What is the chemistry of photosynthesis, and how will it be affected by rising atmospheric CO₂?

Crops could be affected by increased atmospheric CO₂ in multiple ways. Higher CO₂ levels generally lead to increased photosynthesis. This is especially true for C₃ plants, which are more sensitive than C₄ plants to CO₂ levels. Because increased photosynthesis leads to greater plant growth, C₃ crops such as wheat and rice will tend to grow more. However, it is unclear whether this growth will be in the vegetative parts of the plant (stems and leaves) or in the part we eat (grain). To further complicate matters, such increases in plant growth may be counteracted by the effects of increased CO₂ on climate. For example, increased temperatures would increase the rate of photosynthesis and extend the growing season, but might alter rainfall patterns. In some areas of the world there might be less rain, and this could limit plant growth.

Future directions

As the human population rises and land for farming is limited, we are challenged to improve the productivity of crops. A significant increase in overall crop productivity would have global consequences. Increased atmospheric CO₂ will not have a great effect on C₄ plants. But the C₄ pathway accounts for about 25 percent of all net carbon fixed. Some important species, such as corn, sorghum, and sugarcane, have the C₄ pathway, but the major grain crops such as rice do not. An intensive research program is under way to understand the genetic control of the development of the C₄ pathway, including not just the enzymes involved but also how the distinctive leaf anatomy develops. This research may lead to greater use of the C₄ pathway, and its related increase in productivity in more plant species in a higher-CO₂ world.