The researchers studied how proteins and three enzymes related to nitrogen metabolism change in different parts of poplar trees. They found that protein levels in bark, xylem, and roots decreased as the trees sprouted. Nitrate reductase activity peaked in leaves and roots in early September and mid-October, then decreased during leaf loss. Glutamine synthetase activity had two peaks in leaves, one in May and one in October, while in roots it peaked during sprouting. Endopeptidase activity in roots increased as vegetative storage proteins broke down during sprouting and continued to rise during leaf loss. There was a clear negative relationship between total protein levels and endopeptidase activity in roots.