Aging alters nanoplastics-propranolol interactions in lettuce: implications for plant toxicity and microbiomes

This study investigated whether UV aging and surface functionalization of polystyrene nanoplastics (NPs) modify their interactions with propranolol (PRO) and affect lettuce physiology and endophytic bacterial communities. Lettuce was exposed for 30 days to pristine or aged NPs (5 or 50 mg L^-1), PRO (100 μg L^-1), or their combinations. The results showed that UV aging enhanced PRO adsorption, potentially owing to increased surface oxidation and specific surface area. Moreover, aged NPs produced numerically higher PRO contents in both roots and leaves than their pristine counterparts, with a maximum increase of 69.4 ± 5.68% observed in leaves. PRO alone reduced chlorophyll a and leaf dry weight by 31.3 ± 1.89% and 21.5 ± 1.59%, respectively, and induced oxidative stress, disrupted nitrogen metabolism, and altered endophytic bacterial communities. NP co-exposure modified lettuce responses to PRO in a concentration- and aging-dependent manner. At 5 mg L^-1, NPs partially alleviated PRO-induced phytotoxicity, whereas at 50 mg L^-1, disturbances in oxidative status, antioxidant defenses, and nitrogen metabolism were more pronounced, particularly in the presence of aged NPs. Microbial analyses revealed tissue-specific responses of lettuce endophytes to NP+PRO co-exposure. Leaf alpha diversity was more responsive to PRO exposure, whereas root endophytic richness and community structure were more responsive to NP aging. Treatment-dependent changes in dominant genera and differentially abundant taxa indicated distinct root and leaf responses to combined exposure. Overall, NP concentration and aging were important factors influencing PRO accumulation and the physiological and endophytic bacterial responses of lettuce.