Rising atmospheric carbon dioxide concentration and the future of C ₄ crops for food and fuel

Crops with the C₄ photosynthetic pathway are vital to global food supply, particularly in the tropical regions where human well-being and agricultural productivity are most closely linked. While rising atmospheric [CO₂] is the driving force behind the greater temperatures and water stress, which threaten to reduce future crop yields, it also has the potential to directly benefit crop physiology. The nature of C₄ plant responses to elevated [CO₂] has been controversial. Recent evidence from free-air CO₂ enrichment (FACE) experiments suggests that elevated [CO₂] does not directly stimulate C₄ photosynthesis. Nonetheless, drought stress can be ameliorated at elevated [CO₂] as a result of lower stomatal conductance and greater intercellular [CO ₂]. Therefore, unlike C₃ crops for which there is a direct enhancement of photosynthesis by elevated [CO₂], C₄ crops will only benefit from elevated [CO₂] in times and places of drought stress. Current projections of future crop yields have assumed that rising [CO₂] will directly enhance photosynthesis in all situations and, therefore, are likely to be overly optimistic. Additional experiments are needed to evaluate the extent to which amelioration of drought stress by elevated [CO₂] will improve C₄ crop yields for food and fuel over the range of C₄ crop growing conditions and genotypes.