Translocation of Nitrogen and Carbon from Leaves to Roots of Different Nodes in Rice Plants-A Double Labeling Study with 15N and 13C

Abstract

The translocation of nitrogen and carbon from leaves to roots of the rice plant at vegetative stage was investigated using 15N and 13C as a tracer, emphasis being placed on the node position of the roots. The whole shoot of the plant at the stage of developing the 12th leaf (12L) and the 9th nodal roots (9nR) was fed with 13C-labeled CO2 gas for 60 min after being sprayed with 15N-labeled urea solution, and the fate of 15N and 13C in the plant was followed over 12 days. A rapid transfer of 15N and 13C from the expanded leaves (3-11L) took place with-in 3 days and 1 day respectively, then a gradual transfer followed during the chase period. 15N and 13C were translocated to all parts of the plant, preferentially to the expanding leaf (12L) and the root system. Among the roots the upper roots (9nR) were the largest sink of 15N and 13C expected by the expanded laves. The lower roots (≤5nR, 6nR and 7nR) received substantial amount of 15N as well as 13C from the leaves, although they did not increase their N and C contents. Within 1 day when both translocated 15N and 13C in each nodal root increased, the ratio of 13C gain to 15N gain was the highest in the upper roots (9nR). The ratio decreased as the node position of the roots lowered. Whereas the ratio of 13C loss to 15N loss in the expanded leaves within 1 day was higher in the upper leaves (9-11L) than in the lower leaves (3-8L). The results indicate that not only the young upper roots but also the old lower roots are the sinks of the nitrogenous compounds translocated with the photosynthetic assimilates from the expanded leaves. The difference in the 13C/15N ratio among the nodal roots suggests that the C/N ratio of the foliar products imported by the roots varies with their node positions; lower roots receive the products containing more rich in N relative to C as compared with upper roots. Each leaf of different nodes seems to play a specific role to supply the root system with the products of varietical C/N ratio; upper leaves supply the products of higher C/N ratio mainly to upper roots, while lower leaves feed the products of lower C/N ratio to lower roots.