Poly-Glutamic Acid Regulates Physiological Characteristics, Plant Growth, and the Accumulation of the Main Medical Ingredients in the Root of Salvia miltiorrhiza Under Water Shortage

To supply information concerning the application of poly-glutamic acid (PGA) in the drought-resistant cultivation of red sage (Salvia miltiorrhiza), we investigated the role of PGA in regulating the physiological characteristics, plant growth, and the accumulation of the main medical components in the root under water shortage. The findings showed that different levels of water shortage (WS) all suppressed the photosynthetic function by reducing the net photosynthetic rate (Pn), Soil and plant analyzer development (SPAD) value, maximum photochemical efficiency of PSII (Fv/Fm), photochemical quenching (qP), and actual photochemical efficiency of PSII (Y(II)), as well as increasing non-photochemical quenching (qN). Compared with WS, PGA plus WS enhanced the photosynthetic function by reducing qN and increasing the other indicators above. For water metabolism, WS increased stomatal limit value (Ls) and water use efficiency (WUE), but decreased transpiration rate (Tr) and stomatal conductance (Gs). Compared with WS, PGA plus WS decreased Ls and increased Tr, Gs, and WUE. Meanwhile, WS enhanced the antioxidant capacity by increasing superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities. However, WS increased malondialdehyde (MDA) content. Compared with WS, PGA plus WS enhanced the above antioxidant enzymes. In this way, PGA reduced MDA content and improved the antioxidant capacity under WS. In addition, WS decreased the shoot and root biomass, but increased the root/shoot ratio. Compared with WS, PGA plus WS further increased the root/shoot ratio and shoot and root biomass. For medical ingredients, WS decreased the yield of rosmarinic acid (RosA) and salvianolic acid B (SalB), but increased the yield of dihydrotanshinone (DHT), cryptotanshinone (CTS), tanshinone I (Tan I), and tanshinone ⅡA (Tan ⅡA). Compared with WS, PGA plus WS increased the yield of these medical ingredients. Our findings clearly suggested that PGA application was an effective method to enhance sage drought tolerance and the yield of the main medical ingredients in sage root. This provides useful information for its application in sage production under WS.