Effects of water and high temperature stress on active oxygen metabolism and total flavonoids accumulation in Mentha haplocalyx seedlings

Shortage of water resources and trend of global warming make medicinal plants face water and high temperature stress at the same time. Mentha haplocalyx seedlings were subject to stress environment in a light incubator, with polyethylene glycol (PEG6000) used as single stress of water, and 40 ℃ as single stress of high temperature, together with compound stress of the two. Pure water and 25 ℃ were used as controls. Production of superoxide anion (\rmO_2\cdot^- ), hydrogen peroxide (H₂O₂), contents of superoxide dismutase (SOD) and peroxidase (POD), catalase (CAT), as well as malondialdehyde (MDA) and flavone levels were measured. Under single and complex stress of water and high temperature, MDA, \rmO_2\cdot^- , H₂O₂, SOD, POD and CAT increased first and then declined, MDA under single stress increased overall, MDA under complex stress increased first and then decreased. Values under complex stress were significantly higher than under single stress between 4 and 12 h, showing synergistic effects. Under 24 h of combined stress, all detected values decreased down to or even lower than controls before single stress, likely due to greater damage by combined stress than single stress. With extended times, a critical point of tolerance was reached and mint seedling death was seen. Flavone accumulation showed increase from 4 to 8 h then decrease from 8 to 12 h, consistent with levels of \rmO_2\cdot^- and H₂O₂ at 4 and 8 h, respectively. Accumulation of flavone under stress was probably up-regulated by active oxygen. Water and high temperature stress could therefore induce antioxidant enzymes to protect peppermint seedlings, to a limited extent. But reactive oxygen species may enhance accumulation of flavonoids. Therefore, not only synergistic effect of multiple stress, but also time effect of stress should be considered to balance “yield” and “quality” of plants.