Exogenous glycine betaine enhances drought tolerance and medicinal quality in Glycyrrhiza uralensis seedlings

Abstract Background Drought stress constrains the growth and medicinal quality of Glycyrrhiza uralensis , but the concentration-dependent physiological basis of exogenous glycine betaine (GB)-mediated stress alleviation remains incompletely understood. Methods Seedlings were grown in a controlled pot experiment with one 45% maximum moisture capacity (MMC) reference treatment and five 35% MMC water-deficit treatments receiving 0, 20, 40, 60, or 80 mmol L − 1 GB. Growth and root morphology, gas exchange, chlorophyll fluorescence, osmotic-regulation-related metabolites, oxidative damage, antioxidant enzyme activities, and major root medicinal compounds were quantified. Results Exogenous GB alleviated water-deficit-induced growth inhibition and oxidative injury, with the strongest overall response occurring at 40 mmol L − 1 . At this concentration, plant height and root morphological traits increased by 49–91%, whereas H 2 O 2 content, MDA content, and relative electrical conductivity decreased by 57%, 30%, and 44%, respectively, compared with water-deficit-stressed plants without GB. SOD and CAT activities increased by up to 400% and 608%, respectively. GB also improved photosynthetic performance and osmotic-regulation-related responses. Across GB treatments, the maximum increases in liquiritin, glycyrrhizic acid, and total flavonoid contents were 71%, 90%, and 63%, respectively. Exploratory PLS-SEM further indicated coordinated associations among oxidative injury, osmotic and antioxidant responses, photosynthetic performance, growth, and medicinal quality. Conclusions Exogenous GB, particularly 40 mmol L − 1 under the present pot conditions, was associated with coordinated improvements in redox homeostasis, osmotic-regulation-related responses, and photosynthetic performance, together with enhanced growth and medicinal-compound accumulation. These findings identify 40 mmol L − 1 as a candidate concentration within the tested range and provide a physiological basis for field validation in water-limited licorice production.