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 Pelargonium
graveolens. 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 P. graveolens 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 P. graveolens.