Changes in Vertical Distribution of Leaf Nitrogen with the Growth Stage and the Influence on Dry Matter Production in Rice

The change in the vertical distribution of leaf N concentration from the vegetative period to maturity were examined to evaluate the effect of the leaf N gradient on dry matter production in rice (Oryza sativa L.). Cultivar ‘Nipponbare’ was grown in culture solutions at two N levels in 1993 and three N levels in 1994. The samples were taken three times in 1993 and six times in 1994 by a stratified clipping method, and leaf area, leaf mass, leaf N concentration on a dry weight basis (NW) and on a leaf area basis (NA) were determined for each stratum. During the active tillering stage, both NW and NA showed a steep vertical gradient with a high N concentration in the upper strata, and the gradient was steeper in a low N solution. The distributions became fairly uniform during the reproductive phase (from panicle initiation to heading), even though leaf area index (LAI) was as high as 6. After heading, NA showed a steep vertical gradient with a concave shape that was similar to the light distribution, mostly because of the presence of thicker leaves in the upper strata. The gradient was the largest at the later stages of grain filling, in which NA of the uppermost leaves was higher than the average by as much as 40%. Nevertheless, the simulated advantage of biomass production due to the non-uniform NA distribution during the ripening period was less than 8% compared with the uniform N distribution in the canopy. The NW profile did not agree with the light distribution : It was convex and conserved over the sampling periods when expressed in relative values. The rate of NW decrease in all the strata after heading could be expressed as a linear function of the initial level of NW around heading, so that NW in the upper leaves decreased faster