Root to shoot ratio of crops as influenced by CO2

Crops of tomorrow are likely to grow under higher levels of atmospheric CO2. Fundamental crop growth processes will be affected and chief among these is carbon allocation. The root to shoot ratio (R:S, defined as dry weight of root biomass divided by dry weight of shoot biomass) depends upon the partitioning of photosynthate which may be influenced by environmental stimuli. Exposure of plant canopies to high CO2 concentration often stimulates the growth of both shoot and root, but the question remains whether elevated atmospheric CO2 concentration will affect roots and shoots of crop plants proportionally. Since elevated CO2 can induce changes in plant structure and function, there may be differences in allocation between root and shoot, at least under some conditions. The effect of elevated atmospheric CO2 on carbon allocation has yet to be fully elucidated, especially in the context of changing resource availability, Herein we review root to shoot allocation as affected by increased concentrations of atmospheric CO2 and provide recommendations for further research. Review of the available literature shows substantial variation in R:S response for crop plants. In many cases (59.5%) R:S increased, in a very few (3.0%) remained unchanged, and in others (37.5%) decreased. The explanation for these differences probably resides in crop type, resource supply, and other experimental factors. Efforts to understand allocation under CO2 enrichment will add substantially to the global change response data base.