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Elevated CO2 Increases Sucrose-Phosphate Synthase Activity in Rice
Gesch, R.W., Vu, J.C., Boote, K.J., Allen Jr., L.H. and Bowes, G.  2002.  Sucrose-phosphate synthase activity in mature rice leaves following changes in growth CO2 is unrelated to sucrose pool size.  New Phytologist 154: 77-84.

What was done
Rice (Oryza sativa L.) was grown in controlled environment chambers receiving atmospheric CO2 concentrations of 350 ppm for about one month.  Thereafter, plants were either maintained at 350 ppm CO2 or switched to atmospheric CO2 concentrations of 175 or 700 ppm for an additional ten days to determine the effects of switching atmospheric CO2 concentrations on photosynthesis, growth and enzyme function in this important agricultural species.

What was learned
Within 24 hours after the CO2 concentration switch, plants placed in air of elevated CO2 displayed significant increases in the activity of sucrose-phosphate synthase, a key enzyme involved in the production of sucrose; while plants moved to air of sub-ambient CO2 exhibited significant reductions in the activity of this enzyme.  Similarly, elevated CO2 significantly increased the activity of ADP-glucose pyrophosphorylase, which is a key regulatory enzyme in starch synthesis, while sub-ambient CO2 significantly reduced its activity.

Sucrose concentrations in mature source leaves of plants decreased following their transfer to air of high CO2 concentration, while sucrose concentrations in sink organs (stems and sheaths) increased.  In fact, at one day post-transfer, sucrose comprised only 43% of the total nonstructural carbohydrates present in these sinks.  However, at ten days post-transfer (the end of the experiment), sucrose made up 73% of the total nonstructural carbohydrates present in stems and sheaths.

Plants switched to air of elevated CO2 concentration immediately displayed increases in their photosynthetic rates, while those switched to sub-ambient CO2 concentrations displayed immediate reductions in their photosynthetic rates.  In fact, at the end of the experiment, plants growing at 700 ppm CO2 still displayed photosynthetic rates that were 31% greater than those exhibited by unswitched controls, while plants subjected to 175 ppm CO2 displayed photosynthetic rates that were 36% less than those exhibited by the same control plants.  Ultimately, plants switched to atmospheric CO2 concentrations of 700 and 175 ppm displayed total aboveground dry weights that were, respectively, 54% greater and 18% less than those exhibited by control plants maintained at 350 ppm CO2.

What it means
As the CO2 content of the air rises, rice plants will likely exhibit increased rates of photosynthesis and carbohydrate production that should ultimately increase their biomass.  Indeed, the data acquired from this study suggest that rice plants may avoid the onset of photosynthetic acclimation by synthesizing and exporting sucrose from source leaves into sink tissues to avoid any photosynthetic end-product accumulation in source leaves.  Through this mechanism, rice plants can take full advantage of the increasing atmospheric CO2 concentration and stimulate their productivity and growth without exhibiting lower growth efficiencies resulting from photosynthetic acclimation.

Reviewed 28 August 2002