Photosynthesis is the process by which plants sense and respond to changes in atmospheric [CO2] concentration, climate, and soil, and therefore, understanding how this process is affected is very important. This book examines the effects of changes in temperature, [CO2], light intensity and nitrogen nutrition on photosynthesis, based on experiments involving gas exchange and fluorescence techniques on plants grown at different climate and nitrogen nutrient condition in a controlled growth chamber. Short-term responses of photosynthesis and long-term acclimation to [CO2] and temperature are examined. Scaling up from the leaf to the canopy level is simulated using the sun-shade and big-leaf model presenting algorithm for calculating diurnal canopy assimilation rate using parameters from the single leaf light and [CO2] responses, the canopy extinction coefficient for light and nitrogen, canopy leaf area index, canopy nitrogen concentration , daily solar irradiance and daily maximum and minimum temperatures; taking into account the significance of diffuse radiation in canopy assimilation rate and efficiency.