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 show complex physiological responses that cannot be predicted from single stress factors. This study examined the interactive effects of temperature stress and Erwinia amylovora infection on pear (Pyrus pyrifolia) leaves using chlorophyll fluorescence analysis and RGB imaging. Photosynthetic performance remained stable under temperature stress alone at 25℃, while pathogen inoculation reduced chlorophyll content and impaired PSII energy dissipation. Disease progression, assessed by the area under the disease progress curve (AUDPC), peaked at 25℃, followed by 30℃, 15℃, 10℃, and 35℃. Distinct PSII responses were observed under combined stresses: photoprotective NPQ activity at 10 ℃, partial efficiency loss at 15 ℃, severe impairment with elevated non-regulated energy dissipation (ФNO) at 25℃, enhanced oxidative damage at 30℃, and suppressed pathogen activity with partial photoprotection at 35℃. Linear mixed-effects modeling confirmed significant main effects of temperature and infection on PSII parameters, with ФNO particularly sensitive, while three-way interactions (T:I:D) in Fm, Fv, and Fp highlighted the interdependent impact of combined stresses. These findings reveal temperature-dependent resilience strategies of pear leaves against fire blight and underscore the value of chlorophyll fluorescence for assessing tolerance mechanisms and informing cultivar selection and disease management in complex stress environments.