Mitigation of Drought Stress in Licorice (Glycyrrhiza glabra L.) by Seaweed Extract (Ascophyllum nodosum): Effects on Morphological Traits, Pigments, and Biochemical Properties

The application of biostimulant seaweed extract (SWE) has received significant attention as innovative strategies to alleviate the detrimental effects of drought stress. This research utilized a completely randomized factorial design to investigate the effects of irrigation regimes and seaweed extract (SWE) application on licorice (Glycyrrhiza glabra L.). Irrigation treatments imposed graded moisture deficits relative to field capacity (FC): 100% FC (control), 80% FC (slight stress), 60% FC (mild stress), 40% FC (moderate stress), and 20% FC (severe stress). Concurrently, SWE was applied at concentrations of 0 (control), 3, and 5 g/L. Drought stress severity caused significant declines in most morphological traits, whereas SWE application significantly improved all traits, mitigating the adverse effects. The application of SWE demonstrated a beneficial impact on the concentration of rutin, with the peak measurement reaching 3.10 mg/g DW observed under conditions of extreme drought stress combined with a treatment of 5 g/L of SWE. The maximum concentrations of total phenols and total flavonoids were recorded in conditions of severe drought stress when supplemented with 5 g/L of SWE, resulting in measurements of 24.79 mg GAE/g DW and 25.43 mg RE/g DW, respectively. In contrast, the peak level of chlorophyll a was found in non-water stress conditions with the same SWE treatment, measuring 8.98 mg/g FW, whereas the highest concentration of chlorophyll b was noted in non-water stress conditions without the SWE, reaching 2.73 mg/g FW. Seaweed extract application enhanced drought tolerance by eliciting the plant’s defense response, resulting in increased production of antioxidant compounds (including phenols, flavonoids, and pigments).