the free amino acids as well as total nitrogen contents in the leaf blades. Among the free amino acids, decreases in methionine level well correlated with decreases in total nitrogen level of the leaf blades. Furthermore, it was suggested from the removal experiment of ears as sink that glutamine and γ-aminobutyric acid were the main compounds for transport of nitrogen from source to sink during the grain filling period.