Combined Stress from Thiamethoxam and Pyrethroids Exacerbates Metabolic Dysregulation and Quality Deterioration in Pak Choi (Brassica chinensis L.)

The combined stress of neonicotinoid and pyrethroid insecticide mixtures may exert phytotoxic effects distinct from those of individual insecticides. This study investigated the impacts of thiamethoxam, three pyrethroids (lambda-cyhalothrin, deltamethrin, and fenvalerate), and their pairwise combinations with thiamethoxam on oxidative stress, physiological functions, and metabolism in pak choi (Brassica chinensis L.). Here, combined stress inflicted significantly greater metabolic disorder and quality deterioration than individual stress. It compromised antioxidant defenses, induced malondialdehyde accumulation (24.6–55.5%), impaired chloroplast and mitochondrial functionality, decreased phytohormones, and accelerated amino acid catabolism. These perturbations collectively diminished quality parameters in pak choi. Metabolomic analyses validated that combined stress disrupted the tricarboxylic acid cycle, amino acid metabolic pathways, and phenylpropanoid biosynthesis. Enhanced dysregulation of carbon and nitrogen metabolism was identified as a primary mechanism underlying amplified phytotoxicity under combined stress conditions. This study demonstrates that combining insecticides with distinct modes of action exacerbates metabolic and quality deterioration in leafy vegetables.