The partitioning of N during the grain‐filling period of maize (Zea mays L.) was studied under field conditions at differing levels of fertilizer. Two hybrids with differing leaf‐canopy senescence, B73Ht × LHI05 (rapid) and B73Ht × Va35 (slow), were grown with two rates (67 and 257 kg ha−1). Separate plants were supplied with 15N the 14‐leaf stage (VI4), antbesis, or 22 d after anthesis, and the subsequent patterns of 15N partitioning were followed. Independent of time of application, hybrid, or N fertility, ⊄45% of the 15N applied was absorbed within 3 d. Three days after the V14 treatment, the stalk contained 50% of the absorbed 15N, suggesting that it acts as a major sink for N during vegetative growth. With later applications, the 15N was predominantly partitioned to the ear. Although absorption of total N from soil continued during the grain‐filling period, there was significant remobilization of N from vegetative organs (60 to 85% of the total N present at anthesis) to the ear. Leaves and stalk each contributed 45% of total N remobilized into the ear, and 10% was contributed by the roots. Loss of N from the stalk was initiated before anthesis, whereas it began around anthesis with leaves and roots. The results indicate the important role of the stalk in providing N for kernel development. Both N fertility and hybrid influenced the rate of N remobilization from vegetative tissues.

During ear development in maize (Zea mays L.), nitrogenous compounds are translocated from vegetative organs to the kernels. At anthesis, the stalk contains approximately 40% of the total plant N, and contributes 45% of the N remobilized to the ear. Therefore, the stalk has an important function as a temporary reservoir for N. Little is known of the metabolism of maize stalks, and this paper describes initial studies of enzymes of N metabolism. High in vitro activity of glutamine synthetase (GS) in maize stalk samples throughout ear development contrasted with a peak in activity of glutamate synthase soon after anthesis and negligible nitrate reductase. With fresh sections of stalk tissue collected at anthesis, '5N-feeding experiments confirmed high GS and low nitrate reductase activities. Two isoforms of GS were separated from extracts from stalk tissue: GS1, the cytoplasmic form, increased to maximum levels at 2 weeks postanthesis and remained fairly high thereafter; whereas the plastidic form, GS2, declined progressively during kemel development. Westem blot analysis confirmed the presence of constantly high levels of GS protein after anthesis. The levels of GS proteins decreased after transfer of N-starved, hydroponically grown plants to N-rich conditions in order to restrict remobilization of N. In contrast, transfer of plants grown under abundant N conditions to N-free medium, which encourages N remobilization, resulted in a relative increase in GS protein. Because glutamine is the major form of N transported in maize, the results indicate that GS, specifically the GS1 isoform, has a central role in the remobilization on nitrogenous compounds from the stalk to the ear.