Effects of Temperature Regulation on the Physiological Characteristics and Platycodin Synthesis of Platycodon grandiflorum

Platycodon grandiflorum, a dual-purpose herb for food and medicine, is widely distributed in Asia. Although P. grandiflorum has relatively low requirements for its growing environment, temperature remains an important ecological factor affecting its growth, development, and quality formation. In order to explore the effect of different temperatures on P. grandiflorum during their growth period, the diversity in growth physiology, platycodin contents, and gene expression of key enzymes were investigated under constant (8 °C, 18 °C, and 28 °C) and variable (8–18 °C, 8–28 °C, and 18–28 °C) temperature conditions at each of the three levels. The results suggested that both constant and variable temperatures at high levels significantly increased the aboveground fresh weight of P. grandiflorum. However, the low–variable temperature was beneficial for the accumulation of dry and fresh weight in the roots. Regardless of whether temperatures were constant or variable at low levels, this increased the content of soluble sugars, proline, and peroxidase in P. grandiflorum, while upregulating the expression levels of key enzyme genes involved in platycodin synthesis. Meanwhile, a low–constant temperature inhibited the photosynthetic rate of P. grandiflorum. Furthermore, medium–constant and large-scale variable temperatures were conducive to the accumulation of platycodins in the roots. This research provides a theoretical basis and data support for the influence of temperature variations on P. grandiflorum quality formation.