Seasonal Variation in Leaf Mineral Nutrients and the Determination of the Nutritional Diagnostic Period of Paeonia ostii

Paeonia ostii, a significant perennial woody oil crop in China, is notable for its seeds’ high oil content and elevated levels of unsaturated fatty acids. However, there is currently a lack of scientific fertilisation protocols and targeted nutrient management for P. ostii. The concentrations of macronutrients (N, P, K, Ca, and Mg) and micronutrients (Fe, Mn, Zn, and Cu) were determined in the leaves at five distinct growth stages: flowering, initial fruit set, fruit expansion, late fruiting, and foliar senescence. The levels of N and P were found to be at their highest point during the flowering stage, after which they declined significantly. In contrast, the levels of K remained relatively stable throughout the growth phase, while Mg levels increased significantly to peak at fruit expansion. The level of Ca increased, reaching its peak at the late fruiting stage. The annual average content of micronutrients in P. ostii leaves was as follows: Fe > Mn > Zn > Cu. Furthermore, it was observed that the concentrations of Fe and Mn oscillated, while the concentration of Cu decreased significantly after flowering. Additionally, Zn concentrations remained stable throughout the various stages. Multivariate analyses, including PCA, nutrient ratio analysis, and an integrated nutrient stability index, further revealed coordinated shifts in leaf nutrient composition and indicated that May and June were relatively stable periods for nutrient assessment. Considering both the nutrient stability and the phenological relevance, June, corresponding to the fruit expansion stage, was considered a practical sampling window for foliar nutrient diagnosis. These findings contribute to the definition of an appropriate sampling window for foliar nutrient diagnosis, thereby providing a useful basis for nutrient monitoring and future fertilisation studies in P. ostii.