| Summary: | Sulphurous acid derived from sulfur dioxide (SO<sub>2</sub>) emission leads to the pollution of irrigation water and the inhibition of plant growth. The safe concentration threshold of NaHSO<sub>3</sub> in plants should be clarified to promote agricultural production. In this study, <i>Orychophragmus violaceus</i> seedlings were used as experimental materials and five NaHSO<sub>3</sub> concentrations (i.e., 0, 1, 2, 5, 10 mmol·L<sup>−1</sup>) were simultaneously sprayed on the leaf surface of different seedlings separately. Leaf physiology responses under different concentrations were analyzed. The NaHSO<sub>3</sub> did not promote photosynthesis in <i>O. violaceus</i> under the 1 and 2 mmol·L<sup>−1</sup> treatments. It was conducive to the net photosynthetic rate (<i>P<sub>N</sub></i>), photorespiration rate (<i>R<sub>p</sub></i>), chlorophyll content, actual photochemical quantum yield (Y<sub>II</sub>) and photochemical quenching (qP) under the 5 mmol·L<sup>−1</sup> treatment. However, quantum yield of regulated energy dissipation (Y<sub>NPQ</sub>) and nonphotochemical quenching (NPQ) were inhibited. Under the 10 mmol·L<sup>−1</sup> treatment, <i>P<sub>N</sub></i>, chlorophyll content, Y<sub>II</sub>, qP, dark respiration rate (<i>R<sub>d</sub></i>) and electron transport rate (ETR) showed significant decreases, while the photorespiration portion (<i>S<sub>p</sub></i>) significantly increased. Our results demonstrated that NaHSO<sub>3</sub> provided a sulfur source for plant growth and interfered with the redox reaction of the plant itself, and its role as a photorespiratory inhibitor might be masked.
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