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Abstract
Root‐to‐shoot and shoot‐to‐root translocation of N was studied in pea (Pisum sativum L.), maize (Zea mays L.), and sunflower (Helianthus annuus L.) plants. These species were chosen, because they show, in that order, an increasing contribution of the shoots to the total NO3‐ reduction in the whole plant. The plants were allowed to take up about equal amounts of NO3”; from half‐strength Hoagland solutions and the fate of absorbed NO3”; was followed in its assimilation, and accumulation of NO3”; and organic N in shoots and roots was recorded. Distribution of in vivo nitrate reductase activity (NRA) over the plant organs and organic:total N ratios in the xylem bleeding saps were determined to delineate flow sheets for the fate of absorbed NO3‐, including downward transport of organic N.
The main conclusions were: total accumulation of organic N in shoots and roots was similar for the 3 plant species, but the distribution of NO3‐ over those organs was very different; in each of the 3 species a considerable shoot‐to‐root translocation of organic N occurred; phloem transport of organic N was more than adequate to supply the requirements of the roots for growth; both phloem and xylem flux of organic N were smaller but phloem:xylem flux ratios increased as the shoots contributed more to the total NO3”; reduction in the whole plant.