Compound Fertilizer Application Improves the Growth of Herbaceous Peony (<i>Paeonia lactiflora</i> Pall.) under High Temperature in Summer

Summer high temperatures have severely impaired the growth of herbaceous peony (Paeonia lactiflora Pall.) in East China. While compound fertilizer application enhances soil fertility and promotes plant growth, its efficacy in maintaining optimal plant performance under summer heat stress remains poorly understood. This study investigated the effects of compound fertilizer application on herbaceous peony growth during summer thermal stress. Results demonstrated that compound fertilizer supplementation significantly improved plant growth under elevated temperatures, manifesting enhanced phenotypic characteristics, elevated antioxidant enzyme activities, and increased nutrient accumulation. Compared to untreated controls, fertilized plants exhibited three key responses: (1) increased chlorophyll content coupled with reduced relative conductivity, malondialdehyde levels, and reactive oxygen species (ROS) accumulation; (2) upregulated activities of four critical antioxidant enzymes and augmented nitrogen, phosphorus, and potassium assimilation, collectively enhancing photosynthetic efficiency; and (3) stimulated expression of chlorophyll biosynthesis-related genes alongside suppressed transcription of chlorophyll degradation-associated genes. These findings establish a theoretical framework for optimizing compound fertilizer strategies to mitigate summer heat stress in herbaceous peony cultivation across East China.