Seasonal variation in phytochemical and essential oil profiles of Eucalyptus camaldulensis leaves and implications for harvest timing

Eucalyptus camaldulensis Dehnh. is an economically important medicinal and aromatic tree widely exploited for its essential oil and bioactive metabolites in pharmaceutical, cosmetic, and food industries. Despite its industrial significance, integrated evaluations of seasonal harvest timing in relation to phytochemical attributes, essential oil composition, and data-driven harvest optimization remain scarce. This study evaluated seasonal dynamics across six harvest periods (February, April, June, August, October, and December) by integrating phytochemical profiling, essential oil characterization, multivariate analyses, and predictive modeling to identify the most suitable harvest time for quality-oriented production of E. camaldulensis leaves. Harvest time significantly influenced all investigated traits. Total phenolic content reached its maximum in February (53.45 mg GAE g⁻¹ DW), whereas total flavonoid content (52.50 mg QE g⁻¹ DW), antioxidant activity (95.28% DPPH inhibition), and essential oil content (2.98%) were highest in June. GC and GC–MS profiling identified 39 compounds representing 97.91–99.44% of the total essential oil. Oxygenated monoterpenes were the predominant chemical group, with 1,8-cineole (47.74–59.85%) identified as the principal constituent, followed by α-pinene (15.93–22.76%), aromadendrene (1.17–4.36%), and globulol (2.65–5.84%). Multivariate analyses (PCA, hierarchical clustering, and heatmap) effectively discriminated harvest time according to their phytochemical and volatile profiles. Generalized additive models revealed significant nonlinear seasonal responses of both phytochemical traits and major essential oil constituents. Random Forest analysis identified total flavonoid content, flavones and flavonols, total phenolic content, and α-pinene as the most influential variables discriminating harvest times. Multi-response optimization using the Derringer–Suich Overall Desirability Index identified June as the most favorable harvest time among the evaluated sampling dates, providing the best overall balance between phytochemical quality and essential oil yield. These findings provide a comprehensive framework for harvest-time optimization and offer valuable insights for quality-oriented production and the development of improved harvest management strategies for E. camaldulensis .