The researchers studied how different types of nitrogen in the soil affect the activity of nitrate reductase in apple seedling roots. They found that nitrate is the best nitrogen source for promoting nitrate reductase activity, while ammonium has a negative impact on the enzyme's function. When both nitrate and ammonium are present, the enzyme activity decreases significantly compared to when only nitrate is available. This suggests that the type of nitrogen in the soil can influence how well plants can use nitrate for growth.