In situ Nitrate Assimilation in Winter Wheat: Peduncle Injection with Nitrogen‐15‐Nitrate at Anthesis 1

Nitrogen accumulation and assimilation following anthesis in winter wheat ( Triticum aestivum L.) depends on the level of soil nitrate and the availability of carbon substrates to support plant transport and assimilation functions. Spike and peduncle nitrate assimilation was investigated by injecting at anthesis 100 µ mol 15 N‐nitrate (99 atom % 15 N) into the hollow peduncles of field‐grown (Typic Paleudalf soil) winter wheat (‘Doublecrop’). Dry weight changes in vegetative and reproductive culm parts and N assimilation and partitioning were followed at 5‐day intervals until maturity, 40 days after anthesis (DAA). Between 10 and 25 DAA, accumulation of spike dry matter and N increased to 56 mg day −1 and 79 µ mol day −1 . Over 80% of the injected 15 N‐nitrate was absorbed through the peduncle cavity walls within 5 DAA. Differential utilization of endogenous nitrate and injected 15 N‐nitrate or dilution of the injected 15 N‐nitrate by soil derived nitrate was observed. Peduncle nitrate atom % 15 N decreased from 90.6 (0 DAA) to 54.4 (10 DAA). Within 5 DAA, 50% of the recovered excess 15 N was partitioned to the spike. In contrast, 20 DAA elapsed before 50% of the total spike N accumulated after anthesis was acquired. Only a small portion of the total excess 15 N in the culm was recovered in the flag leaf (3%) and culm below the peduncle (2%). Collectively, these results indicate that the rapidly absorbed 15 N‐nitrate was not incorporated uniformly with peduncle N, but the absorbed 15 N was readily transported to the spike. Although the relative contribution of nonleaf components to assimilate soil‐derived nitrate absorbed after anthesis is not indicated, it is clear that the spike and peduncle have a considerable in situ capacity to assimilate nitrate immediately following anthesis.