Nitrogen: uptake, distribution and use of N for grain estimated using a point-placement technique

It is often suggested that nitrogen absorbed in the vegetative stage of growth acts as a 'reservoir' to supply the shortfall in demand during the grain filling. The main objective of the work was to investigate how effectively nitrogen absorbed at different stages of the growing season was retained and used for grain growth. The total nitrogen in the grain is the integral of the product of the total nitrogen absorbed at any instant and the eventual allocation of a fraction of that nitrogen to the grain. A point-placement technique was used to deliver small amounts of 15N to the roots of the rice plant at different growth stages and the total nitrogen content of the crop was measured throughout its duration. Using 15N as a tracer enabled the fate of nitrogen absorbed at any time to be determined. In the short term, the labeled nitrogen was distributed between the various organs, depending on their demand for nitrogen during the period of absorption. In the longer term, transfers of 15N occurred between organs, in particular to the developing spikelets. Surprisingly, in the context of rice as an annual plant, the distribution patterns suggested that toward maturity, the perrenial nature of the rice plant led to the competition for nitrogen between the spikelets and the next generation of developing tillers. The results showed that the total nitrogen absorption by the plant was high when the fractional allocation to the grain was low and vice versa. About 30 percent of the total nitrogen in the grain was acquired before panicle initiation (45 d after transplanting, [DAT]) and the leaves acted as the main 'reservoir' for nitrogen. Losses of labeled nitrogen acquired by the plants after 35 DAT were not significant, suggesting that there was no large loss of nitrogen through volatilization, once the nitrogen had been incorporated in the plant biomass.