The effect of timing of nitrogen topdressing to the grain protein content of wheat

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The mechanism by which nitrogen topdressing after anthesis increases the grain protein content of wheat is unknown. We hypothesized that the route of translocation of adsorbed nitrogen to the grain changes from via the leaf to direct to the grain in response to an increase in the grain's capacity as a nitrogen sink. We tested this hypothesis by changing the timing of topdressing of nitrogen from the jointing stage to the ripening period. With topdressing after anthesis, grain protein content was significantly greater than with no topdressing, because nitrogen accumulation in the spikes was greater than the dry matter accumulation at this stage. The rate of translocation of absorbed nitrogen to assimilation organs such as the leaves decreased, and the rate of translocation to the spikes increased, as the time of nitrogen topdressing became later. Nevertheless, at least 40% of the nitrogen absorbed by the wheat plants in the mid-ripening period was immediately translocated to the leaves.