Photosynthetic characterization of three dominant plant species in the saline-alkaline soil of the Yellow River Delta, China

The diurnal variations of photosynthesis of three dominant species, including Glycine soja, Phragmites australis, and Cynanchum chinensis, in the Yellow River Delta in China have been studied under the same natural conditions using a Li-6400 portable photosynthesis system. The results showed that the curves of diurnal variations of net photosynthetic rate (P-N) of the three plants were different. The diurnal variation of P-N on C. chinensis was a midday depression pattern and had two peaks. However, P-N of G. soja and P. australis showed single-peak curves. The transpiration rate (E) of G. soja was significantly higher than that of P. australis and C. chinensis, both showed single-peak curves. In general, the diurnal course of stomatal conductance (g(s)) followed the same pattern of P-N. A similar diurnal pattern of intercellular CO2 concentration (C-i), vapor pressure deficit (VPD), and water use efficiency (WUE) was observed among different species. VPD showed single-peak curves, while WUE was characterized by double-peak curves, which was contrary to C-i. Linear correlations among photosynthetic variables and key environmental factors indicate high positive correlations between P-N and E, P-N and photosynthetic active radiation, P-N and leaf temperature (T-leaf), P-N and VPD, and between P-N and g(s) except C. chinensis. Negative correlations among P-N and relative humidity, P-N and C-i were found. The irradiance response curves derived from the leaves were substantially affected by different species. C. chinensis showed highest apparent quantum efficiency, followed by P. australis and G. soja, while apparent dark respiration (R-d), convexity (k), light saturation point, and maximum gross CO2 assimilation rate (P-max) of G. soja were higher than those of P. australis and C. chinensis. The irradiance response curve of P-N and WUE of different plant species followed the same order: G. soja>C. chinensis>P. australis. They were both higher than most of other species. It was concluded that plant species adapting to the saline-alkaline habitat showed higher photosynthesis. In addition, G. soja is also effective to improve saline-alkaline soil quality.