Abstract: The characteristics of photosynthetic rate in response to light intensity and the rules of diurnal variations in photosynthesis, light use efficiency and water use efficiency of the representative soybean leaves during the filling period in the Northeastern China were determined and investigated under the artificial and field conditions by using the portable photosynthesis system(LI-6400).The results showed that photosynthetic rate in response to the photosynthetically active radiation flow followed a typical nonlinear curve,which could be expressed by Michalis-Menten equation.Under natural light conditions,apparent maximum photosynthetic rate and initial quantum efficiency were 37.25μmol CO2m-2s-1 and 0.0376μmol CO2m-2s-1 respectively.Changes in photosynthetic rate,transpiration rate,stomatal conductance,photosynthetically active radiation,air temperature,light use efficiency and water use efficiency along with the time elapsed could be simulated by sine curves,but the peaks came forth differently in time.The peak of the photosynthesis and stomatal conductance appeared at the forenoon,while the peak of the photosynthetically active radiation appeared at noon and for transpiration and air temperature in the afternoon,which indicated that fall in photosynthesis because of stomatal limit caused by higher air temperature,rather than the light stress.Light use efficiency of soybean decreased with the time elapsed and stabilized between 0.015 and 0.02μmol CO2m-2s-1 at about 10:30.Water use efficiency of soybean differed significantly.It was higher in the forenoon than that in the afternoon,which could also be caused by temperature.

Key words: Soybean, Soybean, Filling period, Photosynthesis, Transpiration, Stomatal limit, Light use efficiency, Water use efficiency