The article explains how plants use nitrogen from nitrate to make amino acids in both light and darkness. By using 15N tracing experiments, scientists discovered the specific enzymes involved in this process, such as nitrate and nitrite reductases, glutamine synthetase, and asparagine synthetase. They found that nitrogen assimilation in leaves and roots can happen in different ways, depending on the enzymes present in each part. The reactions also require electron donor systems that use carbohydrates as an energy source.