As with all C3 crops, current atmospheric concentrations of CO2 are limiting to photosynthesis in tomatoes. In phytotrons or when greenhouses are closed and light is high, CO2 may be depleted below ambient levels, reducing photosynthesis and growth. However, the effects of raising CO2 concentrations above ambient are more complex. Tomatoes are not particularly responsive to high CO2, compared to other C3 plants (Poorter, 1993) ,p