Determination of photosynthetic parameters
Photosynthetic rate (Pn), stomatal conductance (Gs), intercellular carbon dioxide concentration (Ci), and transpiration rate (Tr) were measured using a portable photosynthesis system (Li-6400, LiCor, Lincoln, NE, USA) on the second fully expanded leaf of soybean seedling. The WUE was calculated as the ratio of Pn/Tr. Air temperature, effective photosynthetic radiation (PAR), air relative humidity and CO2 concentration were maintained at 25°C, 800 μmol· m-2 · s-1, 70%, 400 μmol · mol-1, respectively.