Combined effects of difenoconazole and salinity stress on growth, photosynthetic pigments, and oxidative responses in tomato (Solanum lycopersicum L.)

Abstract

Environmental contamination by pesticides and salts is a growing agricultural issue, especially in arid and semi-arid regions. Difenoconazole (DIF), a commonly used fungicide, and sodium chloride (NaCl), a widespread salinity stressor, often coexist in soil and water, yet their combined effects on non-target crops such as tomato (Solanum lycopersicum) are not well understood. This study assessed the individual and combined impacts of DIF (0.5 L ha−1) and NaCl (150 mM) on tomato seedlings by evaluating morpho-physiological, oxidative, and biochemical responses. Results showed significant reductions in shoot and root lengths under DIF (22.7%, 15.8%), NaCl (31.1%, 35.6%), and combined exposure (19.4%, 29.9%) compared to the control. Chlorophyll a and b levels decreased, with chlorophyll b reduced by 84% under co-exposure, indicating synergistic pigment degradation. Carotenoids increased (up to 96.7% under NaCl), suggesting a compensatory antioxidant mechanism. Oxidative stress markers malondialdehyde (MDA) and hydrogen peroxide (H2O2) increased under all treatments but showed antagonistic trends under combined exposure. Detoxification enzymes peroxidase (POD) and glutathione S-transferase (GST) were highly activated under co-exposure, while antioxidant enzymes catalase (CAT) and ascorbate peroxidase (APX) showed partial recovery. Proline accumulation peaked under DIF (389%) but decreased under combined stress (61%), indicating an antagonistic interaction. Flavonoid content (FLV) and phenylalanine ammonia-lyase (PAL) activity increased significantly under co-exposure, reflecting stimulated secondary metabolism. Overall, DIF and NaCl co-exposure triggered complex phytotoxic responses, primarily synergistic, impairing plant growth and metabolism. These findings underscore the need for integrated risk assessments of agrochemical and salinity co-stress, crucial for sustainable agriculture and environmental protection in vulnerable regions.