Effects of progressive drought stress on the physiology, antioxidative enzymes and secondary metabolites of Radix Astragali

The dried roots of Radix Astragali are widely used in traditional Chinese medicine, and flavonoids present in the root of this herb have been implicated in its bioactivity. We subjected 2-year-old Astragalus membranaceus Bge. var. mongholicus (Bge.) Hsiao to a progressive drought stress over 14 days of water withholding and studied the physiological and biochemical responses and secondary metabolite accumulation. Results indicated that drought stress reduced relative water content, reduced yield, but increased electrolyte leakage, malondialdehyde, proline and soluble sugar content. Mild and moderate drought stress enhanced some antioxidative enzyme activity to protect plants from damaging, but antioxidative enzyme activity was limited by severe stress. The calycosin-7-O-b-D-glycoside and ononin content of plant roots was enhanced with degree of drought stress, whereas calycosin and formononetin levels did not differ significantly until 14 days. These results demonstrate that Radix Astragali can adapt to water stress by up-regulating antioxidant enzymes and accumulation of osmotic agents, and a certain degree of drought stress can enhance accumulation of some flavonoids, potentially facilitating higher yields of pharmacological activity of calycosin-7-O-b-Dglycoside production.