The study looked at how different levels of nitrogen affected the growth and photosynthesis of rice seedlings. They found that high nitrogen supply led to increased leaf nitrogen content and photosynthetic rate, but also decreased nitrogen use efficiency. This decrease was due to insufficient CO2 supply under high nitrogen levels, which affected the activity of a key enzyme involved in photosynthesis. Overall, the results suggest that too much nitrogen can actually hinder the plant's ability to efficiently use nitrogen for growth and photosynthesis.