What was done
The authors grew sorghum (Sorghum bicolor) plants in controlled environment chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm for approximately two months to determine the short-term effects of elevated CO2 on this important C4 crop.

What was learned
Elevated CO2 reduced sorghum photosynthesis rates by about 16%, contrary to the results of an earlier study conducted by the authors, where photosynthetic rates nearly doubled with atmospheric CO2 enrichment (Watling and Press, 1997).  Part of this decrease was likely due to an observed 50% reduction in the concentration of leaf
PEP carboxylase, the initial photosynthetic enzyme that converts CO2 into sugars within the mesophyll cells of C4 plants.  In turn, these sugars are transported from mesophyll cells into specialized bundle sheath cells where they are decarboxylated, allowing CO2 to concentrate internally within these cells prior to its conversion back into sugars by the enzyme rubisco.  Interestingly, the bundle sheath cells in the CO2-enriched plants were only about one-half the thickness of that observed in ambiently-grown plants.  Thus, it appears that this C4 species has the ability to acclimate to elevated CO2 in much the same way most C3 plants do: they can reduce their investment in their primary CO2-fixing enzyme (PEP carboxylase) and other related components (bundle sheath cell walls) and use the saved resources for processes that are more limiting to growth.

What it means
As the atmospheric CO2 concentration continues to rise, it is likely that sorghum will respond positively by mobilizing resources away from its photosynthetic apparatus and into other processes that are more limiting to plant growth.  For example, reductions in leaf PEP carboxylase content and the thickness of the walls of bundle sheath cells similar to those observed in this experiment may have contributed to the 36% CO2-induced plant biomass increase reported earlier for sorghum by these authors (Watling and Press, 1997).  Thus, it appears that C4 plants may be more flexible to environmental change than has previously been thought.  In fact, the authors note that this phenomenon "could have consequences for the persistence of C4-dominated communities in response to climate change and rising atmospheric CO2 concentrations."  In other words, their work suggests that C3 plants will not necessarily replace C4 plants in areas where they are currently found as the air?s CO2 content rises, thus tending to preserve the present state of balance between C3 and C4 plants in ecosystems around the world.

Reference
Watling, J.R. and Press, M.C.  1997.  How is the relationship between the C4 cereal Sorghum bicolor and the C3 root hemi-parasites Striga hermonthica and Striga asiatica affected by elevated CO2?  Plant, Cell and Environment 20: 1292-1300.