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Route-Specific Phytotoxicity: Foliar Polystyrene Nanoplastics Inhibit Rice Photosynthesis
Summary
Researchers compared root versus foliar exposure routes for polystyrene nanoplastics in rice plants and found that foliar exposure caused far more sustained damage to photosynthesis. The nanoplastics accumulated in leaves and co-localized with chloroplasts, leading to dramatic reductions in photosynthetic pigments, ATP production, and carbon fixation. The study suggests that airborne nanoplastic deposition on crop leaves may represent an underappreciated route of agricultural harm.
We examine the effects of polystyrene (PS) NPs at 5.4 ± 1.0 nm on rice (Oryza sativa L.) under two scenarios: a comparative 14-day hydroponic exposure (3.6-35.7 μg PS plant-1 d-1) of root vs foliar exposure and a 30-day soil-based foliar exposure (5.3-73.4 μg PS plant-1 d-1) from heading to maturity. PS NPs exhibit route-specific phytotoxicity: while only high-dose root exposure inhibits photosynthesis, foliar exposure causes sustained inhibition. Importantly, soil-based foliar exposure also inhibits photosynthesis without the PS NP translocation to grains. This foliar-specific inhibition correlates directly with PS accumulation in leaves and is mechanistically attributed to its colocalization with chloroplasts (Pearson's r = 0.592), as confirmed by confocal imaging in high-dose (35.7 μg PS plant-1 d-1) exposed leaf. The colocalization impairs both the light-dependent reactions and carbon fixation during photosynthesis, as evidenced by significant reductions in photosynthetic pigments (54.4-61.0%), Hill reaction activity (49.9-70.2%), ATP production (22.1-24.3%), net photosynthetic rate (84.5-89.0%), Rubisco activity (84.6-87.5%), stomatal conductance (47.9-69.5%), and transpiration rate (47.4-48.9%). Transcriptomic analysis identifies the genetic basis of this inhibition, showing significant downregulation of the core photosynthesis pathway and key genes for photosynthetic-antenna proteins, carbon fixation, and carotenoid/porphyrin metabolism. These findings provide mechanistic insights into PS NP-induced photosynthesis inhibition and underscore atmospheric NPs as an emerging threat to global food security.