Objectives The objective of this study is to investigate the effects of different manganese chloride levels on the growth and flavonoid synthesis of Scutellaria baicalensis.

Methods Two experiments were carried out in Haidian District, Beijing, using annual skullcap as the test material, and five treatments were set up: clean water control (Mn0), 0.5 g/L (Mn1), 2 g/L (Mn2), 4 g/L (Mn3) and 8 g/L (Mn4) MnCl₂ solution. The first foliar spray was carried out on August 7 during the peak growth period, and it was sprayed once every 15 days, for a total of 3 sprays. Scutellaria baicalensis was harvested in autumn when it was ripe (October 8), and its root growth indexes, flavonoid compound content, leaf hormone and antioxidant enzyme activities and other physiological indexes were measured.

Results 1) Pot experiment　Root biomass and total flavonoid yield increased first and then decreased and then increased again with the increase of Mn concentration, reaching a peak at Mn1 treatment, which was significantly increased by 35% and 48% respectively compared with Mn0. With the increase of Mn concentration, the activity of superoxide dismutase (SOD) increased first, then decreased and then increased, reaching a peak in Mn1 treatment, which was significantly higher than that of Mn0 by 8%, and Mn3 treatment was significantly lower than that of Mn0 by 21%. Compared with Mn0, the peroxidase (POD) activity of Mn2, Mn3 and Mn4 significantly decreased. With the increase of Mn concentration, the content of indole-3-acetic acid (IAA) decreased first, then increased and then decreased, and Mn3 and Mn4 were significantly lower than Mn0 by 20%−25%. The gibberellin (GA) content increased first and then decreased with the increase of Mn concentration, reaching a peak in Mn2 treatment, which was 8% higher than that of Mn0. With the increase of Mn concentration, the content of abscisic acid (ABA) decreased and increased, and Mn4 was significantly increased by 13% compared with Mn0, and Mn2 was significantly decreased by 6% compared with Mn0. 2) Field experiments　Root biomass increased first and then decreased with the increase of Mn concentration, reaching a peak at Mn1 treatment, which was significantly increased by 53% compared with Mn0. The total flavonoid yield increased first and then decreased with the increase of Mn concentration, reaching a peak in Mn1 treatment, which was significantly increased by 53% compared with Mn0. With the increase of Mn concentration, SOD first increased and then decreased and then increased, except that there was no significant difference between Mn1 treatment and Mn0, and Mn2, Mn3, Mn4 treatment significantly decreased by 8%–12% compared with Mn0. With the increase of Mn concentration, the content of IAA increased first and then decreased, and the Mn1 treatment significantly increased by 6% compared with Mn0, and the Mn4 treatment significantly decreased by 15% compared with Mn0. The GA content increased first and then decreased with the increase of Mn concentration, reaching a peak in Mn3 treatment, which was 15% higher than that of Mn0. With the increase of Mn concentration, the content of ABA decreased first, then increased, and then decreased and then increased, and the Mn4 treatment significantly increased by 4% compared with Mn0. The results of field and pot experiments showed that the performance trend was basically the same. Compared with Mn0, the spraying of manganese concentration in the range of 0.5–8 g/L could promote the accumulation of flavonoids, but the high concentration had little effect on root biomass, which was generally beneficial to Scutellaria baicalensis. The low concentration of Mn1 showed the best effect, significantly promoting root growth, increasing biomass, and significantly increasing the yield of total flavonoids compared with the control. At this concentration, the levels of auxin IAA and gibberellin GA in leaves of Scutellaria baicalensis increased significantly, and the cell division and elongation of plants accelerated, which promoted the increase of biomass of Scutellaria baicalensis. At the same time, the SOD activity in leaves of Scutellaria baicalensis was significantly increased in the early stage of manganese spraying, which enhanced the adaptability of Scutellaria baicalensis to manganese. There was no significant change in root biomass of Scutellaria baicalensis under medium and high concentration manganese spraying treatment, and the content of flavonoids increased.

Conclusions The appropriate concentration of manganese chloride can significantly affect the chlorophyll content, hormone level and antioxidant enzyme activity in the leaves of Scutellaria baicalensis, and then effectively promote the growth and flavonoid accumulation of Scutellaria baicalensis. In field cultivation practice, it is recommended that the concentration range of manganese chloride spraying should be set between 0.5 g/L to 8 g/L, and the concentration effect of 0.5 g/L is the best.