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

USDA-ARS and Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091

^* Corresponding author.

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.

Received for publication July 16, 1990.