The maximum carboxylation rate of Rubisco (V_cmax) and maximum rate of electron transport (Jmax) for the biochemical photosynthetic model, and the slope (m) of the Ball-Berry stomatal conductance model influence gas exchange estimates between plants and the atmosphere.  However, there is limited data on the variation of these three parameters for annual crops under different environmental conditions.  Gas exchange measurements of light and CO₂ response curves on leaves of winter wheat and spring wheat were conducted during the wheat growing season under different environmental conditions.  There were no significant differences for V_cmax, J_max or m between the two wheat types.  The seasonal variation of V_cmax, Jmax and m for spring wheat was not pronounced, except a rapid decrease for V_cmax and J_max at the end of growing season.  V_cmax and J_max show no significant changes during soil drying until light saturated stomatal conductance (g_ssat) was smaller than 0.15 mol m^–2 s^–1.  Meanwhile, there was a significant difference in m during two different water supply conditions separated  by g_ssat at 0.15 mol m^–2 s^–1.  Furthermore, the misestimation of V_cmax and Jmax had great impacts on the net photosynthesis rate simulation, whereas, the underestimation of m resulted in underestimated stomatal conductance and transpiration rate and an overestimation of water use efficiency.  Our work demonstrates that the impact of severe environmental conditions and specific growing stages on the variation of key model parameters should be taken into account for simulating gas exchange between plants and the atmosphere.  Meanwhile, modification of m and V_cmax (and J_max) successively based on water stress severity might be adopted to simulate gas exchange between plants and the atmosphere under drought.