Mitigation of Fomesafen Phytotoxicity in Sugar Beet by Biochar through Modulation of Antioxidant Metabolism and Photosynthetic Gene Regulation

Fomesafen, a diphenyl ether herbicide, imposes severe phytotoxicity on a certain plant through soil residues. To clarify the phytotoxicity mechanism of fomesafen residues on subsequent sugar beet and the mitigation the basis of biochar, this study was conducted on sugar beet (Beta vulgaris L.), a sensitive subsequent crop. At 0.025 mg kg–1, it reduced biomass by 61.79%, specific leaf weight by 33.34%, chlorophyll a and b by 14.88% and 45.23%, and photosynthetic performance (PIABS) by 22.28%, while increasing DIo/RC by 5.80%. Net photosynthetic rate declined by 28.73%, with leaf deformities, yellowing, and oxidative stress indicated by superoxide, hydrogen peroxide, antioxidant enzyme activation, and 95.14% higher malondialdehyde. Biochar (1%) alleviated toxicity by upregulating psbA genes, enhancing glutathione and proline metabolism (BVRB_9g206010, BVRB_2g025900), and improving stress resistance. This study reveals the mechanisms of fomesafen-induced photosynthetic inhibition and oxidative damage, and biochar’s mitigation potential, providing guidance for residue management in rotation systems; future research will integrate field pesticide metabolism analysis and conduct field trials to verify the effect.