Antibiotic toxicity screening on seedling emergence: beyond traditional species

Antibiotics are prevalent environmental pollutants with documented plant uptake and effects. Germination and seedling emergence are critical stages of plant development, making toxicity tests valuable for assessing the terrestrial risk. This study aims to address the need to understand species and antibiotic-dependent effects of 10 antibiotics across 23 plant species, including non-cultivated and geographically diverse species, through a Tier I phytotoxicity screening in which all antibiotics were tested at a single nominal concentration of 1 mg L^-1, an upper-bound environmentally relevant scenario. Results revealed that antibiotic toxicity is highly species and antibiotic-dependent, highlighting the need to evaluate effects on under-studied species. Some global trends were observed; fluoroquinolones (ciprofloxacin, enrofloxacin, ofloxacin) stimulated germination and root growth in legumes and grasses, while black knapweed (Centaurea nigra) consistently exhibits germination reductions (30-53%) and root growth inhibition under all antibiotic exposures. Florfenicol exposure decreased root length and biomass in Chinese cabbage (Brassica rapa subs. Pekinensis) by nearly 100%, contrasting with stimulation observed in rice (Oryza sativa). The importance of assessing sublethal effects, as root and biomass changes, in addition to germination, for a more comprehensive phytotoxicity assessment is demonstrated. Standardized test conditions may overlook species requiring specific germination conditions. Sorption of antibiotics to filter paper affected exposure concentrations, emphasizing the importance of chemical quantification before and after tests. This study highlights the need for adaptive phytotoxicity protocols, careful experimental design to obtain statistically significant results, and inclusion of non-cultivated species as bioindicators to better assess antibiotic risks to terrestrial plants.