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Nitrogen (N) is usually the limiting nutrient for yield of wheat (Triticum aestivum L.). However, spring and winter wheats in the Great Plains respond slightly to N during late developmental stages, suggesting that there is little requirement. Research was conducted to determine the effect of N nutrition during maturation on photosynthesis, the main source of assimilates for growth of grain, and to ascertain why wheat responds only slightly to late applications of N. The cultivar 'Len' was grown in complete hydroponic solution in a greenhouse until the jointing stage, when partial solutions containing 0, 0.15, 1.5, or 15 mM NaNO 3 plus all other nutrients were supplied until the plants matured. Plant growth, photosynthetic rate, and related traits were measured from 5 to 35 d after anthesis and at JOURNAL maturity. High rates of N markedly increased grain, leaf, and stem biomass of the plants as they matured. Photosynthetic rate, chlorophyll and N contents, and area index of leaves were increased simultaneously by N. Stomatal conductance differed greatly during late stages of maturation, when plants grown with low levels of N senesced and plants grown with high levels continued to accumulate assimilates. It was concluded that N greatly influences photosynthetic processes and grain growth during maturation of wheat, but responses in the field might be limited by remobilization of stored N and early senescence of plants from hot, dry conditions.

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