What was done
Rice was grown in SPAR units with atmospheric CO2 concentrations of 350 or 700 ppm for 34 days after planting, at which time half of the plants in each CO2 treatment had their CO2 concentrations switched for ten days (plants grown in elevated CO2 were exposed to ambient CO2, while those grown in ambient CO2 were exposed to elevated CO2) to determine the effects of this non-intrusive alteration of plant source:sink relationship on photosynthetic acclimation.  Leaf photosynthetic rates, rubisco and carbohydrate contents were measured in expanding and mature leaves prior to and at several intervals following the switching of CO2 concentrations.

What was learned
Average leaf photosynthetic rates were approximately 50% greater for plants grown continuously in elevated CO2 regardless of leaf age.  Within 24 hours of switching CO2 concentrations, the plants moved to elevated CO2 displayed a 15% increase in photosynthesis and a 19% reduction in rbcS mRNA, whereas plants put in ambient CO2 exhibited photosynthetic rates that were 10% lower than those of plants grown continuously at ambient CO2 and a 19% increase in the amount of rbcS transcript.  Leaf rubisco content, however, took much longer than the rbcS transcript to respond to switches in CO2.  Ten days after the switch, for example, plants moved to ambient CO2 increased leaf rubisco content by approximately 25%, whereas plants moved to elevated CO2 displayed little change in rubisco.  Switching the CO2 concentration also influenced the amount of leaf soluble carbohydrate content after ten days.  Plants placed in elevated CO2 experienced a 23% average increase in leaf carbohydrate content, while plants moved to ambient CO2 exhibited a 24% average decrease in carbohydrate content.

What it means
These data suggest that rice plants grown continuously in elevated CO2 experienced sink limitations to growth, which led to photosynthetic acclimation as indicated by increased leaf carbohydrate content and decreased rbcS mRNA and rubisco levels.  However, when the CO2 concentration was switched and plants from elevated CO2 were placed in ambient CO2, growth was no longer sink-limited, as indicated by the up regulation of rubisco levels and the decrease in leaf carbohydrate content.  Consequently, as the CO2 content of the air continues to rise, rice plants may exhibit some signs of photosynthetic down regulation, including decreased rubisco levels, but without sacrificing enhanced rates of photosynthesis.  In fact, with less leaf rubisco, more nitrogen becomes available to support sink development; and these sinks can then utilize greater amounts of leaf-produced carbohydrates, which may ultimately reduce --or even totally alleviate-- the degree of photosynthetic acclimation initially experienced.