Photosynthesis and respiration are two central and basic physiological processes in regulation to carbon budget and carbon sink in the terrestrial ecosystem, as well as assessment and feedback of highly variable and fluctuating environmental conditions. Elevated CO₂, salinity, alkalinity, drought, flood, nutrient deficiency and toxicity, extreme cold and heat, and various natural and anthropogenic pollutants are critical challenges in near future and their primary target is to alteration of photosynthetic and respirational processes, which ultimately influence the final yield potential of major agricultural crops. However, plants adapt themselves to these environmental circumstances via complicated changes at physiochemical and molecular levels. The current chapter highlights the underline mechanisms of photosynthetic and respiration in response to climate change at physiological, biochemical, and molecular levels …