Abstract
The change of nitrogen (N) uptake after topdressing was estimated from the N concentration in bleeding sap and bleeding rate of the sap in a lowland rice variety, Koshihikari, grown in a pot and in the field. The N concentration began to increase 6 h after topdressing with ammonium sulfate, reached the maximum at 24 h, and decreased thereafter. It took more than 6 h for the onset of increase in N uptake, which was probably the time required for distribution of ammonium into the soil and movement into the xylem vessel. The N concentration in bleeding sap decreased to the same level as the control at 168 h after topdressing in the field experiment. This suggests that the uptake of top-dressed N by the rice plants completed in 7 days. Approximately 55 % of top-dressed N was utilized by the rice plants in those 7 days. The onset of increase in bleeding rate was more than 12 h later than that in the N concentration of the bleeding sap in both experiments. This time lag suggests that the bleeding rate was promoted not by a simple increase of N compounds in the xylem sap but through an increase of physiological activity in roots caused by the topdressing. The SPAD values of leaf color responded to the topdressing later (i.e., 48 h after topdressing) than the bleeding rate. The analysis of N in bleeding sap offered an effective method to illustrate the dynamics of top-dressed N in detail.
