Labeled‐Nitrate Assimilation and Nitrogen‐15 Export from Leaves of Burley Tobacco

At harvest, burley tobacco ( Nicotiana tabacum L.) leaves frequently contain undesirably high levels of NO 3 , which are usually greater in older leaves lower in the canopy than in the younger leaves of the upper canopy. Substantial NO 3 uptake occurs during the 4‐ wk period between topping (apical bud removal) and harvest. To evaluate N use characteristics of leaves differing in stalk position during this period, the 4th or 10th leaf from the top of field‐grown plants received 15 N‐NO 3 solution (99 atom % 15 N). The 15 N‐NO 3 was fed to one half of the leaf through a lateral vein in the middle of the leaf. Plants were sampled 1, 3, and 7 d later and separated by leaf position for N analysis. Xylem exudate was collected from the base of the plant at 3 and 7 d after labeling. The 15 N‐NO 3 was readily reduced at both leaf positions. Reduced 15 N accumulation by the uppermost three leaves and stalk was greater in plants fed 15 N‐NO 3 at the 10th leaf than at the fourth leaf position; however, the amount accumulated was small relative to the amount of reduced 15 N retained in labeled leaves. Reduced N in the xylem exudate of plants fed 15 N at either leaf position was enriched to 0.16 and 0.078 atom % 15 N excess at 3 and 7 d, respectively. These results indicate that, after topping, both upper‐ and mid‐canopy leaves had similar NO 3 ‐reduction capacities and a portion of the newly reduced N was cycled through the root system. Differences in leaf NO 3 accumulation may be related to greater N supply and uptake during early plant growth rather than to differences in NO 3 assimilation potential of leaves late in the season.