Temporal origin of nitrogen in the grain of irrigated rice in the dry season: the outcome of uptake, cycling, senescence and competition studied using a \u3csup\u3e15\u3c/sup\u3eN-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 grain filling. The main objective of the work described in this paper 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. The total nitrogen content of the crop was measured by growth analysis throughout its duration and the measurements used to calculate the rate of total nitrogen uptake. Using 15N as a tracer enabled the fate of nitrogen taken up at any time to be determined. In the short-term, the labeled nitrogen was distributed between the various plant organs depending on their demand for nitrogen during the period of absorption. In the long-term, transfers of 15N occurred between organs, in particular to the developing panicle (rice inflorescence). The rate of nitrogen absorption of the panicle exceeded the rate of absorption by the whole plant from 68 DAT onwards. Surprisingly, in the context of rice as an annual plant, the distribution patterns suggested that towards maturity, the perennial nature of the rice plant led to competition for nitrogen between the panicles 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% of the total nitrogen in the grain was acquired before panicle initiation (45 days 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