Abstract: In this paper, the changes of photosynthetic characteristics and fluorescence kinetic characteristics of chrysanthemum leaves under different low light levels were discussed, so as to analyze the internal mechanism of photosynthetic attenuation of chrysanthemum leaves under low light conditions, with the purpose of providing reference for the defense against low light disasters and the regulation of light environment in the seedling stage of chrysanthemum. In order to reveal the influence of shading treatments on greenhouse chrysanthemum leaf photosynthetic characteristics, a controlled experiment with varieties of cut-flower chrysanthemum ‘Hongmian’ was conducted in August 2019 to October 2019 in Nanjing University of Information Science and Technology experimental greenhouse. Chrysanthemum seedlings were planted in the greenhouse pools. When the height of seedlings reached 30cm, shelves were set up 1.5m above the ground, and the shelves were covered with black sunshade nets of 60%(L1) and 30%(L2) light transmittance respectively. Treatments lasted 3(T1), 6(T2), 9(T3) and 12(T4) days, contrasting with normal natural light intensity(CK). Photosynthetic parameters and fast chlorophyll fluorescence induction kinetic parameters of chrysanthemum were determined under different treatments. The results showed that, (1)the light saturation point(LSP), photosynthetic rate at irradiation saturation(Pmax), apparent quantum efficiency(AQE), stomatal conductance(Gs) and transpiration rate(Tr) of chrysanthemum leaves decreased with the increase of treatment days under different shading treatments, while the light compensation point(LCP), dark respiration rate(Rd), stomatal limit value(Ls) and water use efficiency(WUE) were opposite. In addition, the variation range of all indicators treated by L2 is larger than that of L1. After 12 days of shading, Pmax decreased to 51.1%, Gs decreased to 62.4%, while LCP increased to 2.75 times, compared with CK. (2)The maximum fluorescence(P) of chrysanthemum leaves increased with the shading time and shading degree. (3)In terms of PSⅡ energy distribution ratio, the effects of shading treatment on chrysanthemum leaves were to increase the quantum yield captured by the reaction center(φPo) and the quantum yield for electron transfer(φEo), while decreasing the quantum ratio for heat dissipation(φRo). Among them, φPo reaches a maximum value of 0.85 after L2T3 treatment, and φRo drops to a minimum value of 0.16 after L1T4 treatment. These results indicated that shading increased the probability that the light absorbed by chrysanthemum leaves captured by PSⅡ reaction center, while the part used for heat dissipation decreased. (4)In terms of PSⅡ reaction center activity, the values of light energy absorbed by the unit active reaction center(ABS/RC), the energy dissipated by the unit reaction center(DIo/RC), energy recovered by the unit reaction center for reducing QA(TRo/RC) and the unit reaction center for electron transfer(ETo/RC) all decreased after shading treatments. Among them, ABS/RC and DIo/RC reached their minimum values under L2T3 treatment, which were 1.176 and 0.198, respectively. This study confirmed that weak light treatments reduce the chrysanthemum leaf intercellular CO2 concentration, the activity of the PSⅡ reaction center and the ability to restore the PQ, QB. The light energy absorbed and captured by the antenna pigment is also reduced. In conclusion, the photosynthesis of chrysanthemum leaves is reduced.

Key words: Chrysanthemum, Weak light, Photosynthesis, Rapid fluorescence, OJIP curve