Effects of Different Planting Densities and Harvesting Periods on the Growth and Major Alkaloids of Anisodus tanguticus (Maxim.) Pascher on the Qinghai–Tibetan Plateau

Anisodus tanguticus (Maxim.) Pascher, a medicinal plant growing in the Tibetan Plateau region with various medicinal values, is mainly used for the extraction of tropane alkaloids (TAs), and the increased demand for A. tanguticus has triggered its overexploitation. The cultivation of this plant is necessary for the quality control and conservation of wild resources. During 2020 and 2021, a split-plot experiment with three replicates was used to study different planting densities (D1: 30 × 50 cm; D2: 40 × 50 cm; D3: 50 × 50 cm; D4: 60 × 50 cm) and different growth periods (first withering period: October 2020; greening period: June 2021; growth period: August 2021; second withering period: October 2021) on the yield and alkaloid content (atropine, scopolamine, anisodamine, anisodine) of A. tanguticus . The results showed that the mass per plant of A. tanguticus was higher at low density, while the yield per unit area of the underground parts (25288.89 kg/ha) was greater at high density, and the mass of the aboveground parts (14933.33 kg/ha) was higher at low density. The anisodamine (0.0467%) and anisodine (0.1201%) content of D2 (40 cm × 50 cm) was significantly higher than that of the other densities during the green period. The content of all four alkaloids was highest during the greening period, and the scopolamine, anisodamine, and anisodine content was higher in the aboveground parts than in the underground parts. The total alkaloid accumulation per unit area of the whole plant reached its maximum value (1.08%, 139.48 kg/ha) in the growth period of D2; therefore, for economic efficiency and the selection of the best overall quality, it was concluded that the aboveground parts also had medicinal value, the growth period was the best harvesting period, and D2 (40 cm × 50 cm) was the best planting density for A. tanguticus .