The multi-barrier fate of strobilurin fungicides in Dendrobium officinale: uptake, sequestration in root cell walls, and high cumulative dietary risk

The high residual levels and prolonged persistence of strobilurin fungicides prompted this systematic investigation into the environmental fate, accumulation mechanisms, and dietary exposure risks of five key compounds: azoxystrobin, kresoxim-methyl, picoxystrobin, pyraclostrobin, and trifloxystrobin. A hydroponic system was employed to simulate post-application uptake. Root exposure resulted in significant accumulation (bioconcentration factors: 1.6-49) but limited translocation to stems (bioconcentration factors: 0.008-0.31, translocation factor < 0.1), whereas foliar exposure led to effective retention in leaves (translocation factor < 1). To elucidate the underlying mechanisms, correlation analyses revealed that translocation was governed by molecular properties: root-to-stem movement correlated with molecular weight, and leaf-to-stem movement with water solubility (R² > 0.71, p < 0.05). Uptake pathway studies indicated that four fungicides primarily entered roots apoplastically, whereas kresoxim-methyl relied more on the symplastic route. Uptake was predominantly passive, though azoxystrobin and trifloxystrobin appeared to involve membrane transporters, suggesting interspecific differences. Critically, subcellular analysis identified root cell walls as the dominant sink, sequestering 55-88% of absorbed fungicides, which fundamentally limited upward mobility. Based on these fate parameters and integrated with consumption data of 73 crops, probabilistic cumulative exposure assessment predicted an unacceptable chronic risk for children under 14 years, with hazard quotients reaching 3896%. This work elucidates a multi-barrier defense mechanism in plants and underscores the necessity of cumulative risk assessment for medicinal crops under multi-pathway exposure.