Integrated humic acid nutrition in damask rose: Linking yield improvement, transcriptional reprogramming of terpene biosynthesis genes, and reduction of essential oil alkanes

Damask rose (Rosa damascena Mill.) is one of the most valuable aromatic plants; however, its cultivation in calcareous, organic‑matter‑poor soils of semi‑arid regions is frequently constrained by hidden hunger, which impairs both flower yield and essential oil quality. This study evaluated the agronomic and phytochemical performance of nine integrated fertilizer treatments over two growing seasons and further employed physiological, biochemical, and molecular analyses at three phenological stages to elucidate the underlying mechanisms. Among the treatments, only the combination of 75% of the soil‑test‑based recommended NPK with humic acid (C₇₅+H) significantly increased total flower weight by 31.5% and flower number relative to the unfertilized control, whereas full chemical fertilization alone did not differ from the control. This treatment also produced the most favorable essential oil profile, with the highest citronellol (51.70%) and total oxygenated monoterpenes (67.82%), the lowest total alkanes (25.97%), and a citronellol/geraniol ratio of 4.3. Mechanistically, humic acid activated the root plasma membrane H⁺‑ATPase (75% increase), enhanced Fe and Zn uptake by 50–60%, improved leaf phosphorus status (47% increase), and raised the leaf NADPH/NADP⁺ ratio (60%), resulting in a twofold increase in geraniol reductase activity. A 90% rise in xylem sap active cytokinins was accompanied by a 2.65‑fold up‑regulation of DXS gene expression and a 38% down‑regulation of FAE gene expression, channelling carbon flux towards monoterpene biosynthesis and away from undesirable alkane formation. Consequently, integrating 75% of the recommended chemical fertilizer with humic acid offers an efficient strategy to simultaneously improve flower yield and essential oil quality in calcareous soils, through proposed rhizosphere acidification, proposed hormonal signaling, and transcriptional reprogramming of biosynthetic pathways.