Effects of Nitrogen Fertilizer Spraying Time on Source–Sink Nitrogen Metabolism and Seed Oil Quality of Paeonia ostii ‘Fengdan’

The spraying time of nitrogen fertilizer is a key factor to consider when fertilizing with an intelligent micro-sprinkler irrigation system. This study aims to investigate the impact of nitrogen fertilizer spraying time on the seed oil quality of tree peony, with the expectation of providing theoretical support for the application of intelligent micro-sprinkler irrigation systems in the production of tree peony. In 2022 and 2023, foliar nitrogen application was conducted on Paeonia ostii ‘Fengdan’ utilizing an intelligent micro-spray irrigation system, with four distinct nitrogen fertilizer spraying times (3:00–4:00, 7:00–8:00, 14:00–15:00, and 19:00–20:00). Based on this, the study assessed nitrogen metabolism indicators in leaves and seeds at various growth stages and the fatty acid composition of seed oil in Paeonia ostii ‘Fengdan’. The results revealed that foliar nitrogen application between 14:00 and 15:00 significantly enhanced the levels of free amino acids (FAA), nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthase (GOGAT) activity in both leaves and seeds. Furthermore, the ratio of α-linolenic acid in the seed oil was significantly increased. Correlation analysis demonstrated a positive or highly significant positive correlation between the levels of nitrogen metabolism indicators and the ratio of unsaturated fatty acids. In conclusion, foliar nitrogen application between 14:00 and 15:00 significantly enhances the FAA content and the activity of nitrogen metabolism enzymes within the leaves and seeds and promotes the synthesis of unsaturated fatty acids in seed oil. This study contributes to the efficient and high-quality cultivation of tree peony.