As the dominant species of the riparian forests along the Tarim River in Xinjiang, northwest China, Populus euphratica plays an important role in maintaining the functions of riparian ecosystem. Recent embankments along the river have altered the soil, water and salinity conditions of the riparian zones, which is expected to impact the survival and growth of P. euphratica seedlings as well as the regeneration of the riparian forests. In order to assess the potential effects of these alterations on P. euphratica, the photosynthesis of P. euphratica seedlings under different soil salinity gradients was explored. The results indicated that the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and water use efficiency (WUE) of the seedlings were increased with the increase of the photosynthetically active radiation (PAR) in a range of 0-3 500 μmol·m-2·s-1, whereas the change of intercellular CO2 concentration (Ci) was reverse. Photosynthesis of seedlings was restrained, which was evidenced by the decrease of the mean values of Pn, Gs and Tr under the mild salinity stress (0.5 mol·L-1). However, the WUE of seedlings was high to reduce water loss through regulating stomatal closure. Under severe salinity stress (2 mol·L-1), the Tr of seedlings was significantly decreased due to the insensitive stomatal regulation to light, and the average of Pn was decreased by about 75%. With the increase of salinity, the maximal net photosynthetic rate (Pnmax), light-saturation point (LSP), dark respiration rate (Rd) and apparent quantum yield (α) of the seedlings were gradually decreased, but the light compensation point was gradually increased. The results suggested that the capability of light absorption, utilization and conversion of the seedlings was restrained under salinity stress.
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