Abstract：In this study, we selected Egeria najas as the sample plant, which was exposed to different concentrations of Zinc oxide nanoparticles (ZnO NPs) suspensions for six days. The effects of different concentrations of ZnO NPs on photosynthetic processes of Egeria najas were explored respectively, by analyzing the O-J-I-P fluorescence induction dynamics curve and the pulse transient fluorescence induction dynamics curve. ZnO NPs strengthened the connectivity between photosystem Ⅱ (PSⅡ) units, promoted the efficiency of the electron transport at the acceptor side of PSⅡ and the utilization of the absorbed light energy, indicated by the significant decrease (p<0.05) in the net rate of PSⅡ closure （M0）, the relative variable fluorescence intensity at phase J （VJ） and the effective dissipation of an active RC（DI0/RC）, and the significant increase (p<0.05) in the maximum quantum yield of primary photochemistry （ΦP0）, the efficiency with which a trapped exciton can move an electron into the electron transport chain further than Q-A（Ψ0）, the quantum yield of electron transport （ΦE0） and the effective quantum yield of electron transport at PSⅡ （′PSⅡ） after exposure to ZnO NPs suspensions. These results suggested that ZnO NPs improved the photosynthetic performance to some degree. Corresponding concentrations of Zn2+ solution was also used to cultivate Egeria najas. Zn2+ lowered the connectivity between PSⅡ units, inhibited the electron transport at the acceptor side of PSⅡ and the utilization of absorbed light energy and damaged the PSⅡ reaction centers, as indicated by the significant increase (p<0.05) in the net rate of PSⅡ closure, the relative variable fluorescence intensity at phase J, the effective dissipation of an active RC, the effective antenna size of an active RC （ABS/RC）, the energy trapping capacity per active PSⅡ RC (TR0/RC), and the quantum yield of dissipation through fluorescence and basal thermal processes （′NO） and the significant decrease (p<0.05) in the maximum quantum yield of primary photochemistry （ΦP0）, the efficiency with which a trapped exciton can move an electron into the electron transport chain further than Ｑ-A（Ψ0）, the quantum yield of electron transport （ΦE0） and the effective quantum yield of electron transport at PSⅡ （′PSⅡ） after exposure to Zn2+ solution. These results suggested that Zn2+ inhibited photosynthetic processes of Egeria najas. When the sample plant was exposed to ZnO NPs suspensions, the effect of the Zn2+ released from ZnO NPs suspensions on the sample plant was not obvious, which meant that the enhancement was stronger than the inhibition.
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