Comparing with C₄ crops, C₃ crops need more CO₂ because of their high light respiration rate and low photosynthesis rate. When the concentration of CO₂ reached 1 000 mg/kg, production of cucumber, tomato and pepper increased by 83.28%, 69.12% and 49.28% respectively. The increase of CO₂ raised light saturated point, so the photosynthesis rate lifted and sunlight energy use efficient improved. At the same time, light respiration rate of C₃ crops was limited and lose of energy was reduced.