Changes in Phytochemical, Physiological, and Morphological Traits in Pelargonium graveolens as Affected by Drought Stress and Ascophyllum nodosum Extract

Nowadays, the use of natural biological bio-stimulants such as seaweed extract (SWE) is highly considered for alleviating the adverse effects of drought stress in many plant species. This study evaluated the effects of drought stress and foliar application of seaweed extract (SWE) on the morphological, physiological, and phytochemical traits of <i>Pelargonium graveolens</i>. Three levels of water irrigation regimes were used in combination with four SWE concentrations (0, 2.5, 5, and 7.5 mL L^-1). Based on the GC-MS analysis, 83 compounds were identified, of which citronellol, citronellyl formate, α-gurjunene, δ-cadinene, and γ-cadinene were the major constituents of <i>P. graveolens</i> leaves. The highest citronellol content (56.2%) was found under moderate irrigation with 5 mL of L^-1 SWE, while the lowest amount (26.78%) was obtained under full irrigation with no foliar application of SWE. Citronellyl formate and α-gurjunene exhibited their highest relative abundance under non-stress conditions following foliar application of 5 mL L^-1 and 0 mL L^-1 of SWE, respectively. In contrast, δ-cadinene reached its highest value under severe drought stress when treated with 7.5 mL of L^-1 SWE, indicating a stress-responsive shift in essential oil (EO) composition profile. Principal component analysis (PCA) revealed that full irrigation with 7.5 mL of L^-1 SWE and mild drought with 5 mL of L^-1 SWE were the best treatments for ameliorating the EO content and composition. ANOVA revealed that SWE significantly improved the fresh root weight, leaf dimensions, carotenoids, total chlorophyll, protein content, and antioxidant enzyme activities. The 7.5 mL of L^-1 SWE treatment notably increased fresh root weight by 29.16% and enhanced chlorophyll and protein levels under moderate and severe drought conditions. Drought stress reduced shoot biomass but had no significant effect on chlorophyll content. Carotenoid and antioxidant activities were significantly influenced by both drought and SWE, with the highest levels observed at 5 mL of L^-1 SWE. Antioxidant enzymes (CAT, SOD, and guaiacol peroxidase) and total antioxidant activity were enhanced by SWE and its interaction with drought stress conditions. These results suggest that foliar SWE application at 5-7.5 mL L^-1 effectively mitigates drought stress and enhances both growth and EO composition in <i>P. graveolens</i>.