Safety of herbicide Sigma Broad on Radix Isatidis (Isatis indigotica Fort.) seedlings and their photosynthetic physiological responses

• We defined the safety of herbicide Sigma Broad on Chinese medicine plant Radix Isatidis seedlings.
• The recommended dosage (13.48 g ai/ha) of Sigma Broad inhibited the growth of Radix Isatidis significantly.
• We measured the chlorophyll fluorescence and P700 parameters to investigate its photosynthetic physiological mechanisms.
• Unsafety of Radix Isatidis seedlings to Sigma Broad might be associated with damage of PS II and PS I structure.

The study was performed to explore the safety of sulfonylurea herbicide Sigma Broad (mesosulfuron-methyl and iodosulfuron-methyl sodium) on Radix Isatidis (Isatis Indigotica Fort.) seedlings and the photosynthetic physiological response of the crop to the herbicide. Through pot experiment, biological mass, leaf area, photosynthetic pigment content, photosynthetic rate, chlorophyll fluorescence characteristics, and P700 parameters of Radix Isatidis seedlings were analyzed 10 days after Sigma Broad treatment at 5th leaf stage. The results showed that biological mass, leaf area, total chlorophyll, chlorophyll a, chlorophyll b, and carotenoids content, Pn, transpiration rate, stomatal conductance, PSII maximum quantum yield, PSII effective quantum yield, PSII electron transport rate, photochemical quenching, quantum yield of regulated energy dissipation in PS II, maximal P700 change, photochemical quantum yield of PS I, PS I electron transport rate, and quantum yield of non-photochemical energy dissipation due to donor side limitation in PS I decreased with increasing herbicide dosages, whereas chlorophyll a/b, stomatal limitation value, initial fluorescence, quantum yield of non-regulated energy dissipation in PS II and quantum yield of non-photochemical energy dissipation due to accepter side limitation in PS I increased. In conclusion, treatment with Sigma Broad higher than 13.48 g ai/ha was extremely unsafe to Radix Isatidis seedlings, which might be associated with damage of chloroplast, PS II and PS I structure, electron transport and its distribution between PS II and PS I, and decreased photosynthesis caused by the herbicide.

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