Nitrate assimilation and partitioning in detached culms of wheat

Detached wheat culms (Triticum aestivum L. cv. Doublcrop) collected from field-grown plants 16 days after anthesis were cultured for 5 days with nutrient solution containing glutamine (7.5 mM), K15NO3 (15mM), or glutamine (7.5 mM) plus K15NO3 (15mM). Kernel growth rate of cultured culms equaled or exceeded that of intact field-grown plants (1.46 mg kernel-1day-1). Net mobilization of reduced N from the peduncle occurred, but this accounted for less than 23% of the net increase in grain N after 5th day of culture, regardless of N source. Glutamine had no effect on the assimilation of (15N) nitrate or the partitioning of reduced (15N)-labelled products. Peduncles contained less than 4-8% of total reduced (15N) excess recovered in all plant parts. Grain accumulation of reduced (15N) was linear, as was solution depletion, and accounted for an increasing proportion of the total reduced (15N) excess in all culms parts. These results provide direct evidence for nitrate assimilation by spike components. (author)