The researchers studied how different amounts of nitrate affected the growth and nitrate levels in three leafy vegetables. They found that the best plant growth was at a medium nitrate level, with too much nitrate causing a decrease in growth. Nitrate levels increased in the plants with higher nitrate supply, except in the roots. The activity of nitrate reductase, an enzyme involved in nitrate metabolism, was highest at a specific nitrate level. The researchers also confirmed a method for measuring nitrate reductase activity.