Silicon mitigates phthalic acid stress in pepper by coordinating morphological and physiological responses

Continuous cropping obstacles threaten sustainable agriculture. Phthalic acid (PA) is a key autotoxin in pepper. This study investigated the physiological mechanisms and agronomic benefits of exogenous silicon (Si) in alleviating PA stress in pepper. Pepper seedlings grown in soilless substrate culture were exposed to 2 mM PA with or without Si (0-2 mM). Growth, root architecture, photosynthesis and redox metabolism were analysed. A field trial assessed yield and fruit quality. PA stress induced an atypical redox response: suppressed reactive oxygen species accumulation and antioxidant system impairment. Exogenous Si alleviated these effects. The 1 mM Si treatment increased plant height, stem diameter and leaf number by 53.4%, 16.8% and 35.3%, respectively. Si restored antioxidant capacity, with 2 mM Si increasing leaf superoxide dismutase activity by 289.7% and 1.5 mM Si decreasing root H₂O₂ content by over 50%. The 1.5 mM Si treatment also reversed PA-induced photosynthetic suppression, numerically increasing net photosynthetic rate by 381.0% and increasing chlorophyll content by 25.8%. Field validation showed that 1 mM Si reversed PA-induced yield loss (restoring to control levels), improved fruit quality and reduced symptom incidence. Exogenous sodium silicate (Na₂SiO₃) effectively mitigates PA-induced phytotoxicity by rebalancing redox homeostasis, enhancing photosynthesis and improving root growth. However, the specific contribution of Si versus co-supplied Na⁺ requires further verification. This study demonstrates the potential of sodium silicate as a sustainable soil amendment to enhance pepper tolerance under intensive cultivation.