Foliar NPK elicits dose-dependent hormetic responses in biomass, secretory anatomy, phenolic metabolism, and menthol-rich essential oil in Mentha arvensis L.: integrated evidence for balanced nutrition at 1.0%

Mentha arvensis L. is a high-value aromatic crop in which agronomic inputs must be optimized for both biomass and chemical quality. Here, we integrated the dose-response dataset from the supplied manuscript with baseline anatomical, edaphic, elemental, extractive, chromatographic and antioxidant information contained in the accompanying data. The principal experiment used a randomized block design with five foliar NPK concentrations (0, 0.5, 1.0, 1.5 and 2.0% of 18:18:18 NPK), applied at 45 and 60 days after planting and evaluated at 75 days. At 1.0% NPK, plant height, branching, leaf area, fresh weight and dry weight reached 62.4 cm, 21.5 plant⁻¹, 19.8 cm², 92.6 g plant⁻¹ and 21.4 g plant⁻¹, respectively. Total chlorophyll reached 3.24 mg g⁻¹ FW, soluble sugars 42.5 mg GE g⁻¹ FW and soluble protein 34.8 mg BSA equivalents g⁻¹ FW. The same treatment maximized glandular-trichome density (58.2 mm⁻²), trichome diameter (85.6 µm), total phenolics (98.6 mg GAE g⁻¹), total flavonoids (48.4 mg QE g⁻¹), rosmarinic acid (21.2 mg g⁻¹), oil yield (1.25%), menthol (49.2%) and antioxidant performance (DPPH IC₅₀ 18.5 µg mL⁻¹). By contrast, 2.0% NPK increased proline to 7.5 mg g⁻¹ DW and was associated with lower phytochemical and oil quality. The treatment trajectory therefore showed a clear biphasic/hormetic pattern rather than a monotonic fertilizer response. Baseline data characterization independently showed a phenolic-rich hydroalcoholic extract, with rosmarinic acid as the dominant HPLC peak (15.2 mg g⁻¹ extract), and confirmed peltate/capitate glandular structures, nutrient-rich leaf tissue and low detectable toxic-metal burden. Multivariate and regression analyses supported a coupled relationship among balanced nutrition, phenolic accumulation and antioxidant performance. The results identify 1.0% foliar NPK as a strong candidate dose under the tested conditions, while highlighting the need for multi-season validation, nutrient-use-efficiency measurements and molecular confirmation of the proposed phenylpropanoid and monoterpene mechanisms.