Essential oil chemotype diversity and environmental stability in Turkish basil (Ocimum basilicum L.) germplasm: a three-layer field evaluation across contrasting ecologies

The environmental stability of essential oil (EO) chemotype expression in basil (Ocimum basilicum L.) is critical for both industrial quality assurance and food safety, yet remains poorly characterised across the multiple dimensions of environmental variation encountered in production. This study characterises the environmental modulation of EO yield and chemotype composition in 12 basil genotypes representing four chemotypes (linalool, estragole, citral, and methyl cinnamate) using a three-layer experimental design: (i) ecological variation across three contrasting Turkish locations (Bursa, Eskişehir, Tokat) over two years, (ii) ontogenetic variation across three developmental stages at Bursa, and (iii) seasonal variation between summer and autumn harvests at Tokat (144 unique EO profiles). EO yield was highest at Bursa (0.98 mL·100 g⁻¹ dry weight), increased from pre-flowering to onset of flowering, and was greater in autumn than summer harvests. The estragole chemotype (R-10A) exhibited exceptional compositional stability across ecological environments (Layer 1 CV = 2.8%), while the citral (Layer 1 CV = 12.9%) and methyl cinnamate (20.0%) chemotypes showed moderate to variable stability. Within the linalool chemotype, R-16 was the most stable genotype (SD = 5.6%). The results indicate that chemotype stability is primarily genotype-driven and that environment × harvest-stage interactions modulate secondary metabolite accumulation patterns in a chemotype-dependent manner, with implications for cultivar selection and food safety considerations.