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Subambient CO2 Increases Rubisco Content in Rice
Gesch, R.W., Vu, J.C.V., Boote, K.J., Allen, L.H., Jr. and Bowes, G.  2000.  Subambient growth CO2 leads to increased Rubisco small subunit gene expression in developing rice leaves.  Journal of Plant Physiology 157: 235-238.

What was done
Rice (Oryza sativa) was grown in controlled environment chambers receiving atmospheric CO2 concentrations of 350 and 175 ppm to study the effects of subambient CO2 concentrations on photosynthesis in this agricultural species.  Although both chambers were initially subjected to ambient CO2 concentrations, at 34 days after planting one of the chambers had its atmospheric CO2 concentration reduced to only 175 ppm, which was then maintained throughout the remaining ten days of the experiment.

What was learned
One day after lowering the atmospheric CO2 concentration, plants exposed to 175 ppm CO2 displayed photosynthetic rates that were 45% less than those exhibited by control plants growing at the ambient CO2 concentration.  Moreover, at day five of half-ambient CO2 exposure, the photosynthetic reduction become even more severe, reaching 52%.  However, between days five and ten of half-ambient CO2 exposure, plants growing in this unfavorable treatment exhibited a 35% increase in their photosynthetic rates, but subsequent rates were still significantly lower than those observed in plants grown at 350 ppm CO2.  This delayed photosynthetic enhancement was at least partly due to 24 and 19% increases in rubisco activity and content, respectively, within developing leaves.  Thus, low CO2 concentrations induced an up-regulation of rubisco and photosynthetic rates in young leaves of rice.

What it means
Prior to the Industrial Revolution, it is likely that rice had a more difficult time growing than it does today.  At lower atmospheric CO2 concentrations, it is likely that rice plants had to reallocate valuable resources into rubisco and other photosynthetic proteins just to ensure that net carbon uptake could prevail for plant survival.  Thus, it is likely that plants were smaller and produced less yield than they do today.  Therefore, it logically follows that the rise in the air's CO2 content, beginning with the Industrial Revolution and still ongoing, is making carbon uptake and biomass production easier and more efficient for this important agricultural species.

Reviewed 15 November 2000