5-Aminolevulinic Acid Alleviates the Damage Caused by High-Temperature Stress in Herbaceous Peony (Paeonia lactiflora Pall.)

High-temperature stress significantly inhibits the growth, development, and ornamental value of herbaceous peony (Paeonia lactiflora Pall.). 5-Aminolevulinic acid (5-ALA), a natural plant growth regulator, has been reported to play important roles in enhancing plant stress tolerance. Nevertheless, the function of 5‑ALA in enhancing the high-temperature tolerance of P. lactiflora remains to be fully elucidated. In our study, the exogenous application of 5-ALA significantly improved the high-temperature tolerance of P. lactiflora. Exogenous application of 5-ALA resulted in an increase in the leaf water content (LWC) of leaves, as well as a decrease in both relative electrical conductivity (REC) and malondialdehyde (MDA) content. Moreover, exogenous application of 5-ALA enhanced the activity of antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), which effectively cleared reactive oxygen species (ROS) and mitigated oxidative damage. Additionally, exogenous application of 5-ALA facilitated the biosynthesis of chlorophyll and inhibited its catabolism, thereby improving the photosynthetic efficiency of P. lactiflora under high-temperature stress. These findings provided a theoretical foundation and a practical reference for enhancing high-temperature resistance through the application of 5-ALA in plants.