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

Abstract

At harvest, burley tobacco (Nicotiana tabacum L.) leaves frequently contain undesirably high levels of NO3, which are usually greater in older leaves lower in the canopy than in the younger leaves of the upper canopy. Substantial NO3 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 15N‐NO3 solution (99 atom % 15N). The 15N‐NO3 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 15N‐NO3 was readily reduced at both leaf positions. Reduced 15N accumulation by the uppermost three leaves and stalk was greater in plants fed 15N‐NO3 at the 10th leaf than at the fourth leaf position; however, the amount accumulated was small relative to the amount of reduced 15N retained in labeled leaves. Reduced N in the xylem exudate of plants fed 15N at either leaf position was enriched to 0.16 and 0.078 atom % 15N excess at 3 and 7 d, respectively. These results indicate that, after topping, both upper‐ and mid‐canopy leaves had similar NO3‐reduction capacities and a portion of the newly reduced N was cycled through the root system. Differences in leaf NO3 accumulation may be related to greater N supply and uptake during early plant growth rather than to differences in NO3 assimilation potential of leaves late in the season.