Temporal Origin of Nitrogen in the Grain of Tropical Wet-Season Rice
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The total N in the grain is the integral of the product of the total N absorbed at any instant and the fraction of that N eventually allocated to the grain. We investigated the temporal origin of N in the grain of a wet season rice crop and tested the suitability of N-15 nitrate ((NH4NO3)-N-15) as a label for that purpose. The total N content of rice (Oryza sativa L.) plants was measured by growth analysis throughout the duration of the crop and the measurements were used to calculate the rate of total N uptake. A point-placement technique was used to deliver small amounts of N-15 nitrate to roots of the rice plant and this enabled the eventual fate of the total N absorbed at any time to be determined. The rate at which N was acquired by the panicle exceeded that by the whole plant at 64 d after transplanting (DAT); thereafter, N was transferred from the leaves to the panicle. About 60% of N in the grain was acquired before panicle initiation and was transferred from leaves during grain filling. A comparison between the uptake and retention of labeled nitrate and area applied separately at 35 DAT showed that 21 and 58% of the N-15 nitrate and N-15 urea, respectively, were recovered. There were no advantages of using N-15 nitrate as opposed to N-15 urea as a label in such research of irrigated rice.
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