Background
The authors note that photosynthesis by C4 plants is thought to be nearly saturated at today's atmospheric CO2 concentration and that "a rise in atmospheric CO2 presumably may have little impact on C4 photosynthesis and growth." However, they report that numerous experiments have revealed there is "a positive growth response of many C4 plants to elevated CO2, although to a smaller extent than C3 plants."

What was done
In a study designed to see how the C4 crop sugarcane (Saccharum officinarum L.) responds in this regard, Vu et al. grew well watered and fertilized plants from stalk cuttings under field-like conditions in temperature-gradient greenhouses maintained at atmospheric CO2 concentrations of either 360 or 720 ppm, while measuring a number of plant parameters and processes.

What was learned
Relative to plants growing in ambient-CO2 air, the three researchers found that in plants growing in twice-ambient-CO2 air, a number of positive developments occurred: (1) leaf sucrose phosphate synthase was increased by 13% and 37% at 7 and 14 Days After Leaf Emergence (DALE), respectively, (2) leaf sucrose concentration was 31% and 19% higher at 7 and 14 DALE, respectively, (3,4) total chlorophyll and soluble protein were 31% and 15% greater, respectively, at 14 DALE, (5,6,7) Rubisco, pyruvate Pi dikinase and NADP-malate dehydrogenase were up-regulated by 21%, 117% and 174%, respectively, at 14 DALE, (8) leaf carbon exchange rate was 20%, 7% and 10% greater at 7, 14 and 32 DALE, respectively, and (9,10,11) stomatal conductance was 51% lower while there was 39% less transpiration and 26-52% greater water-use efficiency during growth and development. Then, at the end of the study, they determined that elevated CO2 did five other positive things: it (12) augmented leaf area by 31%, (13) enhanced leaf fresh weight by 13.5%, (14) increased stem fresh weight by 55.5%, (15) boosted total above-ground plant fresh weight by 44%, and (16) pumped up stem juice volume by an amazing 83%.

What it means
Vu et al. state that "the up-regulation of the key photosynthesis and sucrose metabolism enzymes at early stages of leaf development," together with "a reduction in leaf stomatal conductance and transpiration and an improvement in leaf water use efficiency and plant water status, could lead to an enhancement in leaf area, plant biomass accumulation and sucrose production for the CO2-enriched sugarcane plants," which was, in fact, what they observed in their experiment. Hence, since sugarcane is one of the four most important C4 crops in the world (Brown, 1999), and since about 20% of global gross primary productivity is provided by C4 plants (Lloyd and Farquhar, 1994; Cerling et al., 1997; Ehleringer et al., 1997), these findings bode well indeed for humanity and nature alike in a CO2-enriched world of the future.

References
Brown, R.H. 1999. Agronomic implications of C4 photosynthesis. In: Sage, R.F. and Monson, R.K. (Eds.), C4 Plant Biology. Academic Press, San Diego, CA, pp. 473-507.

Cerling, T.E., Harris, J.M., MacFadden, B.J., Leakey, M.G., Quade, J., Eisenmann, V. and Ehleringer, J.R. 1997. Global vegetation change through the Miocene/Pliocene boundary. Nature 389: 153-158.

Ehleringer, J.R., Cerling, T.E. and Helliker, B.R. 1997. C4 photosynthesis, atmospheric CO2 and climate. Oecologia 112: 285-299.

Lloyd, J. and Farquhar, G.D. 1994. ¹³C discrimination during CO2 assimilation by the terrestrial biosphere. Oecologia 99: 201-215.