Interplay of Stress Responses in Pear Tree Revealed by Chlorophyll Fluorescence Under Combined Erwinia amylovora Infection and Temperature Stress

Plants exposed to combined abiotic and biotic stresses often exhibit complex physiological responses that cannot be predicted from single stress factors. In this study, we evaluated the interactive effects of temperature stress and Erwinia amylovora infection on pear (Pyrus pyrifolia) leaves under five temperature conditions (10, 15, 25, 30, and 35 °C) with or without pathogen inoculation, using chlorophyll fluorescence analysis and RGB imaging over a 7-day period. Photosynthetic performance remained optimal at 25 °C under single temperature conditions, whereas pathogen inoculation alone caused PSII damage and reduced energy dissipation. Under combined stress, PSII responses exhibited temperature-dependent patterns: at 10, 15 °C, photoprotective mechanisms were partially maintained; at 25, 30 °C, severe structural and functional damage occurred; and at 35 °C, pathogen activity was suppressed while partial recovery of PSII was observed. By integrating chlorophyll fluorescence analysis with a linear mixed-effect model (LMM), distinct patterns of sensitivity were identified among fluorescence parameters, with ΦNO responding to single stress factors, and Fm, Fv, Fp, Fv/Fo, and qL showing significant three-way interactions. These findings highlight temperature-dependent strategies of pear leaves to cope with fire blight and emphasize the utility of chlorophyll fluorescence analysis for evaluating photosynthetic resilience. From an applied perspective, chlorophyll fluorescence could serve as a rapid, non-destructive tool for screening pear cultivars with enhanced tolerance to bacterial fire blight, contributing to more efficient orchard management strategies.