Effects of exogenous calcium on physiological characteristics and medicinal quality of Platycodon grandiflorus seedlings under drought stress

Abstract [Objective] Drought stress seriously affects the yield and quality of medicinal plants. This study aims to reveal the physiological mechanism of exogenous calcium in alleviating drought-induced damage in Platycodon grandifloras, which can provide a basis for using exogenous calcium to improve drought resistance and medicinal quality of P . grandiflorus in arid and semi-arid area. [Methods] With P . grandiflorus seedlings as materials, a pot experiment was conducted to explore the effects of exogenous calcium on the growth, photosynthetic gas exchange parameters, antioxidant enzyme activity, and medicinal quality of P . grandiflorus seedlings under drought stress by foliar spraying of 10 mmol/L CaCl2. [Results] (1) Exogenous calcium significantly increased root length and dry and fresh biomass of P . grandiflorus under drought stress. It significantly increased the stomatal aperture, chlorophyll a content, total chlorophyll content, carotenoid content, net photosynthetic rate, stomatal conductance, and transpiration rate by 30.28%, 57.67%, 44.44%, 100.33%, 89.53%, 60.00%, and 83.11%, respectively, in P . grandiflorus leaves under drought stress. (2) Exogenous calcium effectively reduced the content of malondialdehyde and hydrogen peroxide in P . grandiflorus leaves under drought stress, which was significantly reduced by 13.82% and 18.66%, respectively, compared to drought stress. Exogenous calcium increased the superoxide dismutase, superoxide dismutase, and catalase activities in P . grandiflorus leaves by 25.43%, 7.90%, and 33.92%, respectively, and also increased the soluble protein content in roots compared with drought stress. (3) Exogenous calcium significantly increased the levels of latycodin-D, polysaccharides, total flavonoids, and free amino acids in P . grandiflorus roots by 35.34%, 34.87%, 4.19%, and 6.52%, respectively, under normal conditions. Under drought stress, the levels were significantly increased by 10.94%, 7.53%, 6.07%, and 5.78%, respectively. (4) N, P, K, Ca, Cu, and Mn contents in the aboveground and underground parts of P . grandiflorus were significantly reduced under drought stress, while they were increased to varying degrees under exogenous calcium treatment. [Conclusion] Spraying 10 mmol/L CaCl2 could enhance the antioxidant system activity and osmotic regulation ability in P . grandiflorus leaves in arid environment, increase the content of photosynthetic pigments and photosynthetic performance, synergistically promote the accumulation of secondary metabolites and mineral elements, improve the quality of medicinal parts, and alleviate the damage of P . grandiflorus seedlings under drought stress.