Upstream-downstream pollution gradient of carbendazim in Pazarsuyu (Giresun) stream: Spectral shift signatures and synergistic genotoxic and meristematic responses in Alliumcepa.

Carbendazim (CBZ) is a systemic fungicide frequently detected in aquatic environments, where it coexists with multiple pollutants that may alter its toxicity. This study aimed to determine the environmental distribution of CBZ along an upstream-downstream pollution gradient in the Pazarsuyu Stream (Türkiye) and to evaluate its individual and synergistic toxic effects using the Allium cepa bioassay. Surface water samples were collected from five stations representing different agricultural impact zones, and CBZ concentrations were determined using dispersive liquid-liquid microextraction method. Allium bulbs were germinated in these natural water samples and in a laboratory-prepared CBZ solution, and oxidative stress parameters, physiological growth, cytogenetic damage, comet assay endpoints, and meristematic tissue integrity were comprehensively assessed. CBZ concentrations increased progressively from upstream (non-detectable) to downstream (8.53 ± 1.47 ng/mL), confirming cumulative pollution input. Exposure to contaminated water and CBZ alone induced significant oxidative stress, evidenced by increased malondialdehyde, elevated superoxide dismutase and catalase activities, and depletion of glutathione. Comet test results showed that both environmental water samples and CBZ applied at equivalent environmental doses significantly increased DNA damage, but the damage observed in Pazarsuyu samples was higher compared to CBZ alone. Similarly, increases in the frequency of chromosomal abnormalities were found to be more pronounced in environmental samples. This indicates that environmental toxicity stems not only from CBZ but also from synergistic effects with other pollutants. The correlation between spectrally determined DNA destabilization and cytogenetic damage findings reveals that CBZ can directly interact with DNA and disrupt genetic integrity even at environmental doses. Overall, the findings demonstrate that environmentally relevant CBZ concentrations disrupt cellular redox homeostasis, genomic stability, and plant development, and that its ecological risk is substantially amplified in complex environmental matrices. These results emphasize the importance of incorporating synergistic mixture effects into pesticide risk assessment frameworks to better reflect real environmental conditions.