Effect of Planting Density on Absorption, Distribution and Utilization of ¹⁵N by Flue-cured Tobacco

Characteristics of nitrogen absorption, utilization and distribution as well as effect of planting density, of flue-cured tobacco were studied using a ¹⁵N tracer. The experiment employed K326 (a cultivar of Nicotiana tabacum) for the cultivation with 3 planting densities, i.e., D₁ (16 592 plants·hm^-2), D₂ (18 182 plants·hm^-2) and D₃ (20 202 plants·hm^-2).The results showed that nitrogen derived from fertilizer(Ndff, %) in different parts of a plant was in the order of lower leaves > middle leaves > stem > upper leaves > roots at round top stage. It was indicated that lower and middle leaves had higher ability to absorb and transport nitrogen fertilizer. However, with the development and maturity of tobacco plants, the ability of various parts to absorb and transport nitrogen fertilizer was gradually reduced.Planting density had significant effect on the Ndff ratio in different parts of the tobacco plant. Increment of planting density increased the proportion of Ndff in all parts at round top stage. However, in the middle and late period, with the increase of planting density, the Ndff ratio of tobacco plants decreased significantly, indicating that when the planting density was too large, it was not conducive to maintain the ability to absorb nitrogen in the late growth stage of tobacco plants.The ¹⁵N distribution rate of different parts was in the order of lower leaves > middle leaves > upper leaves > stem > roots at round top stage. On average, 76.95% of ¹⁵N were shown in leaves, 15.36% in the stems, and 7.68% in the roots. Harvesting the lower and middle leaves decreased the proportion of ¹⁵N in the leaves while increased those in the stems and roots. Comparatively, the ¹⁵N distribution to the roots and stem was higher than to the leaves under D₂ resulting in a stronger plants.The nitrogen utilization efficiency (NUE) of flue-cured tobacco decreased as the plant aged. NUE of plants grown under D₂ was the highest in various periods among all treatments, but the difference between D₂ and D₃ was not significant in the late growth period. As the planting density was raised, the tobacco leaf yield increased, but the percentage of high-grade leaves decreased significantly. The output value per unit area of D₂ was the highest, followed by D₃ and D₁.