Effects of drought duration on terpene profiles, physiological responses, and terpene-related gene expression in rosemary

Drought is a major environmental constraint limiting plant growth and productivity. This study investigates the effects of drought on rosemary (Salvia rosmarinus Spenn.) plantlets subjected to 5, 10, and 15 days of irrigation withholding, with control plants watered regularly every five days. Growth, physiological parameters, and antioxidant enzyme activities were examined. The results revealed reductions in chlorophyll content. Antioxidant enzyme activities—including catalase (CAT), superoxide dismutase (SOD), polyphenol oxidase (PPO), and secondary soluble peroxidase (SPO)—increased, whereas phenylalanine ammonia-lyase (PAL) and ascorbate peroxidase (APX) decreased. Furthermore, the types and quantities of terpenes and other phytochemical compounds produced by rosemary plantlets under drought stress at different time points were analyzed using GC–MS. A total of 710 phytochemical compounds were identified across the following samples: control 5 days, control 10 days, control 15 days, drought 5 days, drought 10 days, and drought 15 days. Overall, the total percentage of monoterpenes decreased under drought, whereas sesquiterpenes, diterpenes, and triterpenes increased. Additionally, RT-qPCR was used to quantify the expression of twelve terpene biosynthesis genes (SrBDH, SrGPS, SrFPPS, SrGGPP, SrCINS1, SrCINS2, SrTPS-Pin, SrHUMS, SrKSL2, SrFS2, SrCPS1, and SrTPS1) under drought conditions relative to the control, to elucidate the relationship between gene expression and terpene type and abundance. In summary, this study highlights the impact of drought on the composition of terpenes in S. rosmarinus plantlets, providing valuable insights that may be leveraged to enhance rosemary nutritional value and productivity under drought stress.