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

Crops with the C 4 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 CO 2 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 C 4 plant responses to elevated CO 2 has been controversial. Recent evidence from free-air CO 2 enrichment (FACE) experiments suggests that elevated CO 2 does not directly stimulate C 4 photosynthesis. Nonetheless, drought stress can be ameliorated at elevated CO 2 as a result of lower stomatal conductance and greater intercellular CO 2 . Therefore, unlike C 3 crops for which there is a direct enhancement of photosynthesis by elevated CO 2 , C 4 crops will only benefit from elevated CO 2 in times and places of drought stress. Current projections of future crop yields have assumed that rising CO 2 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 CO 2 will improve C 4 crop yields for food and fuel over the range of C 4 crop growing conditions and genotypes.