Background
Plants grown in elevated CO2 for extended durations often, but not always, exhibit some degree of photosynthetic acclimation or down regulation, which is typically characterized by reduced rates of photosynthesis resulting from decreased activity and/or amount of the primary plant carboxylating enzyme rubisco.  When this phenomenon occurs, leaf nitrogen content often decreases, as nitrogen previously invested in rubisco is transferred to other parts of the plant.

Photosynthetic acclimation to elevated CO2 can be induced by insufficient plant sink strength that leads to carbohydrate feedback inhibition of photosynthesis.  Acclimation can also result from the physical constraints of growing plants in pots or by limiting their access to important nutrients such as nitrogen.  In this study, the authors tested the hypothesis that acclimation to elevated CO2 is primarily a response to nitrogen availability, and not a direct physiological effect of elevated CO2.

What was done
The authors conducted two different experiments on wheat to investigate the role of nitrogen supply in photosynthetic acclimation to elevated CO2.  In one experiment, plants were grown in pots placed within growth chambers receiving 350 or 650 ppm CO2; and they were irrigated with fixed amounts of low or high nitrogen solutions on a regular basis, which is standard protocol for experiments utilizing potted plants, as pots can only hold a finite amount of any solution.  In the other experiment, plants were grown hydroponically at 350 or 650 ppm CO2 to eliminate any root restriction effects on growth.  These plants were placed in nutrient solutions containing low or high concentrations of nitrogen, which were continually increased to match the rising demand of the growing plants.  All plants were grown for approximately five weeks and harvested.

What was learned
Elevated CO2 caused no photosynthetic down regulation in hydroponically-grown wheat plants regardless of nitrogen supply.  In fact, light-saturated rates of photosynthetic carbon uptake in plants grown at elevated CO2 were 56% greater than rates measured for plants grown at ambient CO2.

In potted plants, on the other hand, wheat exposed to elevated CO2 experienced photosynthetic down regulation resulting from reduced amounts of active rubisco.  Moreover, at the fixed low nitrogen supply, leaf nitrogen content was dramatically reduced, and this effect was exacerbated by elevated CO2, even though the low nitrogen pot-grown plants received more total nitrogen over the duration of the experiment than did their hydroponically-grown counterparts.

These data suggest that low nitrogen fertilization may not lead to photosynthetic acclimation in elevated CO2, as long as the nitrogen supply keeps pace with the relative growth rate of the plants.  Consequently, it is important for researchers who use potted plants to increase plant nutrient supply in proportion to plant growth as their experiments progress, in order to avoid inducing acclimation via the dilution of nitrogen that typically results from enhanced carbohydrate and biomass production in elevated CO2.

What it means
As the CO2 content of the air slowly but steadily rises, plants may not exhibit photosynthetic down regulation, even under conditions of low soil nitrogen; for if a plant can maintain a balance between its sources and sinks for carbohydrates at the whole-plant level, acclimation should not be necessary.  And because earth's atmospheric CO2 content is rising by an average of only 1.5 ppm per year, most plants should be able to adjust their relative growth rates by the small amount that would be needed to prevent low nitrogen-induced acclimation from ever occurring.