Elevated CO₂ and 28-homobrassinolide alleviate galaxolide (HHCB) stress in Eichhornia crassipes via photosynthetic protection and rhizosphere microenvironment modulation.

Elevated CO2 (eCO2), a major driver of climate change, is altering aquatic environments. Meanwhile, galaxolide (HHCB) accumulates in water bodies, threatening aquatic plants. Foliar 28-homobrassinolide (HBL) can enhance plant stress resistance. However, how eCO2 and HBL interact to regulate photosynthesis, carbon allocation, and rhizosphere processes under HHCB stress remains unclear. This study therefore investigated how eCO₂ alone alleviates HHCB stress in Eichhornia crassipes and how the combination of eCO₂ and HBL shifts the plant's defense strategy towards the rhizosphere. Results showed that HHCB reduced the biomass of E. crassipes by 36.4%. eCO2 alleviated the stress by promoting the tricarboxylic acid cycle, fatty acid metabolism, and photorespiration. Combined eCO2 and HBL alleviated stress by improving rhizosphere environment. 13C tracing results showed that, relative to the combined eCO2 and HHCB, added HBL increased root 13C atom% by 4.0%, indicating upregulated root metabolic activity. This drove a Rhodanobacter dominated community, potentially reducing HHCB translocation into plant tissues. These findings clarify how eCO2 alone and eCO2 with HBL alleviate HHCB stress, aiding prediction of primary producer responses to combined climate and pollutant pressure.