Route-Specific Phytotoxicity: Foliar Polystyrene Nanoplastics Inhibit Rice Photosynthesis

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.