Interactive Effects of Organic Manure and Planting Density on Growth, Essential Oil Production, and Chemotype Expression in Iranian Thymus daenensis Čelak. Genotypes Under Dry Farming

The cultivation of Thymus daenensis, an endemic Iranian medicinal species rich in phenolic monoterpenes, requires optimization under rainfed conditions to ensure sustainability and productivity. This study investigated the interactive effects of organic manure application (۰ and ۵۰ t ha⁻¹), planting density (۴, ۶, and ۸ plant/m), and genotype (four distinct genotypes) on biomass production, essential oil (EO) yield, and EO composition in a dry farming system in western Iran. A split-plot factorial experiment (CRBD, three replications) was conducted over the ۲۰۲۱–۲۰۲۲ growing seasons. Results indicated that plant density, genotype, and manure application significantly influenced fresh and dry biomass yields as well as EO yield. The combination of medium planting density (۶ plant/m) with manure application generally maximized productivity, yielding up to ۱۶,۳۹۸ g/m fresh biomass and ۷,۱۲۲ g/m dry biomass. EO concentration was less variable but ranged from ۴.۳۰% to ۶.۰۷%, while EO yield increased markedly with manure and optimal density, reaching ۴۳۵.۳ Kg/ha. GC and GC–MS analyses identified thymol, carvacrol, γ-terpinene, p-cymene, and α-terpineol as major compounds. Thymol content varied widely (۵۹.۶۶–۷۲.۷۶%) and was highest under low-density, unfertilized conditions. In contrast, monoterpene hydrocarbons such as p-cymene and γ-terpinene increased under manure application and higher densities. Multivariate analyses (PCA, correlation, clustering) revealed a consistent trade-off: agronomic practices that enhanced biomass and EO yield promoted monoterpene-rich oils, whereas resource-limited conditions (low density, no manure) favored the accumulation of high-value phenolic monoterpenes like thymol and carvacrol. Significant genotype × fertilization × density interactions underscored the need for genotype-specific management strategies. We conclude that medium planting density combined with organic manure optimizes biomass and EO yield under rainfed conditions, while lower inputs enhance EO quality by increasing phenolic content. These findings provide a basis for tailoring cultivation practices according to production goals-whether maximizing yield or obtaining phenolic-rich essential oils for pharmaceutical and industrial uses.

کلیدواژه ها:

Rainfed agriculture ، phenolic monoterpenes ، biomass allocation ، Organic fertilization ، Chemotype plasticity