Abstract:Phytolacca americana L.(Phytolaccaceae) is a new manganese hyper-accumulator plant from Southern China. In order to estimate the relationships among manganese tolerance of P. americana and photosynthesis and chlorophyll fluorescence parameters, chlorophyll contents, gas exchange and chlorophyll fluorescence parameters were measured by glasshouse experiments. With the manganese concentration in the media increasing, chlorophyll a, chlorophyll b and total chlorophyll contents of P. americana decreased, and net photosynthetic rate(Pn), stomatal conductance(Gs) also gradually declined, but intracellular CO2 concentration(Ci) gradually increased. Chlorophyll a/b ratio and transpiration rate(Tr) was the highest at a manganese supply level of 1000 μmol?L-1. The maximum quantum yield of PSⅡ photochemistry (Fv/Fm), the effective quantum yield of PSⅡ photochemistry(EQY), and the photosynthetic electron transport rate(ETR) were gradually decreased. At the manganese concentration of 5 000 μmol?L-1 in the solution, the photochemical quenching(qP) reached the maximum, and non-photochemical quenching(NPQ) also reached the maximum when photosynthetic active radiation(PAR)was higher than 239 μmol photons?m-2?s-1. It is indicated that P. americana has photosynthetic protection in some degree under manganese stress.