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Uptake, Distribution, and Remobilization of ¹⁵N-labeled Urea Applied to Maize Canopies¹

Field-grown maize (Zea mays L.) plants (hybrid B73 X Mo17) were sprayed with ¹⁵N-enriched urea to determine recovery and subsequent distribution within the plant. Plants were sprayed with ¹⁵N enriched urea at 7 days pre- (treatment 1) and post-anthesis (treatment 2) to provide a total of 22.3 kg N ha^–1 (350 mg N plant^–1). Unsprayed plants were used as controls. Plots were located on Flannigan silt loam (fine, montmorillontic mesic Aquic Argiudoll) at stands equivalent to 63 800 plants ha^–1. Timing of spray treatment had no effect on grain yield, stover weight, visible leaf senescence, or N content of the plant parts, relative to controls. About one-third (29% in treatment 1, 30% in treatment 2) of the foliarly applied urea was absorbed and incorporated by the plant. The amounts and distribution patterns within plant parts of foliarly applied N and of total N (N derived from soil uptake and foliarly applied urea N) indicated that leaves were the organs of uptake, and that the stalk served as a conduit for passage of urea-derived N from leaves to the ear rather than as a storage organ for foliarly applied N. The greater initial rates of loss and the final extent of depletion of N from leaves was greater for foliarly applied N than for total N. This indicated that the foliarly applied N was incorporated into a different pool of N than had been formed earlier by unenriched soil derived N. This observation provides possible explanation for the failure of foliar N applications to increase grain yields of maize.

Key Words: Zea mays L. • Yield • Senescene • N content

² Graduate research assistant (now research associate, Univ. of Illinois), graduate research assistant (now plant physiologist (USDA), Univ. of Kentucky, Lexington, KY 40546), plant physiologist (USDA), and professor of agronomy, Univ. of Illinois, Urbana, IL 61801.

Received for publication July 25, 1984.

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