Field-Based Climate Resolution Reveals Seasonal Drivers of Essential Oil Productivity and Antioxidant Functionality in Melaleuca bracteata: Implications for Harvest Optimisation

Essential oils from aromatic plants are gaining traction as naturally derived preservative and antioxidant ingredients, yet the quantitative relationships between field climate conditions and both oil yield and food relevant bioactivity remain poorly characterised. Here, we characterised the leaf essential oil of Melaleuca bracteata F. Muell. “Revolution Gold” across a complete annual cycle using a fixed plant, multiscale spatio temporal sampling framework. Leaf samples were collected at four seasonal time points (March, June, September, and December) and four diurnal time points (06:00, 12:00, 16:00, and 21:00) from a single field individual in Meizhou, Guangdong, China. Gas chromatography–mass spectrometry (GC-MS) profiling identified 51 volatile constituents, with methyl eugenol dominating the composition (up to 93.67% in summer). Oil yield peaked in summer (2.43 mL kg−1 dry weight) and was lowest in spring (1.28 mL kg−1 dry weight). DPPH radical scavenging and ABTS radical cation decolorisation assays revealed that antioxidant activity was highest in summer harvested oils, with IC50 values of 7.68 mg mL−1 (DPPH) and 8.85 mg mL−1 (ABTS), consistent with peak methyl eugenol accumulation. Permutation-based multiple regression (999 permutations; R2 = 0.947) identified seasonal precipitation as the strongest positive predictor of oil yield (β = 11.22, p < 0.05), while temperature exerted a significant negative influence (β = −11.21, p < 0.05). Non-metric multidimensional scaling (NMDS) and permutational multivariate analysis of variance (PERMANOVA) confirmed highly significant seasonal clustering of compositional profiles (F = 15.258, p = 0.001) against negligible diurnal structuring (F = 0.178, p =0.991). Redundancy analysis (RDA) attributed 71.03% of total compositional variance to the climatic predictor set. Pearson correlation analysis established significant positive associations between methyl eugenol content and antioxidant capacity (r > 0.80, p < 0.05). These findings provide an integrated, climate resolved basis for harvest timing optimisation of M. bracteata and identify summer as the strategically optimal harvest window for yield and bioactive functionality.