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Rice

Temporal Origin of Nitrogen in the Grain of Tropical Wet-Season Rice

^a IRRI, DAPO 7777, Metro Manila, Philippines
^b Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE 68583
^c Dep. of Animal and Plant Sciences, Univ. of Sheffield, Sheffield S10 2TN, UK
^d Dep. of Agronomy, Univ. of the Philippines, Los Baños, Philippines

^* Corresponding author (j.sheehy{at}cgiar.org)

Received for publication March 25, 2004. 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 ¹⁵N nitrate (NH₄¹⁵NO₃) 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 ¹⁵N 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 urea applied separately at 35 DAT showed that 21 and 58% of the ¹⁵N nitrate and ¹⁵N urea, respectively, were recovered. There were no advantages of using ¹⁵N nitrate as opposed to ¹⁵N urea as a label in such research of irrigated rice.

Abbreviations: DAT, days after transplanting • E(x/y), the ratio of the ¹⁵N content of a rice hill at maturity to the maximum amount recovered, following an application of ¹⁵N • IRRI, International Rice Research Institute • N_c, the N content of a rice hill • N_g, the N content of the grain • R_c, recovery of ¹⁵N by a rice hill