Nitrogen absorption rate at different growth stages in relation to grain production of lowland rice (Oryza sativa L.)

The accumulation-efflux patterns of leaf nitrogen suggest that a large fraction of nitrogen captured by the plant is conserved and cycled for use in actively growing tissues. The process of accumulation is important because the nitrogen content of the rice crop at panicle formation has been shown to be an important determinant of yield. The rate at which the developing panicle acquires nitrogen probably exceeds the rate at which the roots can absorb it during grain filling. The first hypothesis was that in high yielding rice, the vegetative parts of the crop act as a nitrogen reservoir during grain growth. In rice, sink size is directly proportional to spikelet number. The maximum number of juvenile spikelets on a developing panicle is observed at the late differentiation stage. Resource availability, in particular the nitrogen content of the plant at the late differentiation stage, influences actual spikelet number, but when nitrogen should be applied to maximize yield in low input systems is not very clear. The second hyphothesis was that potential and actual sink size are strongly influenced by the timing of nitrogen fertilizer application. Field experiments were conducted at the International Rice Research Institute (IRRI) in dry and wet seasons to test the above hypothesis. To test the first hypothesis, the contribution of nitrogen absorbed at different stages of growtn made to the final nitrogen content of the grain in irrigated rice was measured using a quantity of 15 N small enough to be absorbed fully over a period of hours. About 50% of nitrogen in the grain was translocated from N stored in vegetative organs. These studies validated the first hypothesis that nitrogen captured during vegetative stage of growth forms a reservoir that is available to meet the deficit that occurs when the demands for nitrogen during the grain filling stage exceed the supply capacity of the roots. The second hypothesis was tested in an experiment using a constant amount of nitrogen applied in four treatments: (1) no nitrogen, (2) basal and then uniformly throughout the growing season, (3) at and after panicle initiation, and (4) at and after flowering. Greatest spikelet number and yield were obtained in treatment 2. It was concluded that nitrogen taken up early is important for grain formation