The study used a special nitrogen marker to see how tobacco plants absorb and distribute nitrogen from fertilizer. They found that nitrogen accumulation in tobacco plants increased during the growing season, with most nitrogen ending up in the leaves. The roots also kept absorbing nitrogen over time. The plants absorbed more nitrate nitrogen than ammonium nitrogen from the fertilizer. The ability of tobacco plants to accumulate nitrogen was highest 40-60 days after planting and decreased later on. The plants started losing some of the fertilizer nitrogen towards the end of the growing season, even though they could still get nitrogen from the soil.