Water deficit affects plant and soil water status, plant growth, and ginsenoside contents in American ginseng

American ginseng (Panax quinquefolius L.) produces pharmacologically active secondary compounds known as ginsenosides which have been shown to be influenced by both genetic and environmental factors. In a greenhouse experiment, effects of water deficit on ginseng plant growth, predawn leaf water potential (I-Leaf), soil water potential (I-Soil), leaf abscisic acid (ABA) concentration, and root ginsenoside contents as well as photosynthesis-related physiological responses were studied. Three-year-old seedlings, grown in 200 mL volume of plastic pots, were well watered for 45 days prior to the initiation of water deficit treatments. Plants in the water deficit treatments were irrigated every 10 or 20 days for the mild and severe water deficit treatments, respectively, while the control plants were watered every 4 days. The experiment was terminated after 15, 6, and 3 dry down cycles (60 days) for the control, mild, and severe water deficit treatments, respectively. As water deficit progressed, both I-Soil and I-Leaf decreased, but foliar ABA concentration increased. Other physiological responses to water deficit, including transpiration rate, stomatal conductance, and CO2 assimilation rate, were decreased. Water deficit decreased root growth, but unaffected shoot growth. Foliar chlorophyll content was also decreased in the water deficit treatments. The contents of individual ginsenosides Re, Rb1, Rc and Rd, and total ginsenosides were increased in the storage roots of water deficit-treated plants as compared with well-watered controls. Rootlet fresh weight before transplanting (RFWBT) as a covariate had a significant effect on the contents of ginsenoside Rb1, Rc, and Rb2. Overall, the results indicate that water deficit could contribute not only to reducing plant performance but also increasing the levels of ABA and certain ginsenoisdes.