Study on the Regulation of Nitrogen Fertilizer on the Physicochemical Properties and Metabolic Characteristics of Starch in Waxy and Non-Waxy Proso Millet

Proso millet (Panicum miliaceum L.) has the characteristics of being drought-resistant and tolerant to poor soil conditions and a high nitrogen (N) utilization rate. It is an ideal crop for studying stress tolerance and nitrogen utilization. This research examined the regulatory effects of different N fertilizer treatments (0, 75, 150, 225, 300 kg·ha−1) on starch physicochemical properties and metabolic characteristics of waxy (Wutai Red proso millet, P1) and non-waxy proso millet (Ningmi No. 9, P2). The results showed that peak yields for P1 and P2 occurred at N applications of 225 kg·ha−1 and 150 kg·ha−1, respectively. As the amount of N applied increased, the proportion of long chains in P1 amylopectin first increased and then decreased, while P2 continued to rise. As the amount of N applied increased, the peak viscosity of P1 gradually decreased, while P2 showed a trend of first decreasing and then increasing. Metabolomics identified 814 metabolites including nonivamide in P1 and P2 under different N treatments. Under the suitable N fertilizer treatment, 130 metabolites, including myristoleic acid, arachidonic acid, and thromboxane B2, were identified in P1, and 98 metabolites, such as trigonelline, 16-hydroxyhexadecanoic acid, and p-anisaldehyde, were identified in P2. Suitable N regulated P1′s starch physicochemical properties via the tricarboxylic acid cycle, glyoxylate and dicarboxylate, and purine and one-carbon metabolism pathways, and P2′s via purine metabolism pathways. This research provides a theoretical basis for the efficient utilization of N fertilizer in proso millet and the cultivation of high-yield and high-quality millet.