The study looked at how the nitrogen content in soybean leaves and their photosynthetic rate changed over time as new cultivars were developed. They found that newer soybean cultivars had higher nitrogen content in their leaves during certain growth stages, but this didn't always lead to better photosynthetic efficiency. In some stages, newer cultivars had lower nitrogen content but still had higher photosynthetic rates. This suggests that the relationship between nitrogen content and photosynthetic efficiency is complex and can vary depending on the stage of growth. Overall, the study suggests that the nitrogen content in soybean leaves can impact their photosynthetic efficiency, which is important for developing high-yielding cultivars.