Effects of SO₂ stress on photosynthetic physiological characteristics of Cinnamomum camphora seedlings

In order to explore the effects of SO₂ stress on photosynthetic fluorescence and physiological and biochemical characteristics of Cinnamomum camphora seedlings, one-year-old C.camphora seedlings were used as experimental materials, and four treatment groups with SO₂ concentration of 0, 1.0, 10 and 100mg/m³ were set up by simple fumigation device.The differences of physiological and biochemical characteristics, photosynthetic characteristics and photosystem performance of Cinnamomum camphora leaves were compared and analyzed on the 1 st, 3 rd, 7 th, 15 th, 30 th and 60 th days of stress. The results showed that SO₂ caused damage to the cell membrane of C.camphora leaves, resulting in a significant increase in malondialdehyde ( MDA ) content, superoxide dismutase ( SOD ) activity and soluble sugar ( SS ) content to alleviate oxidative and osmotic damage. C.camphora seedlings showed strong resistance to mild SO₂ stress ( 1.0 mg/m³SO₂ concentration ), while the resistance to moderate and severe SO₂ stress decreased in turn. With the extension of SO₂ stress time, the relative chlorophyll content ( SPAD ) of C.camphora leaves decreased significantly, and the net photosynthetic rate ( P_n ), intercellular carbon dioxide concentration ( C_i ) and instantaneous carboxylation rate ( CE ) all showed a downward trend, while stomatal conductance ( G_s ) and transpiration rate ( E ) increased first and then decreased, and instantaneous water use efficiency ( WUE ) showed an upward trend. The activity of PSII reaction center in C.camphora leaves showed different degrees of increase under mild SO₂ stress. Under moderate SO₂ stress, ABS/RC, TRo/RC and ETo/RC increased, while Dio/RC decreased, indicating that the PSII reaction center of C.camphora had a certain tolerance to 10 mg/m³ SO₂ concentration. Under severe SO2 stress, ABS / RC, TRo / RC and ETo / RC of C.camphora decreased significantly, while DIo/RC increased significantly, indicating that more absorbed light energy was lost in the form of heat energy, reflecting that 100 mg/m³ SO₂ concentration caused significant damage to the photosystem. On the whole, C.camphora seedlings showed significant resistance to mild SO₂ stress, and also had certain adaptability to moderate SO₂ stress, but caused obvious damage under severe stress, resulting in a significant decrease in relative chlorophyll content, photosynthetic capacity and photosystem II performance.