Humic Acid Improves High-Temperature Tolerance in Herbaceous Peony

Summer temperature in the middle and lower reaches of the Yangtze River gradually increases, that seriously restricts herbaceous peony (Paeonia lactiflora Pall.) growth which introduced from the north. Therefore, alleviating the damage caused by high temperatures to herbaceous peony becomes a key factor in promoting migration southward and maintaining their ornamental characteristics. In this study, herbaceous peonies were potted in a cultivation environment treated with humic acid (HA), aiming to investigate whether the addition of HA could improve the resistance to high temperature stress. The results showed that HA-treatment was able to maintain the growth of herbaceous peony under high temperature stress by inhibiting chlorophyll degradation. Stress physiological indices showed that, firstly, HA-treatment inhibited ROS accumulation by reducing H2O2 and $${\text{O}}_{2}^{{\bullet - }}$$ contents. Secondly, HÀ-treatment maintains membrane lipid peroxidation levels by reducing relative electrical conductivity and MDA content. Moreover, HA-treatment also maintains photosynthetic capacity and promotes stomatal conductance, which is consistent with scanning electron microscope observation, indicating that the surface of herbaceous peony leaves treated with HA is smoother and can still maintain stomatal opening. Additionally, HA-treatment activates antioxidant enzyme activity and increases antioxidant enzyme content. Finally, we found that antioxidant enzyme activity and photosynthetic parameters were the most important indicators to improve high temperature tolerance through correlation analysis. This study reveals the physiological effects of HA in reducing stress damage on herbaceous peony under high temperature stress. It provides a new approach to address the issue of high summer temperatures faced by herbaceous peony during southward migration.