Dose-Dependent Phytotoxic Mechanism of UV-328 on the Photosynthetic System of the Moss Niphotrichum japonicum (Dozy & Molk.) Bedn.-Ochyra & Ochyra

First detected in environmental matrices in the early 2010s, UV-328 has aroused widespread concern due to its toxic risks to terrestrial vascular plants. However, its impacts on photosynthetic performance of bryophytes remain unclear. In this study, we exposed the moss Niphotrichum japonicum to a range of UV-328 concentrations and examined its morphological changes, oxidative stress, photosynthetic pigments, chlorophyll fluorescence kinetics, and energy allocation. Our results show that UV-328 caused dose-dependent leaf yellowing, surface shrinkage, and papillae collapse. It degraded photosynthetic pigments, disrupted chlorophyll a/b balance, and induced excess reactive oxygen accumulation. Chlorophyll fluorescence data indicated that UV-328 noticeably lowered Fv/Fm, Y(II), and qP. OJIP kinetics confirmed multi-target damage to the photosynthetic chain, including disruption of the oxygen evolving complex, decline of PSII energetic grouping, blockage of electron transfer from QA− to QB, inhibition of plastoquinone pool turnover, and PSI terminal electron transport. Energy flux parameters pointed to fewer active PSII reaction centers and a higher light-harvesting load on each remaining center, which uncoupled light capture from electron transfer. The concentration that caused clear photosynthetic damage fell between 100 and 150 μM. In conclusion, UV-328 impacts moss photosynthesis through oxidative stress, structural damage, blocked electron transport, and energy imbalance.