Translocation, Transformation, and Phytotoxicity of Sulfadiazine and <i>N</i>₄-Acetylsulfadiazine in Rice Plants

This study investigates the uptake, biotransformation, and phytotoxicity of sulfadiazine (SDZ) and its acetyl derivative <i>N</i>₄-acetylsulfadiazine (NASDZ) in rice. Results showed that rice was more tolerant to NASDZ, with lower malondialdehyde and reactive oxygen species levels but higher antioxidant enzyme activities (SOD, POD, and CAT). The maximum accumulations of SDZ in roots and shoots were 19.3 ± 1.0 and 3.6 ± 1.1 μg/g, while NASDZ were 18.6 ± 2.5 and 3.5 ± 1.4 μg/g, respectively. SDZ exposure generated more metabolic intermediates, including deamination, hydroxylation, glycosylation, acetylation, and formylation products, while NASDZ metabolism was documented for the first time. Key genes involved in biotransformation include cytochrome P450, acetyltransferase, glycosyltransferases, and methyltransferase. Density functional theory calculations showed structural differences affecting reactive sites and intermediates. SDZ disrupted lipid metabolism, while NASDZ altered carbohydrate and amino acid pathways, highlighting their selective effects on rice metabolism. Our data help understand sulfonamide biotransformation and phytotoxicity in rice.