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Effects of Elevated CO2 on Gas Exchange in Sorghum
Cousins, A.B., Adam, N.R., Wall, G.W., Kimball, B.A., Pinter Jr., P.J., Leavitt, S.W., LaMorte, R.L., Matthias, A.D., Ottman, M.J., Thompson, T.L. and Webber, A.N.  2001.  Reduced photorespiration and increased energy-use efficiency in young CO2-enriched sorghum leaves.  New Phytologist 150: 275-284.

What was done
The authors grew Sorghum bicolor at atmospheric CO2 concentrations of 370 and 570 ppm within a FACE experiment in Arizona, USA, to study the effects of elevated CO2 on gas exchange within this important agricultural C4 grain crop.

What was learned
Within six days after planting, photosynthetic rates of second leaves of CO2-enriched plants were 37% greater than those of second leaves of ambiently-grown plants.  This CO2-induced photosynthetic enhancement slowly declined over the growing season, stabilizing around 15% for the time period between 23 and 60 days after planting.  In addition, when measuring photosynthetic rates at reduced oxygen concentrations of 2%, the authors observed 16 and 9% increases in photosynthesis for ambient and CO2-enriched plants, respectively, indicating that elevated CO2 was reducing photorespiratory carbon losses.  Even so, this phenomenon could not fully account for the CO2-induced stimulation of photosynthesis.  Thus, after further investigation, the authors suggested that elevated CO2 might have decreased CO2 leakage from specialized bundle sheath cells, which concentrate CO2 internally to promote photosynthetic carboxylation reactions by the enzyme rubisco.

What it means
As the atmospheric CO2 concentration increases, it is likely that the C4 grain crop sorghum will display enhanced rates of photosynthesis due to reductions in photorespiratory carbon loss and, possibly, carbon leakage from specialized bundle sheath cells.  These improvements will likely contribute to increases in marketable grain yield.