چكيده انگليسي :
Ajowan (Trachyspermum ammi L.), a valuable medicinal plant belonging to the Apiaceae family provides significant potential for genetic improvement of seed yield, essential oil content and the production of phenolic compounds. The management of environmental stresses and optimization of agronomic practices can profoundly influence plant metabolic pathways thereby augmenting the biosynthesis of these bioactive compounds. This study was conducted with the primary objective of identifying drought-tolerant populations and evaluating the genetic diversity of morpho-physiological traits among 28 ajowan populations under water deficit conditions. The experiment was conducted as a randomized complete block design (RCBD) with three replications during the two consecutive years. Following principal component analysis, 12 superior populations were selected for an in-depth investigation of their phytochemical traits and secondary metabolite variations. Analysis of variance revealed significant differences among the populations for the majority of the traits under investigation. Additionally, the effects of irrigation levels were significant for all evaluated traits. Drought stress induced a significant reduction in the traits such as plant height, number of branches, inflorescence diameter, number of umbels per plant, canopy area, thousand-seed weight, dry biomass, number of flowers and seeds per umbel, seed yield, chlorophyll a and b contents, total chlorophyll, carotenoid content, and relative leaf water content. Morever, drought stress significantly elevated the activity of antioxidant enzymes (catalase, ascorbate peroxidase, and peroxidase), malondialdehyde, hydrogen peroxide, proline content, essential oil content, total phenolic and flavonoid contents, antioxidant capacity, polyphenolic compounds, and essential oil compounds. For genotypic selection, different indices including Tolerance Index (TOL), Susceptibility Index (SSI), Stress Tolerance Index (STI), Mean Productivity (MP), Geometric Mean Productivity (GMP), and Yield Stability Index (YSI) were investigated that MP, GMP and STI were more suitable indices. Based on these indices, the Esfahgh, Arakkho, and Ardebil populations were identified as the most drought-resistant ones. Gas chromatography–mass spectrometry (GC-MS) analysis revealed that thymol, carvacrol, γ-terpinene, and p-cymene are the principal constituents of ajowan essential oil, with the highest thymol content (89.01%) recorded in the Farsmar population under drought stress. Additionally, High-Performance Liquid Chromatography (HPLC) analysis identified eight important compounds including gallic acid, chlorogenic acid, caffeic acid, p-coumaric acid, rutin, ferulic acid, rosmarinic acid, and apigenin in the phenolic extracts of the selected ajowan populations. Notably, chlorogenic acid, caffeic acid, and p-coumaric acid were predominant in the phenolic extracts, making the Yazd and Arakkho populations valuable antioxidant sources for food and pharmaceutical applications. Furthermore, this study explored the expression of genes involved in the flavonoid biosynthesis pathway in ajowan under drought stress. The results indicated an upregulation of the PAL2 gene, which plays a crucial role in phenolic compound biosynthesis, in the inflorescence and leaf tissues under stress, contributing to cellular protection. Additionally, the expression of CHS1, F3’H1, and F3’5’H genes which are integral to flavonoid and anthocyanin biosynthesis was enhanced under drought stress. However prolonged and intensified stress led to a decline in the expression of these genes due to metabolic constraints. These findings provide a foundation for the identification and utilization of drought-tolerant ajowan populations in breeding and genetic manipulation programs.
