Abstract: Manchurian ash, (Fraxinus mandshurica), typically distributed along lower streams and regenerated under the canopy of natural forests, is an important hardwood silvicultural species in the temperate forest regions of northeast China. On most reforestation sites, however, Manchurian ash plantations exhibit very restricted growth. Therefore, the main objectives of this study were to gain an insight into the physio-ecological mechanisms responsible for the poor growth and low productivity of Manchurian ash in plantations and to analyze the environmental limitations of soil water on photosynthetic activity. Two-year old Manchurian ash seedlings were planted in rainproof soil pots (8 m length × 0.5 m width × 0.3 m depth). When seedlings had restored vigor, soil water potentials were maintained at five treatment levels in the different pots: 0 - -0.02 MPa (W1), －0.02 - －0.04 MPa (W2), －0.04 - －0.06 MPa (W3), －0.06 - －0.08 MPa (W4) and －0.08 - －0.16 MPa (W5). Soil water potential was controlled using a specially designed root-sphere osmotic irrigation technique. After 3 months of soil water conditioning, the diurnal course of gas exchange, PSⅡ photochemical efficiency (Fv/Fm) and rubisco carboxylation activity of Manchurian ash seedlings grown under the different soil water conditions were investigated. All treatments showed a maximum net photosynthetic rate (Pn) in the early morning (approx. 6∶00) and a noontime depression of photosynthesis that was the greatest in the low soil water treatment. The cumulative values of diurnal photosynthesis (mmol CO2·m-2·d-1) from W1 to W5 were in the proportion of 100∶96∶64∶60∶52. The initial depression of Pn in the early morning for all the treatments was caused primarily by lowered stomatal conductance, but the strong inhibition of photosynthesis that occurred from 10∶00 to 14∶00 for W3, W4, and W5 treatments was due predominately to non-stomatal limitations. Simultaneous with the morning-to-noon or soil-water-ordered depression of Pn, there was a corresponding decrease in PSII photochemical efficiency (Fv/Fm) and in rubisco carboxylation activity, particularly for the W3, W4 and W5 treatments. There were no significant differences in diurnal average water use efficiency (WUE) among treatments with the exception of a pronounced lower WUE in W1. The morning to noon depression of Pn observed in the well-watered treatments (W1 and W2) indicated that climatic conditions (e.g. high light levels, high temperature, and low humidity) of the experiment differed from the species natural environment and caused physiological stress on the wet-adapted and shade-tolerant Manchurian ash seedlings. This stress was intensified as soil water deficiency increased from W3 to W5 treatments.