The study looked at how different nitrogen sources affect the amount and distribution of nitrogen and nicotine in tobacco plants. They used a special isotope to track the nitrogen. The results showed that as the plants grew, the nitrogen content decreased in different parts of the plant, while the nicotine content increased. Most of the nitrogen in the plants came from the soil, with only a third from fertilizer. The upper leaves had the highest amount of nitrogen and nicotine, with over half coming from the soil. Overall, the leaves contained most of the nicotine, with the upper leaves having the most.