Effect of drought stress on growth parameters, osmolyte contents, antioxidant enzymes and glycyrrhizin synthesis in licorice (Glycyrrhiza glabra L.) grown in the field

Glycyrrhiza glabra L. (licorice) is a medicinal species rich in the specialised plant metabolite glycyrrhizin. It has been previously proposed that drought, which is increasing in importance due to the climatic change and scarcity of water resources, can promote the synthesis of glycyrrhizin. The effects of slight, moderate and intense drought (70, 35 and 23% of the regular irrigation, respectively) on growth parameters, osmolyte content, oxidative stress markers, antioxidant enzymes, glycyrrhizin biosynthesis genes and root glycyrrhizin concentration and contents, have been assessed in five Iranian licorice genotypes grown in the field. Drought decreased progressively biomass and leaf relative water contents, and increased progressively osmolyte (proline, glycinebetaine and soluble sugars) concentrations in leaves and roots. Drought caused oxidative stress in leaves, as indicated by lipid peroxidation and hydrogen peroxide concentrations, and increased the activities of antioxidant enzymes in leaf extracts (catalase, peroxidase, superoxide dismutase and pholyphenoloxidase). Drought promoted the synthesis of glycyrrhizin, as indicated by the increases in the expression of the glycyrrhizin biosynthesis pathway genes SQS1, SQS2, bAS, CYP88D6, CYP72A154 and UGT73, and increased the root concentrations of glycyrrhizin with drought in some genotypes. However, the large decreases in root biomass caused by drought led to general decreases in the amount of glycyrrhizin per plant with moderate and intense drought, whereas the slight drought treatment led to significant decreases in glycyrrhizin content in only one genotype. Under intense drought two of the genotypes were still capable to maintain half of the control glycyrrhizin yield, whereas in the other three genotypes glycyrrhizin yield was 22-33% of the control values. Results indicate that under intense drought, with only 23% of the normal water dose being applied, an appropriate choice of genotype can still lead to acceptable glycyrrhizin yields.