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Down-Regulation of Photosynthesis in CO2-Enriched Air
Reference
Ainsworth, E.A., Rogers, A., Nelson, R. and Long, S.P.  2004.  Testing the "source-sink" hypothesis of down-regulation of photosynthesis in elevated [CO2] in the field with single gene substitutions in Glycine maxAgricultural and Forest Meteorology 122: 85-94.

What was done
In an experiment designed to test the "source-sink" hypothesis of down-regulation of photosynthesis in CO2-enriched air, soybean lines that vary by a single gene altering either growth habit (determinate or indeterminate) or nodulating capacity (nodulating or non-nodulating) were grown within the SoyFACE facility in central Illinois, USA at CO2 concentrations of 370 and 550 ppm, while photosynthesis measurements were made during the flowering growth phase in 2001 and the pod-filling growth phase in 2002.

What was learned
The authors report that significant acclimation of photosynthesis was only apparent in the non-nodulating Williams-NN and the determinate Williams-dtl varieties, both of which were derived by single gene mutations of the nodulating indeterminate Williams variety, which finding, in their words, "is consistent with the expectation that acclimation only occurs when sink capacity is lowered (Rogers et al., 1996, 1998; Drake et al., 1997; Morgan et al., 2001)."

What it means
Ainsworth et al. say their study demonstrates that "preventing nodulation and restricting main stem growth produce acclimation of photosynthesis to elevated CO2."  Hence, they pretty much prove that as long as there is a substantial and expandable sink for receiving photosynthates, down regulation of photosynthesis will not occur.  Their findings also open the door to the possibility that genetic manipulation may enable plants to take better advantage of the ongoing rise in the air's CO2 content, although they note that "conversion of the determinate Elf variety to indeterminate did not result in an up-regulation of photosynthesis in elevated CO2."  Clearly, more work will be required to fully elucidate this latter subject.

References
Drake, B.G., Gonzalez-Meier, M.A. and Long, S.P.  1997.  More efficient plants: a consequence of rising atmospheric CO2Annual Review of Plant Physiology and Plant Molecular Biology 48: 609-639.

Morgan, J.A., Skinner, R.H. and Hansen, J.D.  2001.  Nitrogen and CO2 affect regrowth and biomass partitioning differently in forages of three functional groups.  Crop Science 41: 78-86.

Rogers, G.S., Milham, P.J., Gillings, M. and Conroy, J.P.  1996.  Sink strength may be the key to growth and nitrogen responses in N-deficient wheat at elevated CO2Australian Journal of Plant Physiology 23: 253-264.

Rogers, A., Fischer, B.U., Bryant, J., Frehner, M., Blum, H., Raines, C.A. and Long, S.P.  1998.  Acclimation of photosynthesis to elevated CO2 under low-nitrogen nutrition is affected by the capacity for assimilate utilization: perennial ryegrass under free-air CO2 enrichment.  Plant Physiology 118: 683-689.


Reviewed 5 May 2004